order of two structures

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.

Other answers here are great. You may be wondering why you can forward declare the struct and then declare a pointer to it -- the compiler knows the size of the pointer, so it can determine the size of a struct which contains it, but it does not know the size of the second struct, so it can't define a struct which contains that. — Heath Hunnicutt, Aug 08 '11 at 16:19

score 8 · Answer 1 · answered Aug 08 '11 at 16:07

I prefer this order, I dont want to switch them.

You have to switch them.

If MY_FIRST_STRUCT has a member variable of type MY_SECOND_STRUCT then MY_SECOND_STRUCT must be defined and complete (not just declared and incomplete) before the definition of MY_FIRST_STRUCT.

Nawaz · Accepted Answer · 2011-08-08T16:21:00.223

3

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. :-)

edited Aug 08 '11 at 16:21

answered Aug 08 '11 at 16:13

Nawaz

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if I declare the member as pointer and add forward declaration as you wrote I still get an error. I have to write **struct MY_SECOND_STRUCT * s1;** than its ok. (my compiler gcc) – Meloun Aug 10 '11 at 13:45
@Meloun: That is what I've written. Or are you programming in C? What is the file extension? `.c` or `.cpp`? – Nawaz Aug 10 '11 at 13:50

score 1 · Answer 3 · answered Aug 08 '11 at 16:10

At the time you cerate your first struct the compiler hasn't come across the second struct yet. It needs to know how bit MY_SECOND_STRUCT is because at that time it needs to decide how big to make MY_FISRT_STRUCT and how to lay out the memory for it. Simply forward declaring struct MY_FIRST_STRUCT is not enough, that doesn't tell the compiler about the size or contents of the struct. Forward declaring would work if you were using a pointer to a struct, but it's not enough when trying to include the actual structure.

Your only real options are to move MY_SECOND_STRUCT up above MY_FIRST_STRUCT or to make MY_FIRST_STRUCT takes a pointer. It might seem strange at first, but there's not much that can be done about it.

order of two structures

3 Answers3