Typedef works for structs but not enums, only in C++

Question

I have the following code:

test_header.h:

typedef enum _test_enum test_enum;
enum _test_enum
{
    red,
    green,
    blue
};

typedef struct _test_struct test_struct;
struct _test_struct
{
    int s1;
    int s2;
    int s3;
};

test.c:

#include <test_header.h>
#include <stdio.h>

int main()
{
    test_struct s;
    s.s1=1;
    s.s2=2;
    s.s3=3;
    printf("Hello %d %d %d\n", s.s1, s.s2, s.s3 );
}

test_cpp.cpp:

extern "C"{
    #include <test_header.h>
}
#include <stdio.h>

int main()
{
    test_struct s;
    s.s1=1;
    s.s2=2;
    s.s3=3;
    printf("Hello %d %d %d\n", s.s1, s.s2, s.s3 );
}

Notice how I am typedef'ing the struct and the enum the same way. When I compile in straight C with gcc -I. test.c -o test it works fine, but when compiled in C++ with gcc -I. test_cpp.cpp -o test_cpp, I get the following error:

./test_header.h:1:14: error: use of enum ‘_test_enum’ without previous declaration

So my question is twofold: why does this work in C but not C++, and why does the compiler accept the struct but not the enum?

I get the same behavior when declaring the struct above the enum. I'm using GCC 4.8.2.

Answer 1

The enum is an integral type and the compiler chooses the exact type according to the range of values of the enumeration. So you can't do a forward declaration of an enum.