Why do you use typedef when declaring an enum in C++?

Tim_Merrifield_36706 0
Viewed 219582

I haven't written any C++ in years and now I'm trying to get back into it. I then ran across this and thought about giving up:

typedef enum TokenType
{
    blah1   = 0x00000000,
    blah2   = 0X01000000,
    blah3   = 0X02000000
} TokenType;

What is this? Why is the typedef keyword used here? Why does the name TokenType appear twice in this declaration? How are the semantics different from this:

enum TokenType
{
    blah1 = 0x00000000,
    blah2=0x01000000,
    blah3=0x02000000
};
enums typedef c++
9 Answers

In C, declaring your enum the first way allows you to use it like so:

TokenType my_type;

If you use the second style, you'll be forced to declare your variable like this:

enum TokenType my_type;

As mentioned by others, this doesn't make a difference in C++. My guess is that either the person who wrote this is a C programmer at heart, or you're compiling C code as C++. Either way, it won't affect the behaviour of your code.

user_550

It's a C heritage, in C, if you do :

enum TokenType
{
    blah1   = 0x00000000,
    blah2   = 0X01000000,
    blah3   = 0X02000000
};

you'll have to use it doing something like :

enum TokenType foo;

But if you do this :

typedef enum e_TokenType
{
    blah1   = 0x00000000,
    blah2   = 0X01000000,
    blah3   = 0X02000000
} TokenType;

You'll be able to declare :

TokenType foo;

But in C++, you can use only the former definition and use it as if it were in a C typedef.

mat_420830

You do not need to do it. In C (not C++) you were required to use enum Enumname to refer to a data element of the enumerated type. To simplify it you were allowed to typedef it to a single name data type.

typedef enum MyEnum { 
  //...
} MyEnum;

allowed functions taking a parameter of the enum to be defined as

void f( MyEnum x )

instead of the longer

void f( enum MyEnum x )

Note that the name of the typename does not need to be equal to the name of the enum. The same happens with structs.

In C++, on the other hand, it is not required, as enums, classes and structs can be accessed directly as types by their names.

// C++
enum MyEnum {
   // ...
};
void f( MyEnum x ); // Correct C++, Error in C
user_365650

Holdover from C.

user_107550

The actual answer to the "why" question (which is surprisingly ignored by the existing answers top this old question) is that this enum declaration is probably located in a header file that is intended to be cross-compilable as both C and C++ code (i.e. included into both C and C++ implementation fiules). The art of writing such header files relies on the author's ability to select language features that have the proper compatible meaning in both languages.

user_1876900
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