CODEWITHSUNDEEP
c++rttitypeinfo
Paul Manta
Paul Manta

Reputation: 31597

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?

Upvotes: 20

Views: 7338

Answers (4)

Xion
Xion

Reputation: 22780

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.

Upvotes: 1

Dietmar K&#252;hl
Dietmar K&#252;hl

Reputation: 154025

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.

Upvotes: 9

Yakov Galka
Yakov Galka

Reputation: 72539

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";
}

Upvotes: 29

Bo Persson
Bo Persson

Reputation: 92361

It gives an ordering.

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

Upvotes: 1

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