是std :: pair< int，std :: string>订购明确？ [英] Is std::pair<int, std::string> ordering well-defined?

问题描述

看来我可以排序std :: vector< std :: pair< int，std :: string>>，它会根据int值排序。这是一个明确的事情吗？ std :: pair是否有基于它的元素的默认排序？

It seems that I can sort an std::vector<std::pair<int, std::string>>, and it will sort based on the int value. Is this a well defined thing to do? Does std::pair have a default ordering based on it's elements?

推荐答案

std :: pair 使用词典比较：它将基于第一个元素进行比较。如果第一个元素的值相等，它将根据第二个元素进行比较。

std::pair uses lexicographic comparison: It will compare based on the first element. If the values of the first elements are equal, it will then compare based on the second element.

C ++ 03标准中的定义（第20.2.2节）是：

The definition in the C++03 standard (section 20.2.2) is:

template <class T1, class T2>
bool operator<(const pair<T1, T2>& x, const pair<T1, T2>& y);

Returns: x.first < y.first || (!(y.first < x.first) && x.second < y.second).

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