C++0x: noexcept(ndebug) for testing?

Question

I read about concern that the overly use of noexcept may hinder a testable library.

Consider:

T& vector::front() noexcept {
    assert(!empty());         // <- this may throw in some test-frameworks
    return data[0];
}

With the annotation with noexcept the compiler may optimize exception-code out, which would/could prevent proper handling of assert() (or whichever function the author wants to use here for his tests).

Therefore, I wonder, if it is feasible in a library to never use an unconditional noexcept but to always "link" it with a am-I-in-a-test-condition. Like this:

#ifdef NDEBUG    // asserts disabled
static constexpr bool ndebug = true;
#else            // asserts enabled
static constexpr bool ndebug = false;
#end

T& vector::front() noexcept(ndebug) {
    assert(!empty());
    return data[0];
}

and then maybe add it as a macro (although, I hate that):

#define NOEXCEPT noexcept(ndebug)

T& vector::front() NOEXCEPT {
    assert(!empty());
    return data[0];
}

What do you think? Does this make any sense at all? Or is it not feasible? Or does it not solve the problem? Or is there no problem at all? :-)

Answer 1

If you cannot prove that a function will not emit an exception then you should not tag it with noexcept. It's that simple.

That said, noexcept(ndebug) seems reasonable to me. I see no reason to hide it behind a macro. Template functions often have verbose noexcept conditions.

If the function within the asset can throw during test modes then your program can spontaneously std::terminate during test mode. (It's a lot like an implicit assert actually.)

The only downside I can see is that if this happens you won't get a line number and hint message. That, and the compiler ought to warn you if you call a noexcept(false) function from a noexcept(true) one, so you might get some noise.