# C++ Keyword inline and .inl Files

While at the bar a group of friends jokingly mocked some of the more silly features of C++. The initial banter consisted of how the STL implemented everything including the kitchen sink, though forgot to implement std::girlfriend.

Wouldn’t std::girlfriend be great? We can plug in any type of girlfriend we want into the template parameters and the compiler will just generate one for us! Why in the world would std::girlfriend be omit from STL?

Oh of course, std::girlfriend was never implemented because everyone is just going to put in way too many specific template types (super hot, not crazy) and it’ll just end in a bunch of “failed to specialize template” error messages. And then the moment too many of the template parameters are removed we’ll just get a bunch of “multiple symbols defined” linker errors! Maybe it was a good idea to never implement std::girlfriend in the first place. After all, a girlfriend prefixed with std might make one thing of something other than C++…

Jokes aside I brought up the fact that inline is totally useless for inlining. The only real reason to use the inline keyword (in my opinion) is to able to define functions within a header. Well, I brought it up as a joke, but not really a joke, and that’s the joke.

The inline keyword and .inl files can actually be a pretty nice organizational tool for code, and I’ve found it helps users that didn’t write the implementation understand the code.

Say we are implementing some kind of algorithm that stores elements in an array. Elements need to refer to one another (perhaps to build intrusive linked lists), although these arrays ought to be relocated in memory without requiring any complex copy routines; a single memcpy should yield a new and valid copy.

One way to do so is to make use of array indices instead of pointers. Usually a myriad of small helper functions will arise to clean up all of the array indexing that usually ensues shortly after this kind of code crops up. It’s a huge pain to look into a .cpp and have to continually navigate passed a lot of tiny and trivial helper functions just to understand the algorithm.

These small helpers can be swept to the side into a .inl file. The .inl file signature immediately tells the user what kind of code resides within (either templates or inlined functions), and usually this kind of code isn’t very necessary to understand the more heavy duty code within the .cpp file.

Here’s a mock example:

Aren’t these example files pretty easy going to read? I’m sure you at least scanned the .inl file briefly, and will probably never really need to look at it again. Time will be well spent in the .cpp file with less code to clog your brain. And who knows, maybe the compiler (or perhaps the linker) actually cares a little bit when we type the inline keyword.

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# Small C++ Reflection Demo

I created a small demonstration program that explains the core ideas behind implementing a custom reflection system for C++. More might be written in this post in the future — for now I’m just storing the demo right here on this webpage:

# What is there to Hate about References?

I find most usage of references annoying cruft. Often the arguments I see or hear that are “pro-reference” make the same lame points that most of the internet makes:

• Pointers are dangerous
• Pointers are ambiguous and confusing
• NULL pointers lead to undefined behavior and crashes

Just google “pointers and references” and you’ll see bad advice everywhere. A new programmer seeing these bullet points is likely to get hyped about using references everywhere. Seeing advice like this just sort of upsets some part of me. Perhaps it’s because when the above statements are plastered onto websites they state them as fact.

In an effort to not make the same annoying mistake as every other article on the internet I’ll present my opinion as an opinion. By stating something as an opinion the reader will immediately begin to read with a certain amount of skepticism. This might coax newer readers into thinking for themselves, which ought to be the goal of writing an educational article on the first place. Writing step-by-step instructions on how not to use “dangerous pointers” is the worst way to write on the topic of pointers and references.

I know I sound pretty bitter. I recall a time when I browsed the internet and looked for advice on this exact topic. It takes time to unlearn bad things, and so this post was born.

## Memory Matters

Where things are in memory is a big deal. Memory access is commonly a bottleneck in real-time applications, and code that has ambiguous memory accesses patterns upsets me. Imagine peering into a large function that is operating on some kind of data. Scanning the middle of the function a few lines of code are encountered:

Just be reading this code it isn’t immediately understandable as what the variable d is. What is happening here? In order to know what the scope of d is some scrolling or manual code navigation will ensue. How long will this take? How much does it cut down on focus while the reader is just trying to understand the code?

Often times for member variables of an internal class or struct will be appended with the m_ prefix. This is nice as readers immediately know the scope and implications of all uses of a member variable. There’s an implicit this pointer being accessed somehow, and the variable’s scope is relative to this class’s definition.

In this case there’s no such nice prefix. d can either be a reference or value type. There’s no way to know without some kind of intellisense. Mousing over a variable to see what the type is, given a nice IDE, is not really that big of a deal. The big deal here is that if you have to mouse over something to get an idea of what sort of memory this represents. Just take a look at this code:

What sort of questions might the user have about this code? Clearly d is a pointer to some memory. This immediately informs the reader about the nature of the code. is likely to be some kind of output, or perhaps a specific element in an array. It is definitely not just a lone variable on the stack. No intellisense is needed to get this information, no mousing over or code navigation is needed just to understand the idea of assigning a value to some non-local stack memory. The programmer reading this code might be able to focus slightly better on understanding the code due to the use of a pointer.

Sure d could technically be a *NULL* pointer (gasp), but in reality this is a non-issue. The only times checking for NULL pointers is important is when user-input is being handled. A lot of code doesn’t deal with user input! For internal code I’d make the argument that memory not on the local stack scope (local to at least the function currently executing” should almost always be referred to by pointer. Internal code can make assumptions on how pointers are used and not care about the NULL case. Internal code often solves difficult problems, and needs to be efficient (in the scope of real-time applications). Anything that fragments reader focus is bad, even taking a moment to mouse of a variable to see if it’s a reference or not.

## Another Example

In the above snippet imagine that the joined AABB is being written to, by finding the AABB that bounds both a and b. Perhaps in this specific case it is fairly obvious that joined is memory that is being written to by the MergeAABBs function. This is probably because joined was quite well named, despite being passed by reference to MergeAABBs. However this function might have been written in a way that returns a new AABB entirely by value, and only operates on a local stack copy of joined. In this case the code would compile and run perfectly fine, but joined would have unitialized memory. This might lead to a crash or assert, thus lower iteration time and programmer focus.

Now lets look at the use of a “dangerous” pointer:

In this code snippet, no matter what the third parameter is named as, it is as obvious as possible that the function MergeAABBs is operating on some memory passed to it, and does not return anything useful in this particular use-case. The contents of the function MergeAABBs is probably obvious as well, I know I can imagine how it’s implemented without even looking; there’s just no ambiguity.

The name of variables should be meaningful to the problem the code is solving. Requiring a naming convention for code clarity simply because of an arbitrary reference function parameter is an unnecessary constraint! Naming things is hard enough without random convention limitations.

Sure if some idiot passed in a NULL pointer to MergeAABBs it will crash, but how often does this happen in practice? What kind of programmers are you hiring that actually get caught up in this kind of problem? Real-life competent engineers aren’t going pass in a NULL pointer and will appreciate good code written with “dangerous and ambiguous” pointers.

When a function takes only floats and writes to a float it’s pretty much worst-case for reference ambiguity. Which float is being written to in the next code snippet?

Is the triangle actually {a, b, c}, or some other combination of parameters? Which of the arguments are float arrays (vectors) or just floats? Which ones are written to, and which are read only? Some kind of code navigation is needed to know for sure. By convention uvw might represent the name of barycentric coordinates for a triangle, but perhaps this specific piece of code was solving a particular problem where the derivation named them something else? It’s just ambiguous without a pre-defined naming convention, of which is imposed in the middle of non-related algorithms.

Here’s the pointer version; note how non-ambiguous this code is:

## Useful References

I currently know of a single use of references that I really like, and that’s a const reference passed to a function as an argument, and sometimes the returning of a const reference.

Passing a const reference to a function means that this is a read-only value, and is definitely not pointing to an array. It is also a pretty common convention for operator overloading. The only downside is that the dot access operator may be mistaken as a value-type access, instead of a pointer access.

Returning a const reference might make sense sometimes, but usually I’m of the opinion that a pointer is better. Returning a pointer just abides by all my previous points about memory access. If the user retrieves a const pointer from a function, the explicit -> access makes it very clear that this memory came from somewhere else!

References are also able to capture temporary rvalues. This can make the life-time of such temporary values more explicit.

Sometimes a million dereferences is just too many. In some cases the lack of the dereference operator is nice and adds to code readability. However, in this case references are just an aid and live only on the stack. The pointer is what is actually kept and stored, in order to keep the code clean and up-front. Here’s an example:

An equivalent, but more verbose and annoying version can be constructed with pointers:

## “Advanced C++” and Generic Programming

“Advanced C++” features (in quotations for sarcasm, like much of the rest of the article) are useful sometimes, there’s no doubt about. Templates, classes, and all the weirdness therein is sometimes necessary. The amount of code duplication and boilerplate that these features can be remove makes them important.

However, a lot of code is type-static and very specific. Code often solves very particular, specific problems. A lot of newer students (me) and colleagues get all caught up in the features and just end up wasting their time. When I say waste time, I mean they were actually trying to finish a project, instead of just learn about C++ and the uses thereof.

One might view C++ from the perspective that all the “advanced features” just can egg-on a programmer into over engineering their code into a weird mess of indirection and verbose templated code. Crazy inlined callbacks, type agnostic code, verbose namespaces and whatnot. Many times it’s just useless cruft, and a specific implementation for a single problem will be simpler, easier to read, and smaller in code size.

All of this ranting comes down the the point of: references let code operate in a slightly more agonistic manner, which is great for templates. The dot operator does it all! This makes sense for code that needs templating, but often doesn’t make sense for a lot of code (which was what the previous portions of the article pointed out!).

However, good generic code is so incredibly difficult to design and come up with that hardly anybody should be doing it. Good code that is used by multiple people is at least an order of magnitude harder to write than good code that has a single specific use case. Templates, references, classes, these things might make it tempting to try out all the features to make a “generic program” that “can be re-used in the future”. I’ll tell it how it is: simple code that is type static, specialized for the specific problem it is solving, and doesn’t use advanced features is probably an order of magnitude more reusable (and performant) than “generic” code, simply because it’s easier to write.

## Form an Opinion

As a reader, think for yourself and make your own judgment calls based on your own experience. This means that a good way to take advantage of the knowledge of an experienced programmer is to try out their advice with an almost skeptical attitude. Just don’t look for step-by-step instructions on how to be a computer scientist. Nobody wants to work with a mindless programmer that writes bad code, because then the good programmers will be busy cleaning it up.

# Cache Aware Components

Special thanks to Danny Frisbie for a nice discussion on the PODHandler implementation!

Let me start off by saying that optimizations really only need to be applied to bottlenecks. In order to know where a bottleneck might occur (especially cache related ones) you’ll probably need some experience. The experience not need be your own, but the experience will come from someone. In my (limited) experience the only bottlenecks I’ve ever seen in any piece of game related software (aside from N^2 loops with a high N) are always due to waiting on things to be placed into the cache. It’s really easy to write bad code, and bad code is usually cache oblivious. Even conceptually clear and understandable code can still be cache oblivious!

I’ve seen some very nice 2D and 3D games, made in C++, that used only rudimentary memory allocation schemes and naive component implementations. They ran at 60 fps just fine. If you’re a hobbyist or just trying to learn, then thinking about how to write the fastest component framework ever might be fun but don’t expect to do it correctly on your first try. Expect to fail, and then iterate.

So when the time comes to actually optimize something, having some sort of idea of where to look to learn how solve cache related problems will be valuable.

## Data Lookups and Cache Lines

In general the cache line size for hardware nowadays seems to be mainly 64 bytes. A cache line is a 64 byte piece of memory that is on 64 byte boundaries. Whenever data is transferred from one cache to another (or to/from main memory (RAM)) the memory is transferred in a cache line. This keeps the memory bus busy. 16 32-bit integers would be the size of a single cache line, or 16 32-bit floating point numbers. This reduces to the size of a 4×4 matrix of 4 element floating point scalars.

How fast a cache line is transferred depends on which level it is being fetched from. In general terms: when a piece of memory is fetched by the CPU from a lower level cache it is hoisted up into the L1 data cache (L1 D, L1 I is the instruction cache for code). If this memory was not in the cache it will take forever (100 to 300 cycles, probably near the 300 range for PC). Here’s a nice diagram by Naughty Dog summarizing the common cache setup for PC CPUs:

When something is loaded into the L1 cache whatever was there before has to be evicted, and will be pushed into the L2 cache (again using the same 64 byte cache line size). This will probably evict something from the L2 cache back down into the L3 cache, and so on and so forth.

The implication here is that whenever a cache line is read it is up to the programmer to try to use as much of that cache line as possible. Even though we might have 8 gigabytes of RAM, if we aren’t running in the cache the CPU will be sitting there waiting. Even if a single byte is read from main memory and entire cache line will be fetched. Reading a single byte from a random location in main memory is about the worst possible way to use memory.

This tends toward the idea of using very compact and concise data structures. If a data structure is packed together in memory it can be operated upon by the CPU very quickly once it arrives to the CPU’s cache.

The cache isn’t very big. Here’s a nice slide by Scott Meyers on the topic:

32KB of L1 data cache is tiny. You don’t even get to use all of it as the operating system does need to do stuff too!

## Prefetching

Prefetching exists to try to hide the latency of fetching memory. A prefetch is when a cache line is preemptively fetched and placed into the cache, such that when the memory is actually requested a cache hit occurs.

Hardware can detect patterns in real-memory accesses, but it can only detect pretty simple patterns like array traversals. Scott Meyers describes (see resources section) that the hardware is made in such a way that it can detect iterating over arrays forwards, backwards and with variable (but constant) element step size. It can also do this for all hardware threads simultaneously. However, if you’re not looping over an array you can’t count on any intelligent prefetching. It will take two or more cache misses in a recognizeable pattern to start automatic prefetching.

Usually compilers provide a specific keyword to hint to the run-time to grab a specific cache line from somewhere in memory. This can be used by programmers to ease out a final bit of performance, given a proper implementation to prefetch for.

## Cache (Un)Aware Components

Hopefully by now readers are convinced that contiguous arrays of data are very friendly to the cache, and where performance matters this knowledge should be exploited.

In a component based game engine architecture looking up and operating on components is often the first bottleneck encountered. It might pay to learn a little about how this might be circumvented.

A memory naive implementation of components will look something like this:

Each component is allocated on the heap explicitly by the operating system, which is going to require a context switch and be very sensitive to memory fragmentation.

Ideally all data of a certain type will be packed together in a tight linear array. When this data needs to be operated upon, the fastest sort of transformation (without manual prefetching) will look something like this (for a generalized example):

Ideally the size of a given element will be below the size of a cache line, and often times this is possible if extraneous data can be removed from the Data type.

The explicit consumption of the data where a local copy is made is probably not necessary and will be compiled away. It is fairly easy to check the assembly by using a compiler to process to an assembly file to double check. However this kind of practice can be very helpful to let the compiler know that multiple pointers cannot possibly alias the same type. For more information search for “C++ aliasing Ericson” to find Christer’s old slides.

Now that the ideal computational situation for transforming a large data set has been described, lets look at a common (albeit contrived) data transformation that we’ve all been guilty of while first learning:

Conceptually this code is very concise and easy to reason about. Though the code readability and dynamic niceties aren’t very efficient. Random calls to delete occur, the inner loop contains a branch, and called Update on an object will go to who knows where in memory. All of these things are basically punching the cache right in the gut. Even the branch can be annoying for the CPU pipeline as it may have to eject code out of the L1 I cache if a branch is mispredicted!

How can this be solved? The first step is to make sure as much data is packed together in memory as possible. In the above code snippet the list can be changed to an array (perhaps std::vector). Okay, pretty trivial change, no big deal. Objects can perhaps just be placement new’d into the array and placement deleted. This will act like a memory pool.

The next step is to identify that the UpdateGameObjects function is performing two types of tasks (assuming the Update function performs a single task); deletion and update calling. This is a result of the container of objects not being sorted. It is a non-homogeneous collection of objects that are both alive and dead. If objects can be separated into sections of dead and alive, only the alive objects need to be looped over.

## Cache Aware Components

One way to implement this would be to have the beginning of the array contain a contiguous line of live elements. The rest of the array can contain “deleted” or “not yet allocated” elements. In order to uphold this invariant it might be best to design objects placed into these sorts of arrays not care if they are moved in memory. Usually this means making your data a plain old data type (POD).

Deleting things from the array is going to be a nice feature to support. A game wouldn’t be interactive for very long if it could only consume more and more memory. A simple and very effective scheme is to move the last element of your array into the location of a deleted element.

However in order to refer to a unique element within an array simple pointers are no longer going to cut it. When an element is moved from the last index into a deleted slot any pointers to the old spot (the last and now empty element) will be dangling. Some form of translation from one data type into a pointer must occur in order to ensure that the correct pointer is retrieved for a unique entry in the array.

Usually this translation comes in the form of a handle. A handle can be implemented as an integer divided into two different sections. The details of how to implement a handle should be known by the reader before continuing on, so please view Llopis as a resource.

Lets create a simple abstract data type that grants access to an array of POD elements, of which supports handle translation, allocation and deletion:

In order to implement these two functions please do refer to the Llopis resource referenced in the last paragraph.

In order to implement Release things start to get tricky. How does the PODHandler update the handle of the element it moved? Somehow the location of the internal handle entry needs to be accessible just by knowing where the element was moved from. The easiest solution is to place a handle inside the type T within each element of the array. However, it would be great if types that are held inside of PODHandlers can fit within a single cache line. Adding a handle to every single element lowers the density of the data in the array. For certain situations this data bloat, though only 4 bytes per elements, will reduce the effectiveness of every single cache line from 64 bytes to 60.

Clearly an alternative could be used! The solution is to yet again separate different types of data into different arrays. The internal array of the PODHandler should consist of homogeous data! Rip out that intrusive handle and place them all in their own arrayPODHandler can now consist of 3 arrays in total: an array of type T, an array of Handles, and an array of integers.

The array of integers share their indices with the array of PODs. This means a POD in element 3 will correspond to the integer in element 3 of the integer array. The integer array contains indices that map to the handle associated with a given POD element in the handle array m_entries.

Though readers may by now be wondering “wouldn’t three different arrays potentially have worse locality of reference than just two arrays?”, and this would be a good thing to wonder. It is true that an intrusive handle would be preferable if handle translations are extremely frequent. If they are, the original intrusive handle implementation may be ideal.

If an engine is architected to focus on cache utilization for transforming large data sets with expensive operations, then a homogeneous array will be preferential. Or in other words if you want to loop over a lot of stuff and do expensive math on each element, that array better be dense. This means that handle translations are more infrequent since the code focuses on looping over the data array itself rather than picking out individual elements at random.

## Open Source Implementation

The idea PODHandler represents is important. My implementation is just my own manifestation of the concepts described in this post. My implementation is not important! The concepts are important. Hopefully by allowing readers another piece of reference, in the form of some source code, the ideas presented here can be better realized.

PODHandler Source: Link (not up yet)

# Sane Usage of Components and Entity Systems

With some discussion going in a previous article about how to actually implement some sort of component system for a game engine, without vague theory or dogma, a need for some higher level perspective was reached, and so this article arose.

In general an aggregation model is often useful when piecing together bits of functionality or data to create something new. The ability to do so is very useful for writing game-specific gameplay code due the flexibility of code granted by aggregation. However as of late there’s been tremendous talk about OOP, Entity Systems, Inheritance, and blah blah blah within the online indie development community. More and more buzzwords get tossed around by big name writers and the audience really just looks for some guidelines to follow in hopes of writing good code.

Sadly there isn’t going to be a set of step by step rules for writing a game engine or coming up with a good architecture. Like many of said before me, writing a game is a specific task requiring specific solutions. Why do you think game engine developers such as Epic or the Unity guys have so many people working on the product? Because a generic game engine is a huge piece of software that requires a lot of features. Some features exist simply to let users add in custom features easily.

Components, aggregation, Entity Component Systems, Entity systems, these are just words and have various definitions (depending on who you ask).

## Some Definitions

To hopefully avoid silly arguments and confusion lets define some terms. If you don’t like the definitions here feel free to express so, I’m all up for criticism and debate.

• Component Based Architecture
• A preference for aggregation over inheritance. Is just a concept and does not lead to a single specific implementation. A game object is a collection of components. A component defines data and/or functionality for a concept.
• Entity Component System (ECS)
• A specific implementation of Component Based Architecture. A game object would be an ID (an integer). The ID is used to form an aggregate. Usually an ECS implies an implementation similar to a database, where components are entries into a database that are looked up through some identifier. The main goals of this implementation are efficiency and simplicity. Often times the term “ECS” is used just to describe a Component Based Architecture, often leading to confusion.
• Aggregation
• I like to think of this as a “has-a” relationship over an “is-a” relationship. Aggregation refers to one object “having” another object, which implies an aggregate is a collection (data structure) of other objects.

## Some Truth and History

Aggregation is useful from a game design perspective. It frees functionality from arbitrary classification (classes and inheritance). Classes were originally created in C++ to let a programmer tie together a piece of data and some functionality to represent some sort of real-life concept. This is in simplest terms the essence of Object Oriented Programming (OOP). Over time more features were added to help engineer relationships between classes, one such feature came in the form of inheritance.

There’s nothing inherently wrong with OOP and it makes sense in a lot of code. Problems can arise when there’s a mis-application of OOP that has implications that aren’t fully understood at the time of implementation that cause negative affects down the road. I’m sure we’ve all seen the code migration and mega-class example so commonly thrown around in articles arguing against OOP and inheritance abuse.

In response to such an abuse a new paradigm became popularized which focused on aggregation of functionality to form an object. This might be called a “component based architecture”. In general aggregation can be considered an appropriate alternative to inheritance.

## OOP Diatribe

Usually when an article spews forth caustic attacks against OOP it’s directed at naive implementations that disregard implications of how memory is accessed. Perhaps in the past the bottleneck of most everything was processor speed, so a lot of literature focuses on this. Nowadays CPUs on the PC have an architecture that have ridiculous computational power with extremely limited memory access. In general one might consider accessing memory from RAM 300 times slower than multiplying two floats together. Of course this last statement is extremely anecdotal without any evidence, but exists just to give a rough perspective of reality in many current (2014) cases.

If objects with associated code (classes) are just allocated and deallocated on the heap at will then a performance bottleneck of memory access is going to rear its ugly face, likely long before other performance issues are even on the radar. This is where much of the diatribe comes from.

It should be noted that pretty much all code bases that make use of the C++ language use classes and structures in some form or another. As long as a programmer has an understanding of memory, how it’s accessed, and what implications arise from given implementations, nothing will go wrong. Alas, actually doing these things and writing good code is super hard. It doesn’t matter if a class has some implementation code within it, so long as that bit of code makes sense for the purposes it is serving.

## Implementing Components, a First Draft

The most immediate implementation would be to make use of multiple inheritance. This has a clear definition of where the data goes, and it all goes in one class -the derived class. Multiple inheritance itself can get a bit tricky when dealing with pointer typecasting between derived and base types, though the C++ language itself handles the details much of the time.

Inheritance alone doesn’t provide a good mechanism to query whether a base class is apart of a specific derived aggregation and so the dynamic cast operator is born. Since the dynamic cast is a branching operation, usually implemented (afaik) by inspecting the vtable, it is avoided in general.

Multiple inheritance also does all sorts of work to member function pointers, and is just a sad part of C++. Additionally there isn’t any language feature that allows for dynamic dispatch for combinations of base classes, so if the need arises a custom solution will need to be implemented anyway.

Memory accessing, although defined, isn’t ideal. Multiple inheritance forms a blob of different data, and usually only a single piece of the blob is needed at any given time, meaning locality of reference will be poor in general. This leads to the idea of inheriting from multiple interfaces in order to decouple memory aggregation from functionality aggregation, which leads to the next draft.

## Second Draft – Run-Time Aggregation

Instead of using multiple inheritance on interfaces, which is a compile-time feature, run-time support can be added. Object aggregates can be formed during run-time, and modified thereafter. This is appealing for data driven applications, and game-design friendly development iteration speed.

So lets assume that some programmer wants to implement components, but doesn’t think much about memory access patterns the implications therein. Using a vector of pointers an implementation of components becomes super simple. Each pointer can point to an interface exposing a few functions like Update, Init and Shutdown.

Searching for a particular component is as simple as linearly looping over each pointer until a matching type is found. If these pointers are ordered in some way a search can be performed, perhaps a binary search could suffice. If the identifier of a component is hashable a hash table lookup can be used.

The implementation so far is an excellent one except that there is no definition of how memory is allocated and accessed! In the most naive of implementation each game object and each component will be allocated on the heap with separate calls to malloc.

Despite having no clear memory definition there are some nice benefits that have arisen. Data driving the composition of an aggregate becomes quite trivial as each component of an aggregation can have an entirely isolated lifetime. Adding, removing, modifying, or even creating new components at run-time are all now possibilities. This dynamic aggregate architecture is great for improving game development and design iteration time!

## Aggregation and Components and the Entity System Paradigm (ES/ECS)

As stated in the definitions section, an ECS is just a specific implementation of a component based architecture. A component based architecture game engine architecture would be a custom implementation of multiple inheritance. A clearly defined ECS can impose restrictions on how a component architecture is implemented and used in hopes of avoided poor memory access patterns, or in hopes of keeping code simple and orderly.

If a component is designed as a piece of memory without any code, and a game object defined as an integer ID then performance specifications can be easily imposed. Rules about where in memory components lay, and how components are actually accessed can be clearly defined in simple terms. Code can be written that operates upon arrays of components, transforming arrays linearly. This idea is actually a type of Data Oriented Design (DOD), which makes sense as DOD is just an idea! ECS is an application of the idea of DOD.

So with this type of implementation the benefits of dynamic composition can be paired with well-defined memory layout and access patterns. Suddenly prefetching and parallelism become much simpler to support.

## Aggregatize all the Things!

There’s a problem. Blindly shoving the idea of an ECS implementation into every nook and cranny of an engine during development is just silly (or any complex system, not just game engines or libraries). Often times a particular system is not best implemented with a component or aggregate paradigm in mind.

An obvious case is that of a physics engine. Often times a physics engine developer is worried about collision detection, solving systems of linear equations, rigid body mechanics and allowing the engine to easily be integrated into existing code bases. These details involve a lot of math and good API design. A developer of a physics engine is going to have their focus employed in full force in solving problems specific to physics engines. This means that the engineer’s focus is finite, so the implementation that is best is one that the engineer can actually bring to completion. An implementation that can come to completion is one that makes sense for the specific details of whatever is going on inside the physics engine. The specific paradigms used are often not aggregation or component based!

In order for a physics engine to run fast it needs to have efficient memory access patterns and memory usage, on modern PC hardware, requires some form of DOD. Since this complex (often black boxed) physics engine will have it’s own specific implementation and optimization it doesn’t make sense to force a component based model to its very core with some sort of idealistic zeal. It gets really bad when strict rules are imposed (like banning all code from classes and structures that define components) on the component model (like with an ECS) and the rules start permeating the deep recesses of the entire code base.

The same thing goes for any sort of complex system. The core facilities of a game engine often times just don’t really care about components or aggregation. This means that an engine architecture that implements components will usually have to deal with middleware graphics/physics engines/libraries that don’t subscribe to a component based model (simply because it’s easier to use a library than to write your own custom things, especially if those custom things religiously follow some silly methodology like ECS or even OOP). In practice light wrapper components can be created to let the functionality of such systems be presented in a component format, ready to be used in an aggregate object.

## What does this all mean? What should we all do?

Use components where it makes sense in code. Use inheritance where it makes sense in code. Use databases where they make sense. Use all the things where they should. This is a pretty sad answer but it’s the right one. There is no silver bullet paradigm that solves all the problems in the game engine architecture world, and there are no steps to follow to achieve a result that works in all cases. Specific problems require specific solutions. Good code is hard to write, and will require a lot of judgement calls. In order to make good judgement calls a lot of experience and perspective is required.

I recommend using aggregation where it really matters. Dynamic aggregation is important for gameplay specific code. Gameplay specific code, in this article, would refer to code that would not easily apply or work at all in a different game. It’s code that is your game and doesn’t define an isolated system or functionality.

Dynamic aggregation and the component based model are extremely important for game and object editors. Game design flourishes best when iteration times are driven to zero, and the ability to create new things from a composition of fundamentals is very valuable! Clearly composition is useful, but how it’s to be used is the hard part.

## What Components to Make?

I recommend making components concerned with providing access to game-independent functionality to be quite large. Every 3D game engine has a concept of a mesh, and will usually have some sort of file format to associate with, like FBX. Every 2D game engine will have the concept of a sprite. Each game using Box2D will have colliders and rigid bodies, and possibly joints. These fundamental pieces of functionality don’t change very often, so static compile-time relationships aren’t a bad thing since iteration time isn’t really all that relevant.

A 3D game might have a single Mesh component for example. A Mesh component can have renderable vertices, and possibly all the skeletal and animation information as well. There may be a single Rigid Body component, which encapsulates the idea of colliders or shapes, as well as the functionality of rigid body mechanics. The Rigid Body component might even contain all necessary code and data to hold multiple joints! Or joints may be a component themselves.

For high level and gameplay related features components can become much more granular (or not if you so choose). Gameplay should be iterated, tested and changed frequently, so having small and decomposed components will probably make a lot of sense in a lot of cases. Large components that encompass more broad ideas will be useful in many cases too. Even in the gameplay world judgement calls are essential.

Usually efficiency isn’t so important for much gameplay code, so any implementation that is decently performant will suffice. Scripting languages, dynamic memory allocation and virtual dispatch, or what have you can all work. The decisions of what requires flexibility, what requires performance and all between can be difficult to make. Please see the references section for some concrete examples.

We live in a world of opinions and it takes time to sift through them! If you have recommendations please comment below :)

## Reference Source Code

The best reference I know of is an open source game engine in progress (stalled until I graduate) I myself am developing. Please do send me your recommendations on references!

# Simple and Efficient Singleton Pattern

For game engines the singleton pattern is pretty commonly used for various global accesses, like for the core engine or for specific systems. Often times things like texture managers, the graphics implementation or a physical simulator are represented as singular entity that can be accessed globally.

A singleton pattern can be used to help facilitate and assert the existence of only one of these systems at any given time. This is important for a lot of code that is created under the assumption that only one given instance will be alive at once.

The biggest reason singletons are used is for code clarity. Any programmer that realizes something is a singleton is instantly informed of how it should be used. Beyond conceptual aids a singleton can also be an efficient means of allowing global access of an object. Often times in games everything is owned by something, otherwise referred to as the “Ownership Pattern”.

If a game consists purely of things owning other things (except for the core Game or Engine object), retrieving different systems from global access might be hard. This is because the Engine would be the only global thing. Code like this:

is long and annoying and also inefficient; there are many unnecessary levels of indirection. Instead a singleton can be used to solve such a problem.

The traditional approach to creating a singleton is to utilize some code like so:

This approach does in fact ensure that only a single instance of a given class is alive at any given time, and can be especially effective if the constructor and destructor are declared in the private section.

### Drawbacks

There is a pretty big drawback to this traditional style: construction and destruction order. C++ makes no guarantee about the construction and destruction order of objects on global (file) scope. This means that the code run for each destructor of every singleton instance (despite construction time) can run in any order.

This poses a huge problem for systems that depend on one another. What if the TextureManager class contained a pointer to the Graphics class? What if the destructor of the TextureManager tries to access the Graphics pointer and the Graphics singleton has already destructed?

Such an issue does have workaround solutions, but it might be best to have some form of singleton that controls construction and destruction explicitly.

## A Better Singleton

Here’s a simple way to implement a singleton and allow explicit construction and destruction:

This singleton is actually quite safe due to the simple assertion in the constructor. The nice thing about this assertion is that it can be compiled away during release builds. This might be considered an advantage against the traditional approach, as the traditional approach will often times have a boolean flag to test for previous construction (in the assembly of the compiled C++).

Since it is a slight inconvenience to add the instance handling to every class you wish to be a single the use of a template mixin can help. Making such a utility can be tricky due to multiple inheritance. I will leave solving the multiple inheritance issue as an exercise for the reader.

I myself use this sort of singleton (I don’t even have a utility) and enjoy it. I actually don’t even use a Get function but just have a global extern’d pointer in my header.

I’ve also seen this exact implementation in a few areas, one of which is in an article by Scott Bilas in the first Game Programming Gems book (he covers the multiple inheritance issue),

# C++ Enumeration Reflection

Welcome to the third post in a series of blog posts about how to implement a custom game engine in C++. As reference I’ll be using my own open source game engine SEL. Please refer to its source code for implementation details not covered in this article. Files of interest are EnumData.h, Enum.cpp and Enum.h.

## Crazy Viking Studios

Lets thank the Crazy Viking Studios guys for their generous contribution in knowledge on this topic! One day as a student I emailed them about their enumeration editing in their awesome editor for Volgarr the Viking. They responded with a bunch of source code in a demo! The techniques here have been learned from Taron their programmer.

## Introduction

Enumerations in C++ are a pretty nice feature. They provide type safety and a very readable way to name a lot of various types of constants. However there could be so much more added on top of enumerations in C++ to make extremely useful.

Lets take a trip through our imagination and imagine a game editor. In this editor you can create arbitrary constants with a name and associated integral value. This would be great for some sort of scripting or game logic.

This here can be implemented in C++ (during coding time as a compile-time constant) through enumerations. However there are some features that can be added to this to allow an editor to manipulate things:

• Modify existing entries
• Delete entries

## Basic Enumeration Editing

In order for an editor to manipulate this information within a C or C++ file some run-time memory is required to store a representation of the actual enumerations in code. Data tables (structs) will work well for this. Lets imagine a structure to contain one of these enumerations; we’ll need string representations of all of the enumeration entries:

If an Enum instance were created to contain identical string representations of the entries with the Spells enum, then a constant-time conversion of enumeration to string could be achieved just by indexing the Enum vector with a value. 

Converting a string back to an enumeration would best be done with a small hash table. This will keep string to enum conversions const-time.

### Much to be Desired

This is all fine and good, however if a user creates a new entry as a string this won’t update the actual C++ enumeration entries -new entries only exist until the editor shuts off. Additionally there isn’t an easy way to lookup a particular Enum struct. It would be nice to be able to lookup an Enum struct in various ways, such as by string name or template type. It would also be cool to be able to serialize enumerations to/from file.

It might be fairly simple to actually modify the source code containing a particular enumeration in C++ whenever an entry is modified, deleted or added. This would let programmers actually use enumerations created in an editor within their code (after a recompile). It is also possible to hookup the new entries to be loaded in the Enum struct as a string literal.

## Automation

As you can imagine a lot of manual labor is going to be needed in order to upkeep all of this crazy editing and modifying of enumerations. Some generalization and automation is needed to keep dev-work at an absolute minimum.

This is the time when I reference an old project I created to demonstrate a simple idea for serialization in C. The trick is use a source file and include it multiple times with various macro definitions. The source file to be included fills out the macros, but the macros are interpreted differently depending on when it was included. This allows you to write data files and interpreters using the preprocessor.

This is exactly what we need for building up some automated reflection and editing of enumerations.

Imagine a data file like so (a header without multiple inclusion guards and some macro invocations):

Lets take this data file and create a normal enumeration:

As you can see the macros from the data file are going to interpret the data as an enumeration. It is important to just always #undef all the macros in case they were previously defined.

After the preprocessor runs and the macros expand we will end up with something like:

Now the key part comes with defining the macros again to interpret the data in an all new way. Here’s an example to automate the creation of the Enum struct containing string literals:

The idea here is to construct an array of const char * literals and pass them to the Enum struct’s constructor. The struct can loop over them until the sentinel NULL value is found. When expanded by the preprocessor this file might look like:

While the Enum struct is looping over the literals passed to it in the constructor, it can also be adding the strings to a hash table to lookup appropriate indices.

## Editor Support

Now that a great scheme for automation of generating the actual enumeration data is setup, all that is required is to make sure that an editor can easily find an appropriate C++ file to modify when entries are modified. My solution was just to cram all enumerations into a single C++ file. This C++ is detailed with a nice comment saying something like: WARNING: This file is auto-generated by the Enum Editor.

This actually works pretty well but has a single drawback: editing an enumeration causes a global recompile of the project. There are no separate namespaces or naming schemes in my own implementation, meaning that each enumeration has to be unique to avoid compilation errors.

From here it’s just a matter of writing to your C++ data file.

## Tree Heirarchy

Wouldn’t it be great to be able to say “This enum is a subset of this entry”? That might have sounded confusing, here’s an example:

The idea is to allow each enumeration entry to contain an enumeration by creating a tree hierarchy.

This would be great for all sorts of game logic or general organization! It’s also possible to implement a really fast IsA function, so you could go if(type->IsA( Dragon )). Implementing this would just be a matter of traversing the tree hierarchy.

## Enumeration Features

I implemented a bunch of rag-tag features in my game engine SEL and would like to cover a couple of the more useful ones. Just take a quick look at an example declaration of the Enum struct:

I’m sure most readers can imagine how these methods are useful and how to implement them.

However looking up a specific enumeration by name (useful for macros) or by template type is something that is a little harder to implement. Please see SEL for a working reference on how to accomplish these. The idea is to use the multiple-inclusion trick on the data file to define some template specializations.

## Serialization and Introspection Registration

Serializing enumerations should be really straightforward for both binary and string formats. For binary the numerical representations can be utilized. And string format uses the string arrays constructed at compile-time.

The rest is just a matter of writing some string to/from file routines.

Some introspection techniques rely on the user to register various types within the reflection system. In this case it turns out this registration can also be automated with multiple-file inclusion on the data file! Just define a routine to register each enumeration type. There’s not much to it!

## Conclusion

I certainly hope this helps someone out there! Please do comment or ask questions right here on the post, I always enjoy reading them.

# Component Based Design – Lua Components and Coroutines

Welcome to the second post in a series of blog posts about how to implement a custom game engine in C++. As reference I’ll be using my own open source game engine SEL. Please refer to its source code for implementation details not covered in this article.

I would like to start this post off with a big thank you to Trent Reed for providing great advice in implementing various aspects of Lua integration for a game engine based upon component based design.

Since Lua is such an awesome scripting language it would be great if we could give every game object a component whose sole purpose is just to hold some code. This code could be a type of AI brain, or a little bit of game logic.

I always bring up this one particular back and forth patrol AI as a great example. Imagine you could do this in C++:

When this component is updated the code within a script is substituted for C++ code. Imagine if you could write AI code like so:

This enemy patrols left, right and then throws a bomb. This type of code is actually realistic with a simple feature of Lua’s called Coroutines. I am sadly unable to find my original reference for creating a nice C++ Coroutine wrapper, but I do have a nice wrapper within SEL you can look at. UPDATE: Game Programming Gems 5 has the exact article I used when building my own Coroutine implementation. I believe the section is called “Building Lua into Games”.

The entire purpose of a Coroutine is to allow Lua to pause the state of a function call and resume it exactly where it left off from at a later point in time. This lets you write code that can take pauses and resume later, allowing for extremely readable and easy to create game logic.

Creating and using Coroutines is pretty simple and there exist a lot of references on the internet of how to do so. If you like you can view my own implementation within my SEL game engine.

However there does come a time when one actually thinks about how to store these scripts as components in a simple way. As recommended in one of my other posts, your GameObject should look something like this in an engine utilizing Component Based Design:

There rises an issue of storing components whose only difference is a string representing the script name; what if we want to hold any number of these? One simple idea is to create a single LuaComponent type in C++ of which is stored in a slightly different manner; a separate vector of LuaComponents can be utilized to separate the game logic components from the rest of the core engine components.

This allows LuaComponents to accessed via string lookup:

## Start Update and Finish

It would be really nice if each component’s script name just referred to a sinle Lua file. If this were true then a naming convention can be established: a Lua component might only be a .lua file that contains functions Start, Update and Finish.

This sounds nice but a method for calling these various functions must be concocted. One simple can’t place all LuaComponent function defintions for Start, Update and Finish into the global Lua environment.

The idea is to create a unique Lua table for each GameObject that contains a LuaComponent. Within this table an isolated Lua environment can be constructed to define the 3 base functions. This also adds the benefit that all global variables within a LuaComponent file are local to each individual LuaComponent instance. This is especially important when you have lots of LuaComponents of a single script type. You don’t want globals in the LuaComponent file to be shared between all instances.

Implementing this is pretty easy if your game objects already have unique identifiers (preferable integers). I’ll take a slight detour on unique ids for a moment.

### Unique Object IDs

The simplest way to implement unique IDs for your game objects is to keep track of a single integer. This integer starts at 0, and each time a new object is created the integer is incremented after assigning the object’s ID as the value of the integer.

This works so long as your integer overflow is very high. Luckily 32 bits of precision is more than enough for any game.

This can be taken farther with handles as detailed in one of my other posts.

## Implementing Script Environments

Given a unique id for a game object a table in Lua can be constructed especially for this object:

The idea here is to create a new table instance for a game object if no table already exists. Then a new environment is created within this table (Environments in Lua are just tables).

The next step is to somehow get our .lua file definitions into this environment. In Lua 5.1 (and some lesser versions) there is a nice setfenv function which sets the environment of a Lua function. This is perfect for our cause as files loaded from .lua files are made into chunks, which are just nameless function objects! All that needs be done is to load the script and set it’s environment to the fresh new environment given to our object instance, and run the loaded chunk.

In Lua 5.2 and beyond there’s no nice setfenv function. Instead we must change the first upvalue of the chunk, which is the environment of the chunk. There are a couple ways to do this and I ended up choosing the easiest to implement. Here is my finished loader in Lua:

I decided to make use of the debug library. This allows me to inject a chunk’s definitions into an environment without fetching data from file. First implementations are likely to make use of lua’s loadfile function, as it actually does have a parameter to specify an environment. However loading from file is really slow, so ideally one would just keep a reference to the loaded chunk and run it on different environments as needed.

## Coroutines, or Not?

I myself haven’t experienced this, but around the internet and through word of mouth I’ve heard that coroutines aren’t as fast as we’d all like them to be. This is too bad, but can be dealt with. One way is with recycling of coroutines. I will likely mess with this myself later (when I need to), but I haven’t yet found it necessary.

Instead my LauComponents in SEL contain a boolean to determine if the component will be run as a coroutine or normal Lua function call. A normal Lua function cannot have fancy WaitSeconds or WaitFrames calls, but it is still Lua. This way developers can have control over the amount of overhead a given LuaComponent actually  imposes.

## Lua File System

I highly recommend using Lua File System (LFS). LFS is extremely small and has the same license as Lua itself. This is great in case you need to modify the source. It’s also extremely useful.

I recommend compiling LFS into Lua itself, whether or not you’re making a dynamic library or static library. I had good results doing this  myself.

Here’s an example of some of my code used to load all scripts within a folder (traverse all sub-folders recursively):

There’s great support for querying file extensions, names, paths and differentiating between files and folders.

I actually use LFS as my standard file directory traversal tool in general, not just for LuaComponents.