30 Nov 2023 • C++ tricks: STL-free initializer list

Credit for this one to https://nitter.net/Donzanoid/status/1611315409596071936.

C++'s initializer_list is dogshit:

> cat a.cpp
#include <initializer_list>
> cl.exe /std:c++20 a.cpp /P > /dev/null; and wc -l a.i
Microsoft (R) C/C++ Optimizing Compiler Version 19.36.32534 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

a.cpp
8988 a.i
> g++ -E a.cpp -std=c++20 | wc -l
115
> clang++ -E a.cpp -std=c++20 | wc -l
393

There's actually no compiler magic involved here and initializer_list is pretty much just span so very simple to implement, but you need a few tricks to get it to actually work:

• GCC doesn't let you do this at all
• MSVC/clang have different constructors
• You need to define some header guards so stray STL headers don't try to redefine it

On that last point you may be wondering why bother with this at all. You can't keep a codebase completely STL-free, be it because of certain things where the non-STL way is extremely painful (e.g. MSVC atomics), or unavoidable third party libraries pulling in STL headers (e.g. metal-cpp). So we need to make this not clash with the real thing.

#if COMPILER_GCC

// GCC refuses to compile if the implementation doesn't exactly match the STL
#include <initializer_list>

#else

#define _INITIALIZER_LIST_ // MSVC
#define _LIBCPP_INITIALIZER_LIST // clang

namespace std {
    template< typename T >
    class initializer_list {
    public:

#if COMPILER_MSVC
        constexpr initializer_list( const T * first_, const T * one_after_end_ ) : first( first_ ), one_after_end( one_after_end_ ) { }
#else
        constexpr initializer_list( const T * first_, size_t n ) : first( first_ ), one_after_end( first_ + n ) { }
#endif

        const T * begin() const { return first; }
        const T * end() const { return one_after_end; }
        size_t size() const { return one_after_end - first; }

    private:
        const T * first;
        const T * one_after_end;
    };
}

#endif

You may also want to add your own header guards against the STL defines, or you can just make sure you always include this first.