C - 枚举的前向声明？

Question

Forward declaration of enums in C does not work for me. I searched the internet and stackoverflow but all of the questions regarding forward declarations of enumerators refer to c++. What do you do for declaring enumerators in C? Put them at the top of each file (or in an include) so that all functions in the file can access them? Thanks

Answer 1

Put them in a header so that all files that need them can access the header and use the declarations from it.

When compiled with the options:

$ /usr/bin/gcc -g -std=c99 -Wall -Wextra -c enum.c
$

GCC 4.2.1 (on MacOS X 10.7.1) accepts the following code:

enum xyz;

struct qqq { enum xyz *p; };

enum xyz { abc, def, ghi, jkl };

Add -pedantic and it warns:

$ /usr/bin/gcc -g -std=c99 -Wall -Wextra -pedantic -c enum.c
enum.c:1: warning: ISO C forbids forward references to ‘enum’ types
enum.c:5: warning: ISO C forbids forward references to ‘enum’ types
$

Thus, you are not supposed to try using forward declarations of enumerated types in C; GCC allows it as an extension when not forced to be pedantic.

Answer 2

You can't "forward-declare" enums because the compiler won't know the size of the enum. The C standard says " Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined, but shall be capable of representing the values of all the members of the enumeration ".

Answer 3

I came here having the same error, but there is not really much information provided here on the code/error.

My Makefile-flags are: -Wall -Wextra -Werror -pedantic -std=c17

In my header I have the following enum :

typedef enum 
{
  IS_HEAD = 1, 
  IS_VALUE = 2,
  IS_SIDE
} CoinResult;

The tutorials here and there

Would recommend to use something like this:

enum CoinResult cr;
cr = IS_SIDE;

This results in the error stated by the OP.

Solved by using:

CoinResult cr = IS_SIDE; 

Not sure which C-Standard, Code or reference OP was using, but I somewhat agree: Most tutorials and solutions for this relatively simple issue are kinda ambiguous.

Answer 4

CoinResult isn't an enum, it's a type. If you had

enum CoinResult {
    IS_HEAD = 1,
    IS_VALUE = 2,
    IS_SIDE,
};

then

    enum CoinResult cr;

would be correct.