在C中使用前向声明的typedef stuct

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.)

Answer 2

The forward declaration allows you to use its name in context where an incomplete type is allowed. Declaring a struct member is not one of such cases, the complete definition must be known as it contributes to the struct layout.

Answer 3

for a struct definition you should always use complete types for all members in a structure... but this is not the case with UNITS units in struct Data ,which declares a variable named units of type struct Units which is never declared before the struct Data... this reflects an error.. you should place the Units definition above Data definition.. and all will work fine..

and regarding forward declaration this does not work since whenever a struct variable is defined, the compiler first allocates the memory required to the struct ( struct members donot have a memory allocated to them, unless they are linked to a struct type of variable .. thats why struct variables cant be initialized inside struct template).. :)

Answer 4

There is no prototype for struct. This is because compiler needs to know size of struct before using it. You could use pointer on struct, because pointers have known size no matter which type they point to.