why need typedef in the C++ enum definition

Question

I saw the following in the header file

typedef enum Test__tag {
    foo,
    bar
} Test;

I am wondering why using typedef; I can just use

enum Test{
    foo,
    bar
};

is that right?

Using the name `Test__tag` is not a good idea, because all names with double underscores are reserved for the implementation. — Zyx 2000, Jun 10 '13 at 09:22
@Zyx2000 I thought only those which *start* with a double underscore (or single underscore and captial letter). — Christian Rau, Jun 10 '13 at 13:53
@Christian Rau All names with double underscores anywhere are reserved. See http://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier, where the standard is quoted in the first answer. — Zyx 2000, Jun 10 '13 at 16:14

score 11 · Accepted Answer · answered Jun 10 '13 at 09:17

11

It's for C users. Basically when you go the typedef way, in C you can say

Test myTest;

whereas when you just used enum Test, you'd have to declare a variable like this:

enum Test myTest;

It's not needed if only C++ is used though. So it might also just be written by a C programmer who is used to this style.

answered Jun 10 '13 at 09:17

Botz3000

3

Or maybe it's a header that's meant to be included by both C and C++ code. – StoryTeller - Unslander Monica Jun 10 '13 at 09:25
@StoryTeller Yes, of course. – Botz3000 Jun 10 '13 at 09:26

score 3 · Answer 2 · answered Jun 10 '13 at 09:22

Yes, in C++ you declare x without the typdef just by writing

 enum Test{
    foo,
    bar
};
Test x;

However if you were to try the same thing in C you would need to declare x as

 enum Test x;

or use the typedef.

So the the typedef is probably a habit left over from C (unless the header file is in fact C).

2 Answers2