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

It seems, unfortunately, not possible within the C++ compiler: ie, it seems that only the preprocessor may help here. Note: Johannes has a proposal, so there is hope!

However I would note that:

• friendship does not require you to actually use it
• friendship is a pure compile-time construct (like access specifiers) and does not incur any runtime penalty on any major compiler

there is no reason not to have unconditional friendship, but only use it if some conditions (static or dynamic) are met.

Note: in the future, this is something that the static_if proposal could cover.