枚举 class 声明中的冒号是什么？

Question

I just found this weird piece of code in the align_val_t definition of Visual Studio 2019's standard library:

namespace std
{
    enum class align_val_t : size_t {};
}

What does the colon mean?

enum class align_val_t : size_t {};
//                     ^
//                 this thing

Answer 1

_{All standard references below refers to N4659: March 2017 post-Kona working draft/C++17 DIS .}

---

The enum Name: UnderlyingType {} syntax defines an enum that is said to be fixed , with an explicitly specified underlying type . From [dcl.enum]/5 :

Each enumeration defines a type that is different from all other types. Each enumeration also has an underlying type . The underlying type can be explicitly specified using anenum-base . For a scoped enumeration type, the underlying type is int if it is not explicitly specified. In both of these cases, the underlying type is said to be fixed . [...]

---

 enum class align_val_t: size_t {}; // ^ // this thing

In this particular example, an enum named align_val_t is defined (in the std namespace) as fixed with explicitly specified underlying type size_t ( std::size_t , to be precise).

---

Scoped and unscoped enumerations & underlying types

// Unscoped enumeration; [enum Name {}]
// - underlying type not fixed.
enum UnscopedUnfixed { a, b };
auto uu_a = a;

// Scoped enumeration; [enum class Name {} / enum struct Name {}]
// - underlying type implicitly fixed to int.
enum class ScopedImplicitlyFixed { c, d };
auto sif_c = ScopedImplicitlyFixed::c;

// Scoped enumeration; [enum class Name : TYPE {} / enum struct Name : TYPE {}]
// - underlying type explicitly fixed.
enum class ScopedExplicitlyFixed : unsigned int { e, f };
auto sef_e = ScopedExplicitlyFixed::e;

Answer 2

 enum class align_val_t /*HERE----->*/:/*<------HERE*/ size_t {};

That colon is (an optional) part of the syntax of an enum (class) definition. It separates the name of the enum (class) and the underlying type.

Answer 3

Since C++11, this is part of the syntax of enum s, for which you can (but don't have to) specify the underlying type.

It's saying that every member of the enum has the type std::size_t ; exactly that type, nothing else. In the olden days, it wasn't always quite so clear what the type would be, at least not by simply looking at the code.