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c++rttitypeinfo

What is `type_info::before` useful for?

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?

Solution

  • 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";
}