Question

In C++, I see this code.

public: 
    Ports& GetPorts();
    const Ports& GetPorts() const;

Why is it necessary to have another method with const?
How can a compiler decide which method is called?

Answer 1

If you call x.GetPorts() and x is a non-const object, the first version will be called. If x is a const object, on the other hand, the second version will be called. That kind of code is saying “if the object is modifiable, allow the result of GetPorts() to be modified; if the object is const, don’t allow the result to be modified.” The compiler will prefer the first version if it matches; however, it will not be present if the object is const and so the second version will be used instead.