> declare it as private and just leave an empty implementation?
ideally, declare it as private and do not define it.

> Is this common/useful?
useful, yes. common, not all that much.
if there is a class that you write (for example a window), and want its instances not to have value semantics, you need to suppress any compiler-generated copy constructor and assignment operators.

c++ Syntax (Toggle Plain Text)

struct window
{
    // ...
    private:
       window( const window& ) ; // do not define this
       void operator= ( const window& ) ; // do not define this
    // ...
}

struct window { // ... private: window( const window& ) ; // do not define this void operator= ( const window& ) ; // do not define this // ... }

see: http://www.boost.org/doc/libs/1_37_0...ss_noncopyable