Initializing multiset with custom comparison function in C++

Question

Consider following comparison function:

bool compare(std::shared_ptr &lhs, std::shared_ptr &rhs){
   return lhs->val         


        

Answer 1

Because the set needs a comparison functor to work with. If you don't specify one, it will make a default-constructed one. In this case, since you're using a function-pointer type, the default-constructed one will be a null pointer, which can't be called; so instead, you have to provide the correct function pointer at run time.

A better approach might be to use a function class type (a.k.a. functor type); then the function call can be resolved at compile time, and a default-constructed object will do the right thing:

struct compare {
    bool operator()(std::shared_ptr &lhs, 
                    std::shared_ptr &rhs) const {
        return lhs->value < rhs->value;
    }
};

std::multiset, compare> myset;