A way to do c++ "typedef struct foo foo;" for c

Question

Going by gcc version 4.4.2, it appears that saying

typedef struct foo foo;
// more code here - like function declarations taking/returning foo*
// then, in its own source file:
typedef struct foo
{
    int bar;
} foo;

is legal in C++ but not in C.

Of course I have a body of code that compiles fine in C++ by using the foo type but it appears I must make it use struct foo (in the header file) to get it to work with some C code another developer wrote.

Is there a way to predeclare a struct typedef foo foo in gcc C without getting a "redefinition of typedef 'foo'" error when compiling for C? (I don't want the marginally illegal and less clean underscore solution of struct typedef _foo foo)

Duplicate, see http://stackoverflow.com/questions/1492735/differences-between-struct-in-c-and-c/ — joshperry, Dec 30 '09 at 15:03

Mike Weller · Answer 1 · 2009-12-30T14:11:56.183

8

Is this what you need?

// header (.h)
struct foo;
typedef struct foo foo;

foo *foo_create();
// etc.

// source (.c)
struct foo {
    // ...
}

I also tend to prefix my struct name with an underscore when typdefing to make its privateness clear and prevent possible name clashes.

edited Dec 30 '09 at 14:11

answered Dec 30 '09 at 13:59

Mike Weller

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2

beware reserved names when using underscores, see http://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier – Gregory Pakosz Dec 30 '09 at 14:14

score 5 · Accepted Answer · answered Dec 30 '09 at 20:32

One of the differences between C++ and C is that in C++ it is legal to make a repetitive typedef in the same scope as long as all these typedef are equivalent. In C repetitive typedef is illegal.

typedef int TInt;
typedef int TInt; /* OK in C++. Error in C */

This is what you have in your above code. If you are trying to write a code that can be compiled as both C and C++, get rid of the superfluous second typedef and just do

typedef struct foo foo;  
...
struct foo  
{  
    int bar;  
};

(although in C++ the first typedef is superfluous as well).

Success with that, just write `typedef struct foo foo` in a common header file or a forward-declaration header. — moala, Feb 14 '12 at 17:21

score 1 · Answer 3 · answered Dec 30 '09 at 15:48

I'm not sure why GCC rejects this code, but it appears it only objects because you're defining the same typedef twice.

This works:

typedef struct foo foo;

struct foo {
    int bar;
};

And this works too, with the same effect:

typedef struct foo {
    int bar;
} foo;

A way to do c++ "typedef struct foo foo;" for c

3 Answers3