Frequently Asked Questions

Frequently Asked Questions

Table of Contents

• Introduction
• FAQ
  • Why is my debug build on Windows so slow?
  • How can I represent hierarchies with my components?
  • Custom entity identifiers: yay or nay?
  • Warning C4307: integral constant overflow
  • Warning C4003: the min, the max and the macro

Introduction

This is a constantly updated section where I'll try to put the answers to the most frequently asked questions.
If you don't find your answer here, there are two cases: nobody has done it yet or this section needs updating. In both cases, try to open a new issue or enter the gitter channel and ask your question. Probably someone already has an answer for you and we can then integrate this part of the documentation.

FAQ

Why is my debug build on Windows so slow?

EnTT is an experimental project that I also use to keep me up-to-date with the latest revision of the language and the standard library. For this reason, it's likely that some classes you're working with are using standard containers under the hood.
Unfortunately, it's known that the standard containers aren't particularly performing in debugging (the reasons for this go beyond this document) and are even less so on Windows apparently. Fortunately this can also be mitigated a lot, achieving good results in many cases.

First of all, there are two things to do in a Windows project:

• Disable the /JMC option (Just My Code debugging), available starting in Visual Studio 2017 version 15.8.

• Set the _ITERATOR_DEBUG_LEVEL macro to 0. This will disable checked iterators and iterator debugging.

Moreover, the macro ENTT_DISABLE_ASSERT should be defined to disable internal checks made by EnTT in debug. These are asserts introduced to help the users, but require to access to the underlying containers and therefore risk ruining the performance in some cases.

With these changes, debug performance should increase enough for most cases. If you want something more, you can can also switch to an optimization level O0 or preferably O1.

How can I represent hierarchies with my components?

This is one of the first questions that anyone makes when starting to work with the entity-component-system architectural pattern.
There are several approaches to the problem and what’s the best one depends mainly on the real problem one is facing. In all cases, how to do it doesn't strictly depend on the library in use, but the latter can certainly allow or not different techniques depending on how the data are laid out.

I tried to describe some of the techniques that fit well with the model of EnTT. Here is the first post of a series that tries to explore the problem. More will probably come in future.

Long story short, you can always define a tree where the nodes expose implicit lists of children by means of the following type:

struct relationship {
    std::size_t children{};
    entt::entity first{entt::null};
    entt::entity prev{entt::null};
    entt::entity next{entt::null};
    entt::entity parent{entt::null};
    // ... other data members ...
};

The sort functionalities of EnTT, the groups and all the other features of the library can help then to get the best in terms of data locality and therefore performance from this component.

Custom entity identifiers: yay or nay?

Custom entity identifiers are definitely a good idea in two cases at least:

• If std::uint32_t isn't large enough as an underlying type.
• If you want to avoid conflicts when using multiple registries.

These identifiers are nothing more than enum classes with some salt.
To simplify the creation of new identifiers, EnTT provides the macro ENTT_OPAQUE_TYPE that accepts two arguments:

• The name you want to give to the new identifier (watch out for namespaces).
• The underlying type to use (either std::uint16_t, std::uint32_t or std::uint64_t).

In fact, this is the definition of entt::entity:

ENTT_OPAQUE_TYPE(entity, std::uint32_t)

The use of this macro is highly recommended, so as not to run into problems if the requirements for the identifiers should change in the future.

Warning C4307: integral constant overflow

According to this issue, using a hashed string under VS could generate a warning.
First of all, I want to reassure you: it's expected and harmless. However, it can be annoying.

To suppress it and if you don't want to suppress all the other warnings as well, here is a workaround in the form of a macro:

#if defined(_MSC_VER)
    #define HS(str)\
        __pragma(warning(push))\
        __pragma(warning(disable:4307))\
        entt::hashed_string{str}\
        __pragma(warning(pop))
#else
    #define HS(str) entt::hashed_string{str}
#endif

With an example of use included:

constexpr auto identifier = HS("my/resource/identifier");

Thanks to huwpascoe for the courtesy.

Warning C4003: the min, the max and the macro

On Windows, a header file defines two macros min and max which may result in conflicts with their counterparts in the standard library and therefore in errors during compilation.

It's a pretty big problem but fortunately it's not a problem of EnTT and there is a fairly simple solution to it.
It consists in defining the NOMINMAX macro before to include any other header so as to get rid of the extra definitions:

#define NOMINMAX

Please refer to this issue for more details.