为什么不reinter尺寸相同类型的pret_cast力copy_n的强制类型转换？

问题描述

根据， reinter pret_cast ：

But wait, that's a lie cause it only works in these cases:

According to that list an illegal example would be:

auto foo = 13LL;
auto bar = reinterpret_cast<double&>(foo);

So the only acceptable way to make that cast is to copy the memory:

auto foo = 13LL;
double bar;

copy_n(reinterpret_cast<char*>(&foo), sizeof(foo), reinterpret_cast<char*>(&bar));

My question is, why doesn't reinterpret_cast handle that for me? Or is there something else available so I don't have to jump through this hoop?

解决方案

One reason is that the size, alignment, and bit representations aren't specified, so such a conversion wouldn't be portable. However, that wouldn't really justify making the behaviour undefined, just implementation-defined.

By making it undefined, the compiler is allowed to assume that expressions of unrelated types don't access the same object, which can allow better optimisation. For example, in the following:

int   & i = something();
float & f = something_else();

const int i1 = i;
f = 42;
const int i2 = i;

the compiler can assume that i1 and i2 both have the same value (i being unchanged by the assignment to f), and optimise them into a single constant. Breaking the assumption will then cause undefined behaviour.

Copying the bytes is the only well-defined way to reinterpret one object type as an unrelated type.

Aliasing with reinterpret_cast or a union might work sometimes (assuming the size etc. match), but might trip you up if the optimiser gets too clever with undefined behaviour.

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