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



3 Answers

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.

Question

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
};



Some people say C doesn't have namespaces but that is not correct. It has three:

  1. enum
  2. struct
  3. Global (everything else)

typedef enum { } XYZ; declares an anonymous enumeration and imports it into the global namespace with the global name XYZ.

typedef enum ABC { } XYZ; declares an enum named ABC in the enum namespace, then imports it into the global namespace as XYZ.

enum XYZ x; declares a variable of type XYZ in the enum namespace.

Ditto all of this for the struct namespace as well.

Some people don't want to bother with the separate namespaces so they typedef everything. Others never typedef because they want the namespacing.




In C, it is good style because you can change the type to something besides an enum.

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

foo(enum e_TokenType token);  /* this can only be passed as an enum */

foo(TokenType token); /* TokenType can be defined to something else later
                         without changing this declaration */

In C++ you can define the enum so that it will compile as C++ or C.




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.




Related