CODEWITHSUNDEEP
cenumstypedef
TanyaV
TanyaV

Reputation: 131

enum causes multiply defined error while typedef enum doesnt

Im compliling C code.

Say i have header file, A.h which contains an enum. I include this file in 2 c files - B.c and C.c

B.c:

#include A.h
//....other code...

C.c

#include A.h
//....other code...

When A.h contains an enum:

A.h

#ifndef A_H
#define A_H

enum my_enum {

  //...enum content...

} my_enum;

//...other code...

#endif

when defined like this, i get a 'symbol "my_enum" multiply defined' error from the compiler, which is understandable- since i defined the enum in the header file.

However, this does not happen when i typedef the enum-

A.h

#ifndef A_H
#define A_H

typedef enum my_enum {

  //...enum content...

} my_enum;

//...other code...

#endif

This of course happens regardless of what the code is in the .c files (other than including A.h)

What is the reasoning behind this? Why is it not a multiple definition when i typedef the enum?

Upvotes: 1

Views: 3133

Answers (3)

pablo1977
pablo1977

Reputation: 4433

In C there are different name spaces.
We have this rules of "interference" (or not) of identifiers:

  • Identifiers in different scopes are not in conflct.
  • Identifiers in the same scope, but belonging to different name spaces are not in conflict.

These name spaces are:

  1. Ordinary identifiers (designating objects, functions, typedef names and enumeration constants).
  2. enum, struct, union tags.
  3. struct/union members.
  4. Labels (used with goto).

Thus, for example, since typedef names, enum tagss, struct members and labels are all in different name spaces, the following code has no any conflicts: 

  int main(void)   // "x" is always in diffrent name spaces and/or scopes
  {
       typedef struct { struct { float x; } x; }  x;
       enum x { zero = 0 } value = zero;

       x:    // Label

      if (value != 0)
           goto x;

      { // This block is another scope

          typedef enum x { one = 1 } x;
          x w = one;
      }
  }

For reference, see Section 6.2.3 of C standard document: Standard C99, TC3

Upvotes: 1

Waman
Waman

Reputation: 1179

When you use typedef it will treat my_enum as just an alias of enum.

i.e. in that case my_enum behaves as a type and not a variable.

If you do not use typedef it will multiple declaration..

Upvotes: 0

Yu Hao
Yu Hao

Reputation: 122493

enum my_enum {

  //...enum content...

} my_enum;

my_enum here is not a type, but a variable of type enum my_enum. Including this in a header leads to multiple definition of the variable my_enum.

typedef enum my_enum {

  //...enum content...

} my_enum;

my_enum here is a type, not a variable. It's a type the same as enum my_enum. That's the function of typedef.

Upvotes: 4

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