依靠ADL的std :: begin（）和std :: end（）？ [英] Relying on ADL for std::begin() and std::end()?

问题描述

当迭代标准容器时，你认为省略 std :: 前缀并依靠ADL找到定义是个好主意吗？示例：

When iterating over a standard container, do you think it's a good idea to omit the std:: prefix and rely on ADL to find the definition? Example:

std::vector<int> vec = get_vec();
// range-based for loop would be preferred here, but just for the sake of example
for (auto it = begin(vec), end = end(vec); it != end; ++it) { /*...*/ }

有什么理由做这个？

推荐答案

如果要使用ADL能够更改容器类型而不更改循环， using std :: begin;使用std :: end; 。这确保它从具有 begin 和 end的其他命名空间找到 std 成员，但在其命名空间中没有自由函数。

If you're going to use ADL to be able to change the container type without changing the loops, then add using std::begin; using std::end;. That makes sure it finds the std functions for containers from other namespaces that have begin and end members, but no free functions in their namespace.

namespace my {
    template <typename T>
    struct container {
        // ... stuff
        iterator begin();
        iterator end();
    };
    // no begin/end free functions
}


my::container<int> vec = get_vec();
using std::begin;
using std::end;
for (auto it = begin(vec), end = end(vec); it != end; ++it) { /*...*/ }
// defaults to std::begin which defaults to .begin() member function

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