What is `type_info::before` useful for?

问题

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

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.

So what is it useful for?

回答1:

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";
}

回答2:

This is useful to define an order on typeinfo objects, e.g. to put them into a std::map. The obvious follow-up question is: why isn't it spelled operator<()? I don't know the answer to this question.

回答3:

It gives an ordering.

That is required if you want to store values in some containers, like std::map.

回答4:

Think of it as less-than (<) operator for type_info objects. If you ever wanted to store in ordered collection - such a set of map - you can use it to make an appropriate comparator. It's a reliable and preferred way, as opposed to, say, using type's name which might not be unique.

来源：https://stackoverflow.com/questions/8682582/what-is-type-infobefore-useful-for

标签

c++

rtti

typeinfo