指针或指针到指针

Question

Before i proceed i wanna tell you that i'm a beginner in c language (6 months) and when I have to use pointers in my programs i'm not 100% sure about them, so please be gentle with me.

Today i was coding something and at one point i realized that i need the use of swap.

Option 1: This is what i used:

#include <stdio.h>

void swap(int *pa, int *pb){
    int temp;
    temp = *pa;
    *pa = *pb;
    *pb = temp;
}


int main(void){
    int a = 5;
    int b = 10;

    printf("Before Swap:\n");
    printf("A = %d\nB = %d\n\n",a,b);

    swap(&a,&b);

    printf("After Swap:\n");
    printf("A = %d\nB = %d\n",a,b);

    return 0;
}

And it works fine for what i need:

 Before Swap: A = 5 B = 10 

After Swap:
A = 10
B = 5

Latter i decided that in the Program has to be involved pointers and i changed the Program like this:

Option 2:

#include <stdio.h>

void swap(int *pa, int *pb){
    int temp;
    temp = *pa;
    *pa = *pb;
    *pb = temp;
}


int main(void){
    int a = 5;
    int b = 10;

    int *pa = &a;
    int *pb = &b;

    printf("Before Swap:\n");
    printf("PA = %d\nPB = %d\n\n",*pa,*pb);

    swap(pa,pb);

    printf("After Swap:\n");
    printf("PA = %d\nPB = %d\n",*pa,*pb);

    return 0;
}

And again its works fine for what i was needed:

Before Swap:
PA = 5
PB = 10

After Swap:
PA = 10
PB = 5

Now come the interesting part. I was proceeding with the coding and i changed the swap Function by using Pointer-to-Pointer as arguments and i got this:

Option 3:

#include <stdio.h>

void swap(int **ppa, int **ppb){
    int temp;
    temp = **ppa;
    **ppa = **ppb;
    **ppb = temp;
}


int main(void){
    int a = 5;
    int b = 10;

    int *pa = &a;
    int *pb = &b;

    printf("Before Swap:\n");
    printf("PA = %d\nPB = %d\n\n",*pa,*pb);

    swap(&pa,&pb);

    printf("After Swap:\n");
    printf("PA = %d\nPB = %d\n",*pa,*pb);

    return 0;
}

And everything was ok:

 Before Swap: PA = 5 PB = 10 

After Swap:
PA = 10
PB = 5

So here i had to use & operator when i was calling the swap function in main. My question is (because i'm confused in here) are Option 2 and Option 3 both Ok to be used or just one of them should be used and why. I am not asking for a better coding, I just do not know if option 2 and 3, both are legal or not and why both are working fine.

LE: @Olaf said that:

And if you add a third or forth star in swap, it also will be fine. Just think about it

Now i am more confused because its not working as he said:

#include <stdio.h>

void swap(int ****pa, int ****pb){
    int temp;
    temp = ****pa;
    ****pa = ****pb;
    ****pb = temp;
}


int main(void){
    int a = 5;
    int b = 10;

    int *pa = &a;
    int *pb = &b;

    printf("Before Swap:\n");
    printf("PA = %d\nPB = %d\n\n",*pa,*pb);

    swap(&pa,&pb);

    printf("After Swap:\n");
    printf("PA = %d\nPB = %d\n",*pa,*pb);

    return 0;
}

Output:

 program.c: In function 'main': program.c:21:14: error: passing argument 1 of 'swap' from incompatible pointer type [-Werror] swap(&pa,&pb); ^ program.c:3:10: note: expected 'int ****' but argument is of type 'int **' void swap(int ****pa, int ****pb){ ^ program.c:21:18: error: passing argument 2 of 'swap' from incompatible pointer type [-Werror] swap(&pa,&pb); ^ program.c:3:10: note: expected 'int ****' but argument is of type 'int **' void swap(int ****pa, int ****pb){ ^ cc1: all warnings being treated as errors 

Answer 1

Nothing is wrong while it is legal and working. The only consideration is how much it is useful and efficient. Passing a pointer to pointer the generated code will include a triple addressing, the first addressing will retrieve the address of the pointer to the variable, the second will retrieve the address of the variable from the pointer and last it will get the variable. In your case you are obliged to pass pointers to make the function swap able to modify the two arguments outside the function scope.

But passing a pointer of a pointer can have a sense if you consider this code:

#include <stdio.h>

void swap(int **ppa, int **ppb){
    int *pTemp = *ppb;
    *ppb = *ppa;
    *ppa = pTemp;
}


int main(void){
    int a = 5;
    int b = 10;

    int *pa = &a;
    int *pb = &b;

    printf("Before Swap:\n");
    printf("PA = %d\nPB = %d\n\n",*pa,*pb);

    swap(&pa,&pb);

    printf("After Swap:\n");
    printf("PA = %d\nPB = %d\n",*pa,*pb);

    return 0;
}

In this case you are swapping pointers, not variables. This is very useful when you need to swap large data, like strings, because is much faster than copying back the whole data between objects.

Answer 2

you should well understand difference between pointers and refrences to got the right results

also you should know well difference between calling by value and by pointer and by refrence in functions :

here some examples should clarify the concept to you :

 int myAge=39;

cout<< "myAge is locate at " << &myAge << endl; // address of myAge in memory

int* agePtr=&myAge;

cout<<"Address of pointer "<< agePtr << endl;

cout<< " Data at memory address "<< *agePtr << endl;

int badNums[5]={4, 13, 14, 24, 34};

int* numArrayPtr = badNums;

cout<< "Address " << numArrayPtr << " Value " << *numArrayPtr << endl;

numArrayPtr++; // get next value in Array

cout<< "Address " << numArrayPtr << " Value " << *numArrayPtr << endl;

cout<< "Address " << badNums << " Value " << *badNums << endl;

here calling function paramaters by pointer and by refrence :

void makeMeYoung(int* age) {

 cout<< " I used to be " << *age << endl;

    *age = 21;
  } 

void actYourAge(int& age){
     age = 39;                                       
  }

makeMeYoung(&myAge);

cout << "I'm "<< myAge << " years old now" << endl; // 21

int& ageRef= myAge;

cout<<"myAge : " << myAge << endl; // 21

 ageRef++;

 cout<<"myAge : " << myAge << endl; // 22

actYourAge(ageRef);

cout<<"myAge : " << myAge << endl; // 39

note that you should use pointers if you dont want to initialize at decleration of variables , also if you need to assign another variable to the pointer so with pointer you will be able to deal with more than one variable , while refrence when you need to use just one refrence to a variable and stuck with it , the idea is when you want to point to only one variable use refrence.