std::bit_cast with std::array

Question

In his recent talk “Type punning in modern C++” Timur Doumler said that std::bit_cast cannot be used to bit cast a float into an unsigned char[4]

Answer 1

Yes.

According to the paper that describes the behaviour of std::bit_cast, and its proposed implementation as far as both types have the same size and are trivially copyable the cast should be successful.

A simplified implementation of std::bit_cast should be something like:

template 
inline Dest bit_cast(Source const &source) {
    static_assert(sizeof(Dest) == sizeof(Source));
    static_assert(std::is_trivially_copyable::value);
    static_assert(std::is_trivially_copyable::value);

    Dest dest;
    std::memcpy(&dest, &source, sizeof(dest));
    return dest;
}

Since a float (4 bytes) and an array of unsigned char with size_of(float) respect all those asserts, the underlying std::memcpy will be carried out. Therefore, each element in the resulting array will be one consecutive byte of the float.

In order to prove this behaviour, I wrote a small example in Compiler Explorer that you can try here: https://godbolt.org/z/4G21zS. The float 5.0 is properly stored as an array of bytes (Ox40a00000) that corresponds to the hexadecimal representation of that float number in Big Endian.