C ++中std :: set的自定义比较运算符和自定义类

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() )
{
}

Answer 2

The problem with your code is that it is not guaranteed to compile. The problem is not the compare struct, so take that out of the picture. It is this:

class My_Class {
public:
    char letter;
    set<My_Class, compare> Children;  // it is the set<My_Class> that is the problem
};

You are defining a std::set of My_Class before the definition of My_Class is known to the compiler. In other words, you're using an incomplete type within the std::set container. There is no guarantee that the code will compile, and even if it did, the behavior now is undefined.

If you want a container that works with incomplete types, you can use the Boost container types here:
http://www.boost.org/doc/libs/1_55_0/doc/html/container.html

Here is the description about Incomplete Types within the Boost documentation: http://www.boost.org/doc/libs/1_55_0/doc/html/container/main_features.html#container.main_features.containers_of_incomplete_types