Compile-time assertions in C++?

Question

I recently came upon the need to have compile-time assertions in C++ to check that the sizes of two types were equal.

I found the following macro on the web (stated to have come from the Linux kernel):

#define X_ASSERT(condition) ((void)sizeof(char[1 - 2*!!(condition)]))

which I used like so:

X_ASSERT(sizeof(Botan::byte) != sizeof(char));

This gets me curious - although this works, is there a cleaner way to do so? (obviously there's more than one way, as it is) Are there advantages or disadvantages to certain methods?

Answer 1

In C++0x, there is a new language feature, static_assert, which provides a standard way to generate compile-time assertions. For example,

static_assert(sizeof(Botan::byte) != 1, "byte type has wrong size");

Visual C++ 2010 supports static_assert, as do g++ 4.3 (and greater) and Intel C++ 11.0.