Thread: How can any data be assigned to a const member of a structure?

I review my study book yesterday. While I was examing a program in the book as you can see below, I noticed something strange. We normally know that we cannot assign data to a variable defined as const type. Namely;

Code:

int *const y;

It represents that Constant Pointer - Non Constant Data.

Code:

const int *const z;

It represents that Consant Pointer - Constant Data.

Code:

const int *x;

It represents that Non Constant Pointer - Constant Data.

Code:

int *t;

It represents that Non Constant Pointer - Non Constant Data.



-Constant pointer means that we cannot modify pointer variable.(value inside pointer variable)

-Constant Data means that we cannot modify data inside a variable which is pointed by a pointer variable.



So how can we assign a data to a const member of a structure in the following code,despite of being constant data?

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h> 

struct card{
    const char *face; // CONST MEMBER!!!!!!!!!!!!!!!CONST DATA!!!!!!!!!!!!!!!
    const char *suit; // CONST MEMBER!!!!!!!!!!!!!!!CONST DATA!!!!!!!!!!!!!!!

};
typedef struct card Card;
void fillDeck(Card *const wDeck,const char *wFace[],const char *wSuit[]);
void shuffle(Card *const wDeck);
void deal(const Card *const wDeck);
int main()
{
    Card deck[52];
    const char *face[]={"Ace","Deuce","Three","Four","Five","Six","Seven","Eight","Nine","Ten","Jack","Queen","King"};
    const char *suit[]={"Hearts","Diamonds","Clubs","Spades"};
    srand(time(NULL));
    fillDeck(deck,face,suit);
    shuffle(deck);
    deal(deck);
    return 0;
}
void fillDeck(Card *const wDeck,const char *wFace[],const char *wSuit[])
{
    int i;
    for(i=0;i<=51;i++){
        wDeck[i].face=wFace[i%13];    // ASSIGN DATA TO A CONST MEMBER OF A STRUCTURE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

        wDeck[i].suit=wSuit[i/13];       // ASSIGN DATA TO A CONST MEMBER OF A STRUCTURE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


    }
}
void shuffle(Card *const wDeck)
{
    int i;
    int j;
    Card temp;
    for(i=0;i<=51;i++){
        j = rand()%52;
        temp = wDeck[j];
        wDeck[j] = wDeck[i];
        wDeck[i] = temp;
    }
}
void deal(const Card *const wDeck)
{
    int i;
    for(i=0;i<=51;i++){
        printf("%5s of %-8s%s",wDeck[i].face,wDeck[i].suit,(i+1)%4 ? " " : "\n");
    }
}

OUTPUT:

Originally Posted by hefese

So how can we assign a data to a const member of a structure in the following code,despite of being constant data?

The members of the structure are non-const pointers to const char, hence you can assign to them.

If a structure member is truly constant, you might not be able to assign it with "=", but you can at initialization-time.

Code:

typedef struct {
    const int test1;
    const int test2;
} Test;

// This doesn't work.
Test t;
t.test1 = 5;  // error: assigning to const value

// Use structure initialization (init members in the order they appear):
Test t = {42, 127};

// Named-element structure initialization. NOTE: this is C99, part of the newer C standard. For gcc, use --std=c99 to get this to work.
Test t = {
    .test1 = 42,
    .test2 = 66
};