What is the underlying type of a c++ enum?

The type of a C++ enum is the enum itself. Its range is rather arbitrary, but in practical terms, its underlying type is an int.

It is implicitly cast to int wherever it’s used, though.

C++11 changes

This has changed since C++11, which introduced typed enums. An untyped enum now is defined as being at least the width of int (and wider if larger values are needed). However, given a typed enum defined as follows:

enum name : type {};

An enumeration of type name has an underlying type of type. For example, enum : char defines an enum the same width as char instead of int.

Further, an enum can be explicitly scoped as follows:

enum class name : type {
    value = 0,
    // ...
};

(Where name is required, but type is optional.) An enum declared this way will no longer implicitly cast to its underlying type (requiring a static_cast<>) and values must be referenced with a fully-qualified name. In this example, to assign value to a enum variable, you must refer to it as name::value.