How to deal with -Wreturn-type for switch over C++11 enum class?

Question

If I have a function that returns based on switching over a enum class, gcc emits warning: control reaches end of non-void function [-Wreturn-type]. Example code:

enum class Test { a, b, c } ;

int foo(Test test) {
    switch (test) {
        case Test::a: return 0;
        case Test::b: return 1;
        case Test::c: return 2;
    }
}

I have thought of two solutions but neither seem correct:

1) have a default case that throws. However, when I add an additional member to the enum class I no longer receive an error that I missed a case.

2) Suppress the warning with a pragma, but then I don't guard against someone passing in a static_cast<Test>(123).

So my question is, how to deal with -Wreturn-type errors for switch over C++11 enum class?

Answer 1

Note: this answer is written with performance in mind (if you don't need it, just ignore this, and put some fatal error after the switch).

I'd recommend to use a compiler-specific feature to mark unreachable code. All major compilers have something like this. For example, GCC/Clang/icc has __builtin_unreachable, MSVC has __assume(false).

Use these tools in release mode. In debug mode, put some fatal error (assert, exception, whatever) there. This way, while in development, you'll catch errors, and in release mode, generated code will be efficient.

Note, there is a proposal, which intends to add std::unreachable() to C++. When this hopefully gets through, you can use it in such codes. Until then, you have to resort to the previously-mentioned compiler-specific solution.