Parameter Passing Techniques in C/C++

There are different ways in which parameter data can be passed into and out of methods and functions. Let us assume that a function B() is called from another function A(). In this case, A is called the “caller function” and B is called the “called function or callee function”. Also, the arguments which A sends to B are called actual arguments and the parameters of B are called formal arguments.

Terminology

• Formal Parameter: A variable and its type as they appear in the prototype of the function or method.
• Actual Parameter: The variable or expression corresponding to a formal parameter that appears in the function or method call in the calling environment.
• Modes:
  • IN: Passes info from caller to the callee.
  • OUT: Callee writes values in the caller.
  • IN/OUT: The caller tells the callee the value of the variable, which may be updated by the callee.

Important Methods of Parameter Passing are as follows:

1. Pass By Value

This method uses in-mode semantics. Changes made to formal parameters do not get transmitted back to the caller. Any modifications to the formal parameter variable inside the called function or method affect only the separate storage location and will not be reflected in the actual parameter in the calling environment. This method is also called call by value.

Example of Pass by Value

C

// C program to illustrate
// call by value
#include <stdio.h>

void func(int a, int b)
{
    a += b;
    printf("In func, a = %d b = %d\n", a, b);
}
int main(void)
{
    int x = 5, y = 7;

    // Passing parameters
    func(x, y);
    printf("In main, x = %d y = %d\n", x, y);
    return 0;
}

C++

// C++ program to illustrate
// call by value
#include <iostream>
using namespace std;

void func(int a, int b)
{
    a += b;
    cout << "In func, a = " << a << " b = " << b << endl;
}
int main(void)
{
    int x = 5, y = 7;

    // Passing parameters
    func(x, y);
    cout << "In main, x = " << x << " y = " << y;
    return 0;
}

// This code is contributed by shivanisinghss2110

Output

In func, a = 12 b = 7
In main, x = 5 y = 7

Note: Languages like C, C++, and Java support this type of parameter passing. Java in fact is strictly call by value.

Shortcomings of Pass By Value:

• Inefficiency in storage allocation
• For objects and arrays, the copy semantics are costly

2. Pass by reference(aliasing)

This technique uses in/out-mode semantics. Changes made to formal parameter do get transmitted back to the caller through parameter passing. Any changes to the formal parameter are reflected in the actual parameter in the calling environment as formal parameter receives a reference (or pointer) to the actual data. This method is also called as call by reference. This method is efficient in both time and space.

Example

C

// C program to illustrate
// call by reference
#include <stdio.h>

void swapnum(int* i, int* j)
{
    int temp = *i;
    *i = *j;
    *j = temp;
}

int main(void)
{
    int a = 10, b = 20;

    // passing parameters
    swapnum(&a, &b);

    printf("a is %d and b is %d\n", a, b);
    return 0;
}

C++

// C++ program to illustrate
// call by reference
#include <iostream.h>
using namespace std;

void swapnum(int* i, int* j)
{
    int temp = *i;
    *i = *j;
    *j = temp;
}

int main(void)
{
    int a = 10, b = 20;

    // passing parameters
    swapnum(&a, &b);

    cout << "a is " << a << " and b is " << b;
    return 0;
}

// this code is contributed by shivanisinghss2110

Output

a is 20 and b is 10

Note: C and C++ both support call by value as well as call by reference whereas Java doesn’t support call by reference.

Shortcomings of Pass by Reference

• Many potential scenarios can occur
• Programs are difficult to understand sometimes

Other Methods of Parameter Passing

These techniques are older and were used in earlier programming languages like Pascal, Algol, and Fortran. These techniques are not applicable in high-level languages.

1. Pass by Result

This method uses out-mode semantics. Just before control is transferred back to the caller, the value of the formal parameter is transmitted back to the actual parameter. This method is sometimes called call by the result. In general, the pass-by-result technique is implemented by copying.

2. Pass by Value-Result

This method uses in/out-mode semantics. It is a combination of Pass-by-Value and Pass-by-Result. Just before the control is transferred back to the caller, the value of the formal parameter is transmitted back to the actual parameter. This method is sometimes called call by value-result.

3. Pass by Name

This technique is used in programming languages such as Algol. In this technique, the symbolic “name” of a variable is passed, which allows it both to be accessed and updated.

Example

To double the value of C[j], you can pass its name (not its value) into the following procedure.

procedure double(x);
  real x;
begin 
  x:=x*2
end;

In general, the effect of pass-by-name is to textually substitute the argument in a procedure call for the corresponding parameter in the body of the procedure. Implications of Pass-by-Name mechanism:

• The argument expression is re-evaluated each time the formal parameter is passed.
• The procedure can change the values of variables used in the argument expression and hence change the expression’s value.

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