Question

I have these two structures…

typedef struct{
    MY_SECOND_STRUCT  s1;
}MY_FIRST_STRUCT;


typedef struct{
    int s1;
}MY_SECOND_STRUCT;

I prefer this order, I dont want to switch them.
But compiler dont know MY_SECOND_STRUCT at the moment and I get error

error: expected specifier-qualifier-list before ‘MY_SECOND_STRUCT’

I´ve tried add declaration to the top

struct MY_SECOND_STRUCT;

also change definition to

typedef struct{
    struct MY_SECOND_STRUCT  s1;
}MY_FIRST_STRUCT;

but it didnt help.

Answer 1

That order is not possible. You have to switch them.

However, if you declare the member as pointer, then switching is not required:

struct MY_SECOND_STRUCT; //forward declaration is required though

typedef struct{
    MY_SECOND_STRUCT  * s1;  //now its a pointer
}MY_FIRST_STRUCT;

Or, in C++ you can use template as:

template<typename MY_SECOND_STRUCT>
struct MY_FIRST_STRUCT_T
{
    MY_SECOND_STRUCT  s1;
};

struct MY_SECOND_STRUCT
{
    int s1;
};

And when you want to use MY_FIRST_STRUCT, just use this typedef:

typedef MY_FIRST_STRUCT_T<MY_SECOND_STRUCT> MY_FIRST_STRUCT;

Use MY_FIRST_STRUCT now. 🙂