是否可能“固定” `std :: pair`的字段没有hack？ [英] Is it possible to "constify" a field of `std::pair` without hacks?

问题描述

在C ++中，编译以下代码：

In C++, the compiling the following code:

std::pair <int, int>  x;
static_cast <std::pair <const int, int>*> (&x);

出现错误：

error: invalid static_cast from type ‘std::pair<int, int>*’ to type ‘std::pair<const int, int>*’

我或多或少地理解为什么会发生这种情况，因为cv-qualifying模板参数列表中的类型原则上可以给出不兼容结果。即使在这种情况下，编译器也无法知道它。

I more or less understand why it happens, as cv-qualifying a type in a template parameter list can, in principle, give an "incompatible" result. And even if in this case it doesn't, compiler has no way to know it.

无论如何，有一个非黑客的方式来执行这个转换？我警惕使用 reinterpret_cast 的任何东西，因为我以前的类型冲突问题。此外，我不能使用临时值，因为这是性能关键代码。

Anyway, is there a non-hackish way to perform this conversion? I'm wary of using reinterpret_cast for anything as I've been by type-punning problems before. Also, I can't use temporaries since this is in performance-critical code.

EDIT：

这里是我在做什么。我实现一个自定义容器接口兼容 std :: unordered_map 。因此，它的 value_type 需要是对< const key_type，mapped_type> 。对于一些优化，我需要内部存储值 pair< key_type，mapped_type> ，而不是 const 。但是，如果我这样做，我不能（不 reinterpret_cast ）在容器上实现迭代器，因为他们需要返回对值的引用，我只引用这些非-const pair。

Here is what I'm doing. I'm implementing a custom container interface-compatible with std::unordered_map. Because of that, its value_type needs to be a pair <const key_type, mapped_type>. For some optimization, I need to internally store the values as pair <key_type, mapped_type>, without const. However, if I do that, I can't (without reinterpret_cast) implement iterators over the container, as they need to return references to values and I have only references to these non-const pairs.

推荐答案

这不是强制类型，但可以执行以下操作：

That's not a cast, but you can do the following:

std::pair<int, int>  x;
std::pair<const int, int> y( x );

这应该根据&20.2.2 / 4。工作

This should work according to §20.2.2/4.

这篇关于是否可能“固定” `std :: pair`的字段没有hack？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！