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

问题:

This may have been answered elsewhere but I could not find a suitable response.

I have this code:

enum enumWizardPage
{
    WP_NONE = 0x00,  
    WP_CMDID = 0x01,    
    WP_LEAGUES = 0x02,  
    WP_TEAMS = 0x04,    
    WP_COMP = 0x08, 
    WP_DIVISIONS = 0x10,
    WP_FORMULAS = 0x20, 
    WP_FINISHED = 0x40, 
};

Which is legacy and I have to modify it by adding a few new values. The issue is each value must be a unique bit so they may be OR combined to a bitmap.

The values are set using the #x## hex format, but I'm wondering if this is the max it can store? What will be the effect, if any, if I change my code to

enum enumWizardPage
{
    WP_NONE = 0x0000,  
    WP_CMDID = 0x0001,  
    WP_LEAGUES = 0x0002,    
    WP_TEAMS = 0x0004,  
    WP_COMP = 0x0008,   
    WP_DIVISIONS = 0x0010,
    WP_FORMULAS = 0x0020,   
    WP_FINISHED = 0x0040,   
};

回答1:

From Standard C++ 7.2/5:

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 enu- merator cannot fit in an int or unsigned int. If the enumerator-list is empty, the underlying type is as if the enumeration had a single enumerator with value 0. The value of sizeof() applied to an enu- meration type, an object of enumeration type, or an enumerator, is the value of sizeof() applied to the underlying type.

回答2:

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.

回答3:

IIRC its represented as int in memory. But gcc has switch -fshort-enum to make it a shortest integer type that fits all the values, if you need to save space. Other compilers will have something similar.

回答4:

The underlying type is the smallest signed integer fitting the biggest/smallest value of your enum.