std :: bit_cast与std :: array [英] std::bit_cast with std::array

问题描述

在他最近的演讲中现代C ++中的类型修剪" Timur Doumler 说说，不能使用std::bit_cast将float强制转换为unsigned char[4]因为无法从函数返回C样式的数组.我们应该使用std::memcpy或等到C ++ 23(或更高版本)定义类似reinterpret_cast<unsigned char*>(&f)[i]之类的东西时.

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] because C-style arrays cannot be returned from a function. We should either use std::memcpy or wait until C++23 (or later) when something like reinterpret_cast<unsigned char*>(&f)[i] will become well defined.

在C ++ 20中，我们能否将std::array与std::bit_cast一起使用

In C++20, can we use an std::array with std::bit_cast,

float f = /* some value */;
auto bits = std::bit_cast<std::array<unsigned char, sizeof(float)>>(f);

而不是C样式的数组来获取float的字节?

instead of a C-style array to get bytes of a float?

推荐答案

是的，这适用于所有主要的编译器，据我观察标准可以确定，它是可移植的并且可以保证正常工作.

Yes, this works on all major compilers, and as far as I can tell from looking at the standard, it is portable and guaranteed to work.

首先，保证std::array<unsigned char, sizeof(float)>是一个集合( https://eel.is/c++draft/array#overview-2 ).由此可见，它内部恰好包含sizeof(float)个char数(通常为char[]，尽管从原则上讲，该标准并未强制要求该特定实现-但它确实说元素必须是连续的)，并且不能具有任何其他非静态成员.

First of all, std::array<unsigned char, sizeof(float)> is guaranteed to be an aggregate (https://eel.is/c++draft/array#overview-2). From this follows that it holds exactly a sizeof(float) number of chars inside (typically as a char[], although afaics the standard doesn't mandate this particular implementation - but it does say the elements must be contiguous) and cannot have any additional non-static members.

因此可以轻松复制，并且其大小也与float的大小匹配.

It is therefore trivially copyable, and its size matches that of float as well.

这两个属性允许您在两个属性之间bit_cast.

Those two properties allow you to bit_cast between them.

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