什么是`type_info :: before`有用？ [英] What is `type_info::before` useful for?

问题描述

根据cplusplus.com， std :: type_info :: before（） function ...

According to cplusplus.com, the std::type_info::before() function...

整理顺序只是一个特定实现保存的内部顺序，不一定与继承关系或声明顺序相关。

Returns true if the type precedes the type of rhs in the collation order.
The collation order is just an internal order kept by a particular implementation and is not necessarily related to inheritance relations or declaring order.

这对于什么有用？

推荐答案

考虑你想把 type_info 对象作为键放入 map< type_info *，value> 。 type_info 没有定义运算符< ，因此您必须提供自己的比较器。从 type_info 接口中保证工作的唯一的事情是 before（）函数，因为 type_info 或名称（）必须是唯一的：

Consider you want to put your type_info objects as keys into a map<type_info*, value>. The type_info doesn't have an operator < defined, so you must provide your own comparator. The only thing that is guaranteed to work from the type_info interface is the before() function, since neither the addresses of type_info nor the name() must be unique:

struct compare {
    bool operator ()(const type_info* a, const type_info* b) const {
        return a->before(*b);
    }
};

std::map<const type_info*, std::string, compare> m;

void f() {
    m[&typeid(int)] = "Hello world";
}

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