Custom comparison operator and custom class for std::set in C++

Question

I would like to create a set containing the objects of my class, I have to determine a custom comparison. Unfortunately, everything I tried did not work.

class My_Class {
    public:
        char letter;
        set<My_Class, compare> Children;
};

Ant then, the compare struct:

struct compare {
    bool operator() (const My_Class& a, const My_Class& b) const{
        return a.letter < b.letter;
    }
};

How can I make this work please?

Currently, the issue displays that identifiers a and b are not declared.

Answer 1

You are trying to use compare structure inside My_Class, which uses My_Class in its method. It is not a trivial case, but forward declaration will help. So this should work:

class My_Class;

struct compare {
    bool operator() (const My_Class &a, const My_Class &b) const;
};

class My_Class {
public:
    char letter;
    set<My_Class, compare> Children;
};

bool compare::operator() (const My_Class &a, const My_Class &b) const
{
    return a.letter < b.letter;
}

Another alternative would be to pass comparator to std::set constructor, rather than specify it as a template parameter:

class My_Class {
public:
    My_Class();
    char letter;
    set<My_Class> Children;
};

struct compare {
    bool operator() (const My_Class& a, const My_Class& b) const{
        return a.letter < b.letter;
    }
};

My_Class::My_Class() : Children( compare() )
{
}