constexpr_assert

constexpr_assert.h

// A compilation of the following posts:
// http://stackoverflow.com/questions/18648069/g-doesnt-compile-constexpr-function-with-assert-in-it
// http://ericniebler.com/2014/09/27/assert-and-constexpr-in-cxx11/
#include <cassert>
#include <utility>

template<class Assert>
inline void constexpr_assert_failed(Assert&& a) noexcept { std::forward<Assert>(a)(); }

// When evaluated at compile time emits a compilation error if condition is not true.
// Invokes the standard assert at run time.
#define constexpr_assert(cond) \
    ((void)((cond) ? 0 : (constexpr_assert_failed([](){ assert(!#cond);}), 0)))

/////////////////////////////////////////////////////////////////////
// Usage example:

inline constexpr int divide(int x, int y) noexcept
{
    return constexpr_assert(x % y == 0), x / y;
}

int main()
{
    constexpr auto a = divide(6, 2); // OK
    constexpr auto b = divide(5, 2); // Compile time error in both debug and release builds

    auto a1 = divide(6, 2); // OK
    auto a2 = divide(5, 2); // Run time assertion (debug build only)
    
    return 0;
}