C++ throwing compilation error on sizeof() comparison in preprocessor #if

Question

I have this which does not compile with the error "fatal error C1017: invalid integer constant expression" from visual studio. How would I do this?

template <class B>
A *Create()
{
  #if sizeof(B) > sizeof(A)
  #error sizeof(B) > sizeof(A)!
  #endif
  ...
}

Answer 1

The preprocessor does not understand sizeof() (or data types, or identifiers, or templates, or class definitions, and it would need to understand all of those things to implement sizeof).

What you're looking for is a static assertion (enforced by the compiler, which does understand all of these things). I use Boost.StaticAssert for this:

template <class B>
A *Create()
{
  BOOST_STATIC_ASSERT(sizeof(B) <= sizeof(A));
  ...
}

Answer 2

Preprocessor expressions are evaluated before the compiler starts compilation. sizeof() is only evaluated by the compiler.