Thread: struct definition

Originally Posted by kiros88

I want both the structs to have pointers of each other but Im worried the order will affect the compiling since in struct 1 the declartion of struct 2 wasnt define wont it just say it doesnt see it.

You can find out if your worry was justified by writing a program to test it.

A problem that may arise is that there is actually no such thing as a "struct Struct2"; a Struct2 is user defined (typedef'd) datatype. But you can do this:

Code:

typedef struct _struct1
{
   char   bug[10];
   struct _struct2 *ptr;    // cannot be Struct2 *ptr
}Struct1;

typedef struct _struct2
{
  int size;
  Struct1 *ptr;      // could also be struct _struct1 *ptr
} Struct2;

You can know declare:

Code:

int main () {
    Struct1 alpha;
    Struct2 beta;
    alpha.ptr = β
    beta.ptr = α

Originally Posted by MK27

A problem that may arise is that there is actually no such thing as a "struct Struct2"; a Struct2 is user defined (typedef'd) datatype. But you can do this:

hmm... I was hoping that with an appropriate program a C compiler would warn about that, but apparently the MinGW port of gcc 4.4.0 does not. Consequently, I would point out an alternative:

Code:

typedef struct Struct2 Struct2;

typedef struct Struct1
{
    char bug[10];
    Struct2 *ptr;
} Struct1;

struct Struct2
{
    int size;
    Struct1 *ptr;
};

Originally Posted by laserlight

hmm... I was hoping that with an appropriate program a C compiler would warn about that, but apparently the MinGW port of gcc 4.4.0 does not.

gcc -Wall -pedantic doesn't either. You can include this even:

Code:

typedef struct {
     struct ThisDoesNotExist *ptr;
} mytype;

As long as it is a struct it's all good...at this point.

Originally Posted by MK27

gcc -Wall -pedantic doesn't either.

Of course that is what I tested with.

Originally Posted by MK27

As long as it is a struct it's all good...at this point.

I think you mean pointer, since a pointer can point to an incomplete type.

Originally Posted by laserlight

I think you mean pointer, since a pointer can point to an incomplete type.

Sure but this will not fly the same way:

Code:

typedef struct {
     ThisDoesNotExist *ptr;
} mytype;

I guess for a sort of unrelated reason, but the effect is still slightly misleading. No big deal.

So that suggests that a definition of a pointer to a struct type serves as a declaration for that struct. Interesting. Laserlight's code does a similar thing with a typedef using a yet undeclared struct.

Is it valid C though, or just an extension?