How to enforce the 'override' keyword?

C++11 almost had what you want.

Originally the override keyword was part of a larger proposal (N2928) which also included the ability to enforce its usage:

class A
{
  virtual void f();
};

class B [[base_check]] : public A
{
    void f();  // error!
};

class C [[base_check]] : public A
{
  void f [[override]] ();  // OK
};

The base_check attribute would make it an error to override a virtual function without using the override keyword.

There was also a hiding attribute which says a function hides functions in the base class. If base_check is used and a function hides one from the base class without using hiding it's an error.

But most of the proposal was dropped and only the final and override features were kept, as "identifiers with special meaning" rather than attributes.

There are few ways to do this in VC++ and equivalent ways with GCC as well.

VC++

Below are the relevant warning numbers in VC++:

C4263 (level 4) 'function': member function does not override any base class virtual member function
C4266 (level 4) 'function': no override available for virtual member function from base 'type'; function is hidden

To enable these two warnings, you can use one of following options:

1. Set warning level to 4 in project settings and then disable the warnings you don't want. This is my prefered way. To disable unwanted Level 4 warnings, go to project settings > C/C++ > Advanced and then enter warning numbers in Disable Specific Warnings box.

2. Enable above two warnings using code.

   #pragma warning(default:4263)
   #pragma warning(default:4266)
   

3. Enable above two warnings in project settings > C/C++ > Command Line and then enter /w34263 /w34266. Here /wNxxxx option means enable xxxx warnings in Level N (N = 3 is default level). You can also do /wdNxxxx which disables the xxxx warning in level N.

GCC

GCC 5.1+ has added new warning suggest-override that you can pass as command line option -Wsuggest-override.

Clang

Clang 3.5+ has -Winconsistent-missing-override, however this only detects cases if some overriding memebers or base classes use override but other overriding members do not. You might want to take a look at clang-tidy tool as well.