EnTT: Crash Course: configuration

EnTT 3.16.0

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Crash Course: configuration

# Table of Contents

- Introduction
- Definitions
  - ENTT_NOEXCEPTION
  - ENTT_USE_ATOMIC
  - ENTT_ID_TYPE
  - ENTT_SPARSE_PAGE
  - ENTT_PACKED_PAGE
  - ENTT_ASSERT
    - ENTT_ASSERT_CONSTEXPR
    - ENTT_DISABLE_ASSERT
  - ENTT_NO_ETO
  - ENTT_STANDARD_CPP

# Introduction

EnTT has become almost completely customizable over time, in many respects. These variables are just one of the many ways to customize how it works.
In the vast majority of cases, users will have no interest in changing the default parameters. For all other cases, the list of possible configurations with which it is possible to adjust the behavior of the library at runtime can be found below.

# Definitions

All options are intended as parameters to the compiler (or user-defined macros within the compilation units, if preferred).
Each parameter can result in internal library definitions. It is not recommended to try to also modify these definitions, since there is no guarantee that they will remain stable over time unlike the options below.

## ENTT_NOEXCEPTION

Define this variable without assigning any value to it to turn off exception handling in EnTT.
This is roughly equivalent to setting the compiler flag -fno-exceptions but is also limited to this library only.

## ENTT_USE_ATOMIC

In general, EnTT does not offer primitives to support multi-threading. Many of the features can be split over multiple threads without any explicit control and the user is the one who knows if a synchronization point is required.
However, some internal static data shared between threads should be atomic when using EnTT from multiple threads, even when dealing with local storage. Define this macro without assigning any value to it to get the job done.

## ENTT_ID_TYPE

entt::id_type is directly controlled by this definition and widely used within the library.
By default, its type is std::uint32_t. However, users can define a different default type if necessary.

## ENTT_SPARSE_PAGE

It is known that the ECS module of EnTT is based on sparse sets. What is less known perhaps is that the sparse arrays are paged to reduce memory usage.
Default size of pages (that is, the number of elements they contain) is 4096 but users can adjust it if appropriate. In all cases, the chosen value must be a power of 2.

## ENTT_PACKED_PAGE

As it happens with sparse arrays, packed arrays are also paginated. However, in this case the aim is not to reduce memory usage but to have pointer stability upon component creation.
Default size of pages (that is, the number of elements they contain) is 1024 but users can adjust it if appropriate. In all cases, the chosen value must be a power of 2.

## ENTT_ASSERT

For performance reasons, EnTT does not use exceptions or any other control structures. In fact, it offers many features that result in undefined behavior if not used correctly.
To get around this, the library relies on a lot of asserts for the purpose of detecting errors in debug builds. By default, it uses assert internally. Users are allowed to overwrite its behavior by setting this variable.

### ENTT_ASSERT_CONSTEXPR

Usually, an assert within a constexpr function is not a big deal. However, in case of extreme customizations, it might be useful to differentiate.
For this purpose, EnTT introduces an admittedly badly named variable to make the job easier in this regard. By default, this variable forwards its arguments to ENTT_ASSERT.

### ENTT_DISABLE_ASSERT

Assertions may in turn affect performance to an extent when enabled. Whether ENTT_ASSERT and ENTT_ASSERT_CONSTEXPR are redefined or not, all asserts can be disabled at once by means of this definition.
Note that ENTT_DISABLE_ASSERT takes precedence over the redefinition of the other variables and is therefore meant to disable all controls no matter what.

## ENTT_NO_ETO

In order to reduce memory consumption and increase performance, empty types are never instantiated nor stored by the ECS module of EnTT.
Use this variable to treat these types like all others and therefore to create a dedicated storage for them.

## ENTT_STANDARD_CPP

EnTT mixes non-standard language features with others that are perfectly compliant to offer some of its functionalities.
This definition prevents the library from using non-standard techniques, that is, functionalities that are not fully compliant with the standard C++.
While there are no known portability issues at the time of this writing, this should make the library fully portable anyway if needed.