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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?
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std::type_info The class type_info holds implementation-specific information about a type, including the name of the type and means to compare two types for equality or collating order. This is the class returned by the typeid operator.
The typeof operator returns the type of its argument, which can be an expression or a type. The language feature provides a way to derive the type from an expression. Given an expression e , __typeof__(e) can be used anywhere a type name is needed, for example in a declaration or in a cast.
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";
}
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.
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