std :: begin和std :: end的const重载的目的是什么？ [英] What is the purpose of the const overloads of std::begin and std::end?

问题描述

对于 std :: begin ，我们有两个容器重载：

For std::begin, we have two overloads for containers:

template< class C > 
auto begin( C& c ) -> decltype(c.begin());
template< class C > 
auto begin( const C& c ) -> decltype(c.begin());

但是可以推导出 C 的常数按照通常的模板推导规则，因此似乎第二次重载是多余的。我缺少什么？

But the constness of C can be deduced by the usual template deduction rules, so it seems like the second overload is redundant. What am I missing?

推荐答案

调用开始是合理的（并且（对于此问题， end ），前提是在销毁容器后不使用生成的迭代器。但是，将右值传递给 T& 形式的参数将不起作用，这是第二个重载起作用的地方。

It's reasonable to call begin (and end, for that matter) on an rvalue, provided we don't use the resulting iterator after the container has been destroyed. However, passing an rvalue to a parameter of the form T& will not work, which is where the second overload comes into play.

但是，很可能是我们正在对以前的基于范围的提案措辞：

However, it may well be that we're dealing with a thoughtless transformation of a former range-based for proposal wording:

在[container.concepts.member]的末尾添加以下内容：

Add the following to the end of [container.concepts.member]:

template<Container C> concept_map Range<C> {
    typedef C::iterator iterator;
    iterator begin( C& c ) { return Container<C>::begin(c); }
    iterator end( C& c )   { return Container<C>::end(c); } };

template<Container C> concept_map Range<const C> {
    typedef C::const_iterator iterator;
    iterator begin( const C& c ) { return Container<C>::begin(c); }
    iterator end( const C& c )   { return Container<C>::end(c); } };

当很明显概念无法实现时到C ++ 11中，对论文进行了修改，并且将所有四个函数类倍体都翻译为等效的命名空间范围函数模板。这具有接受rvalue的（可能是意料之外的）结果，而原始代码仅是为了区分不同合格的容器类型。

When it became clear that concepts weren't going to make it into C++11, papers were amended, and all four function temploids were presumably translated into equivalent namespace-scope function templates. This had the (potentially unintended) consequence of rvalues being accepted, while the original code was simply intended to distinguish between differently qualified container types.

请注意，开始 / end 会改用转发引用-例如

Note that a modern implementation of begin/end would use forwarding references instead - e.g.

template <typename T>
constexpr auto begin(T&& t)
  -> decltype(std::forward<T>(t).begin()) {
    return    std::forward<T>(t).begin();
}

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