如何切换枚举 class？

Question

Enum classes are supposed to be strong enums in the sense that they don't implicitly convert to and from int . For instance:

enum class EC { a, b };

However, when switching over such a "strong enum":

int sw(EC ec) {
  switch (ec) {
    case EC::a: return 0;
    case EC::b: return 1;
  }
}

gcc -Wreturn-type -wants me to add a default clause to the switch even though all legal enum values are covered:

warning: control reaches end of non-void function [-Wreturn-type]

In an old (non-class) enum, this makes sense, because any int could have been accidentally converted to EC . But I had (apparently wrongly) assumed that assigning an invalid enum member to an enum class was UB.

How can I use truly strong enum classes where the compiler realises that functions like sw cover all possible paths? Of course I could just add a default: branch that I know will never be triggered, but I want to make sure that adding more members to EC in the future will trigger a warning in the switch .

Answer 1

You can return a dummy variable, to remove the "control reaches end of non-void function". This way, the warning is removed, and any additions to the enum-class will still trigger a warning in the switch statement:

int sw(EC ec) {
  switch (ec) {
    case EC::a: return 0;
    case EC::b: return 1;
  }

  return 0; //dummy variable
}

Answer 2

"control reaches end of non-void function" is quite different from common "enumeration value 'c' not handled in switch [-Wswitch]" warning. I think compiler is being a bit too cautious here, but this warning may turn out to be handy because it will prevent potential future UB caused by modification of enum and ignoring of -Wswitch warning.

Rewriting this snippet like this would make code future proof:

online compiler

enum class EC { a, b /*,c */ };

int sw(EC ec) {
    int result{};
  switch (ec) { // warning: enumeration value 'c' not handled in switch [-Wswitch]
    case EC::a: result = 0; break;
    case EC::b: result = 1; break;
  }
  return result; // control flow will always leave function properly
}

Answer 3

A enum variable - both old-school one and enum class can hold values that are not one of the members of the enumeration. As long as the integer value fits within the underlying type (with a few more restrictions), it is valid to store it in the enumeration type.

Answer 4

This worked for me:

enum class EC {a, b, c};
int sw (EC ec)
{
    int rc;
    switch (ec)
    {
        case EC::a:
        rc = 0;
        break;
    case EC::b:
        rc = 1;
        break;
    }
    return rc;
}

Answer 5

In GCC/Clang/ICC you can silence this warning with __builtin_unreachable() :

int sw(EC ec) {
    switch (ec) {
        case EC::a: return 0;
        case EC::b: return 1;
    }

    assert(false);
    __builtin_unreachable();
}

In MSVC __assume(0) can be used.