5.4. Parameter Passing by Value

5.4. Parameter Passing by Value
	Chapter 5. Functions

[ fromfile: functions.xml id: valueparm ]

By default, C++ parameters are passed by value.
When a function is called, a temporary (local) copy of each argument object is made and placed on the program stack.
Only the local copy is manipulated inside the function, and the argument objects from the calling block are not affected by these manipulations.
The temporary stack variables are all destroyed when the function returns.
Example 5.11 gives a short demonstration.

Example 5.11. src/functions/summit.cpp

#include <iostream>

int sumit(int num) {
    int sum = 0;
    for (; num ; --num)  
        sum += num;
    return sum;
}


int main() {
    using namespace std;
    int n = 10;
    cout << n  << endl;
    cout << sumit(n) << endl;
    cout << n << endl;  
    return 0;
}
Output:

  10
  55
  10

	The parameter gets reduced to 0.
	See what sumit() did to n.

If a pointer is passed to a function, a temporary copy of that pointer is placed on the stack.
Changes to that pointer will have no effect on the pointer in the calling block.

Example 5.12. src/functions/pointerparam.cpp

#include <iostream>
using namespace std;

void messAround(int* ptr) {
    *ptr = 34;       
    ptr = 0;         
}


int main() {
    int n(12);       
    int* pn(&n);     
    cout << "n = " << n << "\tpn = " << pn << endl;
    messAround(pn);  
    cout << "n = " << n << "\tpn = " << pn << endl;
    return 0;
}
Output:

n = 12  pn = 0xbffff524
n = 34  pn = 0xbffff524

	Change the value that is pointed to.
	Change the address stored by ptr. Better not dereference this!
	Initialize an int.
	Initialize a pointer that points to n.
	See what is changed by messAround().

	Note
To summarize: When an object is passed by value to a function, a copy is made of that object. The copy is treated as a local variable by the function. The copy is destroyed when the function returns.

Note

To summarize:

When an object is passed by value to a function, a copy is made of that object.
The copy is treated as a local variable by the function.
The copy is destroyed when the function returns.


5.3.1. Exercises: Operator Overloading		5.5. Parameter Passing by Reference