typedef stuct with forward declaration in C

Question

I have something like:

typedef struct Data DATA, *DATA_PTR;
typedef struct Units UNITS, *UNITS_PTR;

struct Data
{
    double miscData;
    UNITS units;
};

struct Units
{
    double x[2];
    double y[2];
    double z[2];
};

in my project_typedef.h file.

In another file, I have something like:

void fileInput(DATA_PTR data)
{
     //usual declarations and other things
     data->miscData = 0; //Works!
     data->units.x[0] = 5; //Doesn't work
     //etc...
}

However, this doesn't work since units is declared after data in project_typedef.h (if I switch the order it works). The error that i get is "left of '.x' must have struct/union type". I thought that the forward declaration would fix this issue. Why not?

Answer 1

When you define Data, all members must be complete types. Since UNITS isn't a complete type at that point, this doesn't work. (By contrast, UNITS_PTR would be fine, since pointers to incomplete types are complete types.)

Simply put the Units definition above the Data definition and you should be fine.

(As @cnicutar already noted, you're also using the array x wrong.)