请问g++ trunk在C++2a模式下是否可用constexpr std::bit_cast做编译时检查？

Great question! Let's break down what you need to know for both stable GCC releases and the trunk development branch:

Stable GCC Versions #

• As of GCC 13.x (the latest stable series at time of writing), std::bit_cast is available when compiling with C20 or later (-std=c++20), but it isn't constexpr-capable in these stable builds. The implementation adheres to C20 specs for runtime use, but constexpr support hasn't been backported to stable branches yet.

GCC Trunk (Cutting-Edge Development Build) #

• The good news: GCC trunk does fully support constexpr std::bit_cast when using -std=c++2a (which is equivalent to -std=c++20 now that the standard is finalized, or even -std=c++23). This means you can use it directly for compile-time checks exactly as you intended.

Example: Compile-Time Bit Pattern Check #

Here's a quick code snippet demonstrating how to use constexpr std::bit_cast in GCC trunk for compile-time validation:

#include <bit>
#include <cstdint>

constexpr float pi_float = 3.1415926535f;
constexpr auto pi_bits = std::bit_cast<std::uint32_t>(pi_float);

// Verify the IEEE 754 bit pattern of 3.1415926535f at compile time
static_assert(pi_bits == 0x40490FDB, "Mismatched bit pattern for pi float");

To compile this with GCC trunk, use:

g++ -std=c++2a -c your_file.cpp

Fallback Constexpr Equivalent (If Needed) #

If you run into any temporary issues with the trunk implementation (rare, but possible in active development), you can use a manual constexpr bit_cast alternative for trivially copyable types. GCC allows std::memcpy in constexpr contexts starting with C++20, so this works reliably:

#include <cstring>
#include <type_traits>

template <typename To, typename From>
constexpr std::enable_if_t<
    sizeof(To) == sizeof(From) &&
    std::is_trivially_copyable_v<From> &&
    std::is_trivially_copyable_v<To>,
    To>
constexpr_bit_cast(const From& from) noexcept {
    To to;
    std::memcpy(&to, &from, sizeof(To));
    return to;
}

// Usage example
constexpr auto fallback_bits = constexpr_bit_cast<std::uint32_t>(pi_float);
static_assert(fallback_bits == 0x40490FDB, "Fallback bit_cast failed");

内容的提问来源于stack exchange，提问作者wimalopaan