Are C++ enums signed or unsigned?

Are C++ enums signed or unsigned? And by extension is it safe to validate an input by checking that it is <= your max value, and leave out >= your min value (assuming you started at 0 and incremented by 1)?

Solution 1:

Let's go to the source. Here's what the C++03 standard (ISO/IEC 14882:2003) document says in 7.2-5 (Enumeration declarations):

The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration. It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int.

In short, your compiler gets to choose (obviously, if you have negative numbers for some of your ennumeration values, it'll be signed).

Solution 2:

You shouldn't rely on any specific representation. Read the following link. Also, the standard says that it is implementation-defined which integral type is used as the underlying type for an enum, except that it shall not be larger than int, unless some value cannot fit into int or an unsigned int.

In short: you cannot rely on an enum being either signed or unsigned.

Solution 3:

You shouldn't depend on them being signed or unsigned. If you want to make them explicitly signed or unsigned, you can use the following:

enum X : signed int { ... };    // signed enum
enum Y : unsigned int { ... };  // unsigned enum

Solution 4:

You shouldn't rely on it being either signed or unsigned. According to the standard it is implementation-defined which integral type is used as the underlying type for an enum. In most implementations, though, it is a signed integer.

In C++0x strongly typed enumerations will be added which will allow you to specify the type of an enum such as:

enum X : signed int { ... };    // signed enum
enum Y : unsigned int { ... };  // unsigned enum

Even now, though, some simple validation can be achieved by using the enum as a variable or parameter type like this:

enum Fruit { Apple, Banana };

enum Fruit fruitVariable = Banana;  // Okay, Banana is a member of the Fruit enum
fruitVariable = 1;  // Error, 1 is not a member of enum Fruit
                    // even though it has the same value as banana.