我可以在 C 中 typedef struct aaa struct x_aaa 吗？

Question

I use a public library. It change its struct from struct aaa to struct x_aaa in newer version. I want to keep my code pass compiling in any version of the library. So I want to do such:

#if lib_ver > 20000
typedef struct x_aaa struct aaa;
#endif

And then use strcut aaa in later code. But this not work. How do you solve such problem?

Answer 1

A typedef name must be a single identifier, so you can't do that.
(A typedef has the same form as a variable declaration, with the word "typedef" added in front of it.)

You can use a typedef that depends on the version

#if lib_ver > 20000
    typedef struct x_aaa lib_aaa;
#else
    typedef struct aaa lib_aaa;
#endif

and change your code to use lib_aaa instead of the full struct name.

You could also use a macro, but it's a good idea to avoid them:

#if lib_ver > 20000
    #define aaa x_aaa;
#endif

The renaming sounds like it could be a breaking change even if you get your code to compile, though.
I would be very cautious about this.

Answer 2

You cannot typedef a struct something into struct something_else .

What you can do is to typedef a struct something into something_else (dropping the word struct ).

If this is not what you want, you should use a simple #define :

#define aaa x_aaa

Answer 3

typedef struct aaa bbb;
//      ^^^^^^^^^^      source type
//                 ^^^  new identifier

You can't have the new identifier for an existent type be "struct something".

You maybe need to change the code to reflect that the struct you're using is not the same in all versions of the library

#if lib_ver > 20000
#typedef struct x_aaa libaaa
#else
#typedef struct aaa libaaa
#endif

// in your code use `libaaa` instead of `struct aaa`
//struct aaa foo;
libaaa foo;
//struct x_aaa bar;
libaaa bar;