Is there a way to prevent a method from being overridden in subclasses?

Question

Is anyone aware of a language feature or technique in C++ to prevent a child class from over riding a particular method in the parent class?

class Base { public:     bool someGuaranteedResult() { return true; } };  class Child : public Base { public:     bool someGuaranteedResult() { return false; /* Haha I broke things! */ } }; 

Even though it's not virtual, this is still allowed (at least in the Metrowerks compiler I'm using), all you get is a compile time warning about hiding non-virtual inherited function X.

Answer 1

When you can use the final specifier for virtual methods (introduced with C++11), you can do it. Let me quote my favorite doc site:

When used in a virtual function declaration, final specifies that the function may not be overridden by derived classes.

Adapted to your example that'd be like:

class Base { public:     virtual bool someGuaranteedResult() final { return true; } };  class Child : public Base { public:     bool someGuaranteedResult() { return false; /* Haha I broke things! */ } }; 

When compiled:

$ g++ test.cc -std=c++11 test.cc:8:10: error: virtual function ‘virtual bool Child::someGuaranteedResult()’ test.cc:3:18: error: overriding final function ‘virtual bool Base::someGuaranteedResult()’ 

When you are working with a Microsoft compiler, also have a look at the sealed keyword.

Answer 2

A couple of ideas:

1. Make your function private.
2. Do not make your function virtual. This doesn't actually prevent the function from being shadowed by another definition though.

Other than that, I'm not aware of a language feature that will lock away your function in such a way which prevents it from being overloaded and still able to be invoked through a pointer/reference to the child class.

Good luck!