multiset count method not working with class comparator-CodePudding

I have this struct

struct C {
    int ID;
    int age;
    C(int ID, int age) : ID{ID}, age{age} {}
};

I use a comparator function for a multiset

bool fncomp (const C& lhs, const C& rhs) {
    return lhs.age < rhs.age;
}

multiset<C, decltype(fncomp)*> ms{fncomp};
ms.emplace(1, 15);
...

// this works fine
ms.count(C(1, 15));

However if I use a class comparator, this is no longer working.

struct classcomp {
    bool operator() (const C& lhs, const C& rhs) {
        return lhs.age < rhs.age;
    }
};

multiset<C, classcomp> ms;
ms.emplace(1, 15);
...

// error
// ms.count(C(1, 15));

Anything makes the two different?

CodePudding user response：

Elaborating on my comment above:

multiset::count is a const member function, which means that it operates on a const multiset. This includes the member variables of the multiset. The comparator is a member variable of the multiset.

Since your classcomp::operator() is not marked const, it can't be called on a const object, and so it fails to compile.

This works for the function pointer example, because it's the pointer that is const in that case.

CodePudding user response：

bool operator() (const C& lhs, const C& rhs) const  {
    return lhs.age < rhs.age;
}

This would fix things to compile in this link you provided, courtesy of @Marshall -> https://stackoverflow.com/a/71384594/10630957