Can friend class be declared conditionally in C++03?

Question

I want to declare a friend class only if some (compile-time) condition is true. For example:

// pseudo-C++
class Foo {
    if(some_compile_time_condition) {
         


        

Answer 1

Use friend std::conditional::type; where C is your condition. A nonclass type friend will be ignored.

The conditional template is easily implemented in C++03. However since C++03 does not support typedef friends you need to use the following syntax there

namespace detail { class friendclass {}; }

class Foo {
  friend class std::conditional::type::friendclass;
};

Note that the detail dummy class name needs to match the name of the potential friend in this workaround.