C++ set with customized comparator crashes on insert

Question

STL set can have customized comparator. It can be implemented in several ways, such as define an operator(), use decltype on lambda, etc. I was trying to use a static method of a class and encountered a weird crash. The crash can be demonstrated by the following code

#include <string>
#include <set>

struct Foo {
        static bool Compare(std::string const& s1, std::string const& s2)
        {
                return s1 < s2;
        }
};

std::set<std::string, decltype(&Foo::Compare)> S;

int main()
{
        S.insert("hello");
        S.insert("world");

        return 0;
}

The crash happened on the second insert. Thank you.

Answer 1

You have to pass pointer to compare function to set constructor, otherwise it is null and it is why the code fails.

std::set<std::string, decltype(&Foo::Compare)> S{&Foo::Compare};

By decltype(&Foo::Compare) you only specify the type of comparator, it has to be provided because its default value is null.