cenumsenumerationforward-declaration

Forward declaration of enums in C (not C++)


Forward declaration of enums in C does not work for me.

I searched the Internet and Stack Overflow, 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 a header), so that all functions in the file can access them?


Solution

  • 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 Mac OS X 10.7.1 (Lion)) 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.