C11 anonymous structs via typedefs?

Question

Anonymous structs have been added in the C11 standard, so

typedef struct { 
    struct {int a, b};
    int c; 
} abc_struct;

is valid and stand

Answer 1

Actually your second snippet is fraught with peril and is not equivalent to the first one without explicitly specifying -fplan9-extensions in gcc.

In particular ab_struct; declaration on line 6 does NOTHING (as per gcc warning). Just pasting your second snippet in foo.c generates:

foo.c:6: warning: declaration does not declare anything

And in particular if you were to try:

typedef struct { 
    int a, b;
} ab_struct;

typedef struct { 
    ab_struct;
    int c; 
} abc_struct;


int main() {
    abc_struct abc;
    abc.a = 5;
    return 0;
}

you would get a syntax error on line 13 abc.a = 5; without the -fplan9-extensio.

whereas using the top snippet your anonymous structure will work as you are thinking it should. Namely:

typedef struct { 
    struct { 
            int a, b;
    };
        int c; 
} abc_struct;


int main() {
    abc_struct abc;
    abc.a = 5;
    return 0;
}