正在投射std :: pair< T1，T2>常数& to std :: pair< T1 const，T2>常数&安全？ [英] Is casting std::pair<T1, T2> const& to std::pair<T1 const, T2> const& safe?

问题描述

是否安全（在理论或实践中）reinterpret_cast a std :: pair< T1，T2& const& 转换为 std :: pair< T1 const，T2> const& ，假设程序员没有故意做一些奇怪的事情，例如专门 std :: pair< T1 const，T2> p>

Is it safe (in theory or in practice) to reinterpret_cast a std::pair<T1, T2> const & into a std::pair<T1 const, T2> const &, assuming that the programmer hasn't intentionally done something weird like specializing std::pair<T1 const, T2>?

推荐答案

这不是可移植的。

std :: pair 需求在第20.3节中列出。第17.5.2.3条澄清

std::pair requirements are laid out in clause 20.3. Clause 17.5.2.3 clarifies that

第18至30条和附录D没有规定类的表示，故意忽略类成员的规范。实现可以根据需要来定义静态或非静态类成员或两者，以实现第18至30条和附件D中所指定的成员函数的语义。

Clauses 18 through 30 and Annex D do not specify the representation of classes, and intentionally omit specification of class members. An implementation may define static or non-static class members, or both, as needed to implement the semantics of the member functions specified in Clauses 18 through 30 and Annex D.

这意味着实现包含部分专门化是合法的（虽然不可能），如：

This implies that it's legal (although incredibly unlikely) for an implementation to include a partial specialization such as:

template<typename T1, typename T2>
struct pair<T1, T2>
{
    T1 first;
    T2 second;
};

template<typename T1, typename T2>
struct pair<const T1, T2>
{
    T2 second;
    const T1 first;
};

这是明确的不兼容布局。其他变体包括可能在第一和/或第二之前包含额外的非静态数据成员。

which are clearly not layout-compatible. Other variations including inclusion of additional non-static data members possibly before first and/or second are also allowed under the rule.

现在，考虑布局已知的情况有点有趣。虽然Potatoswatter指出 DR1334 ，它声称 T 和 const T 不符合版面配置，标准提供足够的保证以获得大部分的方式：

Now, it is somewhat interesting to consider the case where the layout is known. Although Potatoswatter pointed out DR1334 which asserts that T and const T are not layout-compatible, the Standard provides enough guarantees to allow us to get most of the way anyway:

template<typename T1, typename T2>
struct mypair<T1, T2>
{
    T1 first;
    T2 second;
};

mypair<int, double> pair1;
mypair<int, double>* p1 = &pair1;
int* p2 = reinterpret_cast<int*>(p1); // legal by 9.2p20
const int* p3 = p2;
mypair<const int, double>* p4 = reinterpret_cast<mypair<const int, double>*>(p3); // again 9.2p20

但是这不适用于 std ::对，因为我们不能应用9.2p20，而不知道第一实际上是未指定的初始成员。

However this doesn't work on std::pair as we can't apply 9.2p20 without knowing that first is actually the initial member, which is not specified.

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