I came across this concept recently and wanted to share it here. In C++, you can change the return type of a virtual function if and only if that return type is derived class of the parent return type. Note: you can always change the return type of a overloaded method in a derived class. The following example compiles with no errors. Notice that the virtual getThis() is overridded with a new return type that is derived from the return type of the parent’s getThis() function. Also notice how the non-virtual getInt() has its return type changed.
class Base {
public:
virtual Base * getThis() { return this; }
Base & getThisRef() { return *this; }
int getInt() { return 1; }
};
class Derived : public Base {
public:
virtual Derived * getThis() { return this; }
Derived & getThisRef() { return *this; }
long getInt() { return 1; }
};
Now notice the next example. This example will not compile because you can only change the return type of a virtual function with a derived type.
class Base {
public:
virtual int getInt() { return 1; }
};
class Derived : public Base {
public:
virtual long getInt() { return 1; }
};