sizeof single struct member in C

Question

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
{
  floa         


        

Answer 1

Use a preprocessor directive, i.e. #define:

#define TEXT_LEN 255

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

typedef struct _child
{
  char flag ;
  char text[TEXT_LEN] ;
  int used ;
} child_t ;

Answer 2

c++ solution:

sizeof(Type::member) seems to be working as well:

struct Parent
{
    float calc;
    char text[255];
    int used;
};

struct Child
{
    char flag;
    char text[sizeof(Parent::text)];
    int used;
};

Answer 3

@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])