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) {
        friend class Bar;
    }
};

I did not find any solution on the internet. I went through all the answers to the question Generating Structures dynamically at compile time. Many of them use the C++11 std::conditional, but I would like to know if it is possible to do this in C++03 without using the preprocessor.

This solution https://stackoverflow.com/a/11376710/252576 will not work because friendship is not inherited ( friend class with inheritance ).

Edit Just to make this more easily visible, as mentioned below in the comment: This requirement is unusual. This is part of a new research project in hardware simulation, that I am working on. The testbench is written in C++, and I want to display the variables in a waveform. I have researched various other options, and figured out that I need to use a friend class, due to practical considerations. The friend will capture the values and generate the waveform, but I would prefer to have the friend only when the waveform is required, and not all the time.

Answer 1

Use friend std::conditional<C, friendclass, void>::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<C, 
    friendclass, detail::friendclass>::type::friendclass;
};

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

Answer 2

[class.friend]/3 tells this :

A friend declaration that does not declare a function shall have one of the following forms:
friend elaborated-type-specifier ;
friend simple-type-specifier ;
friend typename-specifier ;

therefore it is not possible to conditionally declare friends of a class, without a macro.