sizeof single struct member in C

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I am trying to declare a struct that is dependent upon another struct. I want to use sizeof to be safe/pedantic.

typedef struct _parent
{
  float calc ;
  char text[255] ;
  int used ;
} parent_t ;

Now I want to declare a struct child_t that has the same size as parent_t.text.

How can I do this? (Pseudo-code below.)

typedef struct _child
{
  char flag ;
  char text[sizeof(parent_t.text)] ;
  int used ;
} child_t ;

I tried a few different ways with parent_t and struct _parent, but my compiler will not accept.

As a trick, this seems to work:

parent_t* dummy ;
typedef struct _child
{
  char flag ;
  char text[sizeof(dummy->text)] ;
  int used ;
} child_t ;

Is it possible to declare child_t without the use of dummy?

sizeof struct c
9 Answers

@joey-adams, thank you! I was searching the same thing, but for non char array and it works perfectly fine even this way:

#define member_dim(type, member) sizeof(((type*)0)->member) / \
                                 sizeof(((type*)0)->member[0])

struct parent {
        int array[20];
};

struct child {
        int array[member_dim(struct parent, array)];
};

int main ( void ) {
        return member_dim(struct child, array);
}

It returns 20 as expected.

And, @brandon-horsley, this works good too:

#define member_dim(type, member) sizeof(((type){0}).member) / \
                                 sizeof(((type){0}).member[0])
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