Question

If I try to compile:

class Outer 
{
    class Inner 
    {
        int t;
    };
public:
    Inner inner_;
    bool operator ==(Outer rightSide);
};

bool Outer::operator ==(Outer rightSide)
{
    if (inner_ == rightSide.inner_)
        return true;
    return false;
}

I get an error:

/home/martin/Projects/Experimentation/Various-build-desktop/../Various/main.cpp:18: 
error: no match for ‘operator==’ in ‘((Outer*)this)->Outer::inner_ == 
rightSide.Outer::inner_’

Please, is it just me doing something wrong or is this a property of C++

EDIT:

Oh, I never realized that the operator == is never synthesized, I was so convinced that it is synthesized, that I did not bother to check.
Thank you Parapura Rajkumar!

Answer 1

Comparison operators are never implicitly generated. Only these things are:

• Default constructor,
• Destructor,
• Copy and move constructors,
• Copy-assigment and move-assignment operators

If you want to be able to compare your types, you’ll have to write your own comparison operators. If you implement them as members, then they should be declared const; otherwise, it will be impossible to compare constant objects. You might also consider taking arguments as constant references to avoid unnecessary copying; it makes little difference for simple types like these, but can be much more efficient for large or complicated classes. Something like:

bool Outer::operator==(Outer const & rhs) const {
    return inner_.t == rhs.inner_.t;
}

or as a non-member function:

bool operator==(Outer const & lhs, Outer const & rhs) {
    return lhs.inner_.t == rhs.inner_.t;
}