Question

I would like to perform a compile-time check on datatype sizes in a C/C++ project, and error on unexpected mismatches. Simple

#if sizeof foo_t != sizeof bar_t

does not compile – claims that sizeof is not a proper compile-time constant.

The desired scope of platforms – at the very least Visual C++ with Win32/64, and GCC on x86/amd64.

EDIT: compile-time, not necessarily preprocessor. Just not a run-time error.

EDIT2: the code assumes that wchar_t is 2 bytes. I want a compilation error if it’s accidentally compiled with 4-byte wchar’s.

Answer 1

You have two options:

a) static_assert of C++11

b) BOOST_STATIC_ASSERT of boost

I would prefer the first one.

Edit:

The preprocessor is not really part of the language, as the name says it pre-processes a file, it has no knowledge of the language, so it does not know sizeof.

You could use some template to do some compile time code generation, for example:

template <typename T, bool x = sizeof(T) == 4>
class X;

template <typename T>
class X<T, true> {
  T v;
  const char* msg() const {
    return "My size is 4";
  }
}

template <typename T>
class X<T, false> {
  T v;
  const char* msg() const {
    return "My size is NOT 4";
  }
}

X<int> a;
X<short> b;