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Is it legal to initialize a static constexpr const void pointer using reinterpret_cast?

Linda Hamilton
Release: 2024-11-12 03:35:01
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Is it legal to initialize a static constexpr const void pointer using reinterpret_cast?

constexpr and Initialization of a Static const void Pointer with reinterpret_cast: Compiler Differences Explained

In the given code snippet, a static constexpr const void pointer is declared using a reinterpret_cast. This code raises a question about which compiler interpretation is correct according to the standard and the proper way to declare such expressions.

Compiler Rightfulness

The standard dictates that a constexpr variable, such as the static const void pointer in this case, must be initialized with a constant expression. A reinterpret_cast expression, however, is not considered a core constant expression according to the C 11 standard. Therefore, clang is correct in reporting an error for this code.

Proper Declarations

To declare a static constexpr const void pointer correctly, there are a few options:

  1. Use intptr_t instead: Use an intptr_t type and cast it to a void pointer when retrieving the value, as follows:

    static constexpr intptr_t ptr = 0x1;
    // ...
    reinterpret_cast<void*>(ptr);
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  2. GCC/clang Extension: GCC and clang support a little-documented extension using __builtin_constant_p:

    static constexpr const void* ptr = 
      __builtin_constant_p( reinterpret_cast<const void*>(0x1) ) ? 
        reinterpret_cast<const void*>(0x1) : reinterpret_cast<const void*>(0x1) ;
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This expression will be constant-folded by both compilers. However, note that this extension is not part of the standard and may not be supported in future versions of the compiler.

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