Call by address in C/C++, how does it work?

[shivajikobardan]

Homework Statement

How does changes in user defined function gets reflected in main function in call by address?

Relevant Equations

none

I've asked my question in the figure below as picture speaks 1000 words.

Code:

#include<iostream>
using namespace std;
void change(int*,int*);
int main()
{
    int x,y;
    cout<<"Enter values  of x and y"<<endl;
    cin>>x>>y;
    change(&x,&y);
    cout<<"In main()"<<endl;
    cout<<"Values x="<<x<<" "<<"y="<<y<<endl;
    cout<<"Address x="<<&x<<" "<<"Address y="<<&y<<endl;
    return 0;
}
void change(int *a,int *b)
{
    int k=*a;
    *a=*b;
    *b=k;
    cout<<"In change()"<<endl;
    cout<<"Values x="<<*a<<" "<<"y="<<*b<<endl;
    cout<<"Address x="<<a<<" "<<"Address y="<<b<<endl;
    cout<<"Address x="<<&a<<" "<<"Address y="<<&b<<endl;
}

Output:

Enter values of x and y

20

30

In change()

Values x=30 y=20

Address x=0x79709ffdbc Address y=0x79709ffdb8

Address x=0x79709ffd90 Address y=0x79709ffd98

In main()

Values x=30 y=20

Address x=0x79709ffdbc Address y=0x79709ffdb8

 

[shivajikobardan]

the common reason i'm reading is "address gets passed so". but I'm failing to realize it mathematically like above.

 

[shivajikobardan]

I've got it.

int main( void ) {
    int x, y; // two variables, allocated on the stack, say at locations 0x4 and 0x8, values undefined
    change( &x, &y ); // receives 0x4 and 0x8
    return x + y; // accesses memory at 0x4 and 0x8, returning the sum of the values stored there
}

void change( int * a, int * b ) {
    *a = 3; // memory at location 0x4 accessed, value set to 3
    *b = 5; // memory at location 0x8 accessed, value set to 5
}

 

[Mark44]

I've got it.

Yes, it seems like you do.

int main( void ) {
    int x, y; // two variables, allocated on the stack, say at locations 0x4 and 0x8, values undefined
    change( &x, &y ); // receives 0x4 and 0x8
    return x + y; // accesses memory at 0x4 and 0x8, returning the sum of the values stored there
}

void change( int * a, int * b ) {
    *a = 3; // memory at location 0x4 accessed, value set to 3
    *b = 5; // memory at location 0x8 accessed, value set to 5
}

Your understanding seems fine to me. The only comment I would make is that the compiler would never allocate stack variables at the locations you wrote. The actual locations would be very different. Although it's not crucial to know these addresses, you can see them if you use the debugger.