Question

I am trying to write a swap function using pointer(specially a void pointer)by-reference, but my code is not working. Here is my code:

void swap(void *p1,void *p2) 
{
    int temp;   
    temp=*((int*)p2);
    p2=p1; 
    p1=&temp;
}

int main() 
{
    int i=4;
    int j=5; 
    cout<<i<<j<<endl;
    swap(&i,&j); 
    cout<<i<<j<<endl;
    return 0;
}

Where am I going wrong?

Answer 1

The code does not work because you are not dereferencing your pointers on assignments. It should be

*((int*)p2)=*((int*)p1);
*((int*)p1)=temp;

Note that you are making an assumption that void* points to an int, which is obviously not always the case. Essentially, you might as well replace void* with int*, and get rid of the casts.

A more general case of the API should look like this:

void swap(void *p1,void *p2, size_t sz)

Internally, the API should allocate a buffer of size sz, make a memcpy into it, and then make a swap, again using memcpy.