How to specify a class's comparator in a std::multiset - c++

Question

I want to create a multiset of objects of my own Class with a custom comparator, and every example I have seen out there does it without classes.
The code I have so far main.cpp is like this:
multiset<MyClass> myMultiSet;
whereas in Myclass.cpp there is a comparator function:
bool MyClass::operator<(const MyClass c) const {
return (this->data->size() < c.size());
}
Multiset seems to take the less operator by default, and it actually uses the operator< function in the class, but I cannot find a way to specify it to use another operator (such as operator<=)
For example, I would like it to look like this:
multiset<MyClass, MyClass::operator<=> myMultiSet;
bool MyClass::operator<=(const MyClass c) const { // <----- Now it's using this one
return (this->data->size() <= c.size());
}

Answer 1

Member functions are different to functions: they take an the this pointer. As a result, you can't use pointer-to-member-functions directly (and you would need to take the address explicitly as member functions don't decay to pointer-to-member-functions, i.e., &MyClass::operator<=). Also, the parameter is type, not a value. Also, as was pointed out, operator<= doesn't implement a strict weak order, i.e., you can't use that anyway.
The easiest to use an actual operator, whether member of not, is to use the names from <functional>, e.g.
std::multiset<MyClass, std::greater<MyClass>> s;
If the member isn't a known name, I'd think using a lambda is the easiest approach but requires C++20:
std::multiset<MyClass, decltype([](MyClass const&l, MyClass const&r){ return l.member(r); })> s;
Without C++20 I think you can use std::mem_fn although that is slightly annoying as you'll need to provide the pointer to member as a constructor argument:
#include <functional>
#include <set>
struct MyClass
{
bool member(MyClass const&) const { return false; }
};
int main()
{
std::multiset<MyClass, decltype(std::mem_fn(&MyClass::member))> s(std::mem_fn(&MyClass::member));
}