Question

I would expect the following code snippet to complain about trying to assign something other that 0,1,2 to a Color variable.
But the following does compile and I get the output

Printing:3

3

Can anybody explain why? Is enum not meant to be a true user-defined type? Thanks.

enum Color { blue=0,green=1,yellow=2};

void print_color(Color x);

int main(){
    Color x=Color(3);

    print_color(x);
    std::cout << x << std::endl;

    return 0;
}

void print_color(Color x)
{
    std::cout << "Printing:" << x << std::endl;
}

Answer 1

Since you manually cast the 3 to Color, the compiler will allow you to do that. If you tried to initialize the variable x with a plain 3 without a cast, you would get a diagnostic.

Note that the range of values an enumeration can store is not limited by the enumerators it contains. It’s the range of values of the smallest bitfield that can store all enumerator values of the enumeration. That is, the range of your enumeration type is 0..3:

00
01
10
11

The value 3 is thus still in range, and so the code is valid. Had you cast a 4, then the resulting value would be left unspecified by the C++ Standard.

In practice, the implementation has to chose an underlying integer type for the enumeration. The smallest type it can choose is char, but which is still able to at least store values ranging up to 127. But as mentioned, the compiler is not required to convert a 4 to a value of 4, because it’s outside the range of your enumeration.

---

I figure i should post some explanation on the difference of “underlying type” and “range of enumeration values”. The range of values for any type is the smallest and largest value of that type. The underlying type of an enumeration must be able to store the value of any enumerator (of course) – and two enumerations that have the same underlying type are layout compatible (this allows some flexibility in case a type mismatch occurs).

So while the underlying type is meant to fix the object representation (alignment and size), the values of the enumeration is defined as follows in 7.2/6

For an enumeration where e_min is the smallest enumerator and e_max is the largest, the values of the enumeration are the values of the underlying type in the range b_min to b_max, where b_min and b_max are, respectively, the smallest and largest values of the smallest bit-field that can store e_min and e_max . It is possible to define an enumeration that has values not defined by any of its enumerators.

[Footnote: On a two’s-complement machine, b_max is the smallest value greater than or equal to max (abs(e_min) − 1 ,abs(e_max)) of the form
2^M−1; b_min is zero if e_min is non-negative and −(b_min+1) otherwise.]