EnTT: Gaming meets modern C++

Introduction

The entity-component-system (also known as ECS) is an architectural pattern used mostly in game development. For further details:

This project started off as a pure entity-component system. Over time the codebase has grown as more and more classes and functionalities were added.
Here is a brief, yet incomplete list of what it offers today:

Statically generated integer identifiers for types (assigned either at compile-time or at runtime).
A constexpr utility for human readable resource names.
A minimal configuration system built using the monostate pattern.
An incredibly fast entity-component system based on sparse sets, with its own pay for what you use policy to adjust performance and memory usage according to the users' requirements.
Views and groups to iterate entities and components and allow different access patterns, from perfect SoA to fully random.
A lot of facilities built on top of the entity-component system to help the users and avoid reinventing the wheel (dependencies, snapshot, handles, support for reactive systems and so on).
The smallest and most basic implementation of a service locator ever seen.
A built-in, non-intrusive and macro-free runtime reflection system.
Static polymorphism made simple and within everyone's reach.
A cooperative scheduler for processes of any type.
All that is needed for resource management (cache, loaders, handles).
Delegates, signal handlers (with built-in support for collectors) and a tiny event dispatcher for immediate and delayed events to integrate in loops.
A general purpose event emitter as a CRTP idiom based class template.
And much more! Check out the wiki.

Consider this list a work in progress as well as the project. The whole API is fully documented in-code for those who are brave enough to read it.

It is also known that EnTT is used in Minecraft.
Given that the game is available literally everywhere, I can confidently say that the library has been sufficiently tested on every platform that can come to mind.

Code Example

#include <entt/entt.hpp>
 
struct position {
    float x;
    float y;
};
 
struct velocity {
    float dx;
    float dy;
};
 
void update(entt::registry &registry) {
    auto view = registry.view<const position, velocity>();
 
    // use a callback
    view.each([](const auto &pos, auto &vel) { /* ... */ });
 
    // use an extended callback
    view.each([](const auto entity, const auto &pos, auto &vel) { /* ... */ });
 
    // use a range-for
    for(auto [entity, pos, vel]: view.each()) {
        // ...
    }
 
    // use forward iterators and get only the components of interest
    for(auto entity: view) {
        auto &vel = view.get<velocity>(entity);
        // ...
    }
}
 
int main() {
    entt::registry registry;
 
    for(auto i = 0u; i < 10u; ++i) {
        const auto entity = registry.create();
        registry.emplace<position>(entity, i * 1.f, i * 1.f);
        if(i % 2 == 0) { registry.emplace<velocity>(entity, i * .1f, i * .1f); }
    }
 
    update(registry);
}

Motivation

I started developing EnTT for the wrong reason: my goal was to design an entity-component system to beat another well known open source solution both in terms of performance and possibly memory usage.
In the end, I did it, but it wasn't very satisfying. Actually it wasn't satisfying at all. The fastest and nothing more, fairly little indeed. When I realized it, I tried hard to keep intact the great performance of EnTT and to add all the features I wanted to see in my own library at the same time.

Nowadays, EnTT is finally what I was looking for: still faster than its competitors, lower memory usage in the average case, a really good API and an amazing set of features. And even more, of course.

Performance

The proposed entity-component system is incredibly fast to iterate entities and components, this is a fact. Some compilers make a lot of optimizations because of how EnTT works, some others aren't that good. In general, if we consider real world cases, EnTT is somewhere between a bit and much faster than many of the other solutions around, although I couldn't check them all for obvious reasons.

If you are interested, you can compile the benchmark test in release mode (to enable compiler optimizations, otherwise it would make little sense) by setting the ENTT_BUILD_BENCHMARK option of CMake to ON, then evaluate yourself whether you're satisfied with the results or not.

Honestly I got tired of updating the README file whenever there is an improvement.
There are already a lot of projects out there that use EnTT as a basis for comparison (this should already tell you a lot). Many of these benchmarks are completely wrong, many others are simply incomplete, good at omitting some information and using the wrong function to compare a given feature. Certainly there are also good ones but they age quickly if nobody updates them, especially when the library they are dealing with is actively developed.

The choice to use EnTT should be based on its carefully designed API, its set of features and the general performance, not because some single benchmark shows it to be the fastest tool available.

In the future I'll likely try to get even better performance while still adding new features, mainly for fun.
If you want to contribute and/or have suggestions, feel free to make a PR or open an issue to discuss your idea.

Integration

EnTT is a header-only library. This means that including the entt.hpp header is enough to include the library as a whole and use it. For those who are interested only in the entity-component system, consider to include the sole entity/registry.hpp header instead.
It's a matter of adding the following line to the top of a file:

#include <entt/entt.hpp>

Use the line below to include only the entity-component system instead:

#include <entt/entity/registry.hpp>

Then pass the proper -I argument to the compiler to add the src directory to the include paths.

Requirements

To be able to use EnTT, users must provide a full-featured compiler that supports at least C++17.
The requirements below are mandatory to compile the tests and to extract the documentation:

CMake version 3.7 or later.
Doxygen version 1.8 or later.

Alternatively, Bazel is also supported as a build system (credits to zaucy who offered to maintain it).
In the documentation below I'll still refer to CMake, this being the official build system of the library.

CMake

To use EnTT from a CMake project, just link an existing target to the EnTT::EnTT alias.
The library offers everything you need for locating (as in find_package), embedding (as in add_subdirectory), fetching (as in FetchContent) or using it in many of the ways that you can think of and that involve CMake.
Covering all possible cases would require a treaty and not a simple README file, but I'm confident that anyone reading this section also knows what it's about and can use EnTT from a CMake project without problems.

Packaging Tools

EnTT is available for some of the most known packaging tools. In particular:

Conan, the C/C++ Package Manager for Developers.
vcpkg, Microsoft VC++ Packaging Tool.
You can download and install EnTT in just a few simple steps:

$ git clone https://github.com/Microsoft/vcpkg.git
$ cd vcpkg
$ ./bootstrap-vcpkg.sh
$ ./vcpkg integrate install
$ vcpkg install entt

The EnTT port in vcpkg is kept up to date by Microsoft team members and community contributors.
If the version is out of date, please create an issue or pull request on the vcpkg repository.

Homebrew, the missing package manager for macOS.
Available as a homebrew formula. Just type the following to install it:

brew install skypjack/entt/entt

build2, build toolchain for developing and packaging C and C++ code.
In order to use the entt package in a build2 project, add the following line or a similar one to the manifest file:

depends: entt ^3.0.0

Also check that the configuration refers to a valid repository, so that the package can be found by build2:

cppget.org, the open-source community central repository, accessible as https://pkg.cppget.org/1/stable.
Package source repository: accessible as either https://github.com/build2-packaging/entt.git or ssh://git@github.com/build2-packaging/entt.git. Feel free to report issues with this package.

Both can be used with bpkg add-repo or added in a project repositories.manifest. See the official documentation for more details.

Consider this list a work in progress and help me to make it longer if you like.

pkg-config

EnTT also supports pkg-config (for some definition of supports at least). A suitable file called entt.pc is generated and installed in a proper directory when running CMake.
This should also make it easier to use with tools such as Meson or similar.

Documentation

The documentation is based on doxygen. To build it:

$ cd build
$ cmake .. -DENTT_BUILD_DOCS=ON
$ make

The API reference will be created in HTML format within the directory build/docs/html. To navigate it with your favorite browser:

$ cd build
$ your_favorite_browser docs/html/index.html

Tests

To compile and run the tests, EnTT requires googletest.
cmake will download and compile the library before compiling anything else. In order to build the tests, set the CMake option ENTT_BUILD_TESTING to ON.

To build the most basic set of tests:

$ cd build
$ cmake -DENTT_BUILD_TESTING=ON ..
$ make
$ make test

Note that benchmarks are not part of this set.

License

Code released under the MIT license.
Documentation released under CC BY 4.0.
All logos released under CC BY-SA 4.0.