static if in plain c++?

Question

Problem in short:
How could one implement static if functionality, proposed in c++11, in plain c++ ?

History and original prob

Answer 1

Quoting @JohannesSchaubLitb

with my static_if that works on gcc one can do it :)
in some limited fashion

(see also here)

This trick involves a specific GCC interpretation of the specs on Lambdas in C++11. As such, it will (likely) become a defect report against the standard. This will lead to the trick no longer working in more recent version of GCC (it already doesn't work in 4.7).

See the comment thread below for some more details from Johanness

http://ideone.com/KytVv:

#include 
 
namespace detail {
template
struct call_if { template void operator<<(F) { } };
 
template<>
struct call_if {
  template
  void operator<<(F f) { f(); }
};
}
 
#define static_if(cond) detail::call_if() << [&]
 
template
void f(T t) {
  static_if(C) {
    t.foo();
  };
}
 
int main() {
  f(42);
}