C++ Functions

A function is a group of statements that together perform a task. Every C++ program has at least one function, which is main(), and all the most trivial programs can define additional functions.

You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division usually is such that each function performs a specific task.

A function declaration tells the compiler about a function's name, return type, and parameters. A function definition provides the actual body of the function.

The C++ standard library provides numerous built-in functions that your program can call. For example, function strcat() to concatenate two strings, function memcpy() to copy one memory location to another location and many more functions.

A function is known with various names like a method or a sub-routine or a procedure etc.

A function is a set of statements that take inputs, do some specific computation and produces output.

Defining a Function

The general form of a C++ function definition is as follows −

return_type function_name( parameter list ) {
   body of the function
}

A C++ function definition consists of a function header and a function body. Here are all the parts of a function −

• Return Type − A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
• Function Name − This is the actual name of the function. The function name and the parameter list together constitute the function signature.
• Parameters − A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
• Function Body − The function body contains a collection of statements that define what the function does.
• 
  

Example:
Below is a simple C++ program to demonstrate functions.

#include

using namespace std;

  

int max(int x, int y) 

{ 

    if (x > y) 

    return x; 

    else

    return y; 

} 

int main() {

    int a = 10, b = 20; 

  

    // Calling above function to find max of 'a' and 'b' 

    int m = max(a, b); 

  

    cout << "m is " << m; 

    return 0; 

}




Output:

m is 20

Why do we need functions?

• Functions help us in reducing code redundancy. If functionality is performed at multiple places in software, then rather than writing the same code, again and again, we create a function and call it everywhere. This also helps in maintenance as we have to change at one place if we make future changes to the functionality.
• Functions make code modular. Consider a big file having many lines of codes. It becomes really simple to read and use the code if the code is divided into functions.
• Functions provide abstraction. For example, we can use library functions without worrying about their internal working.

Function Declaration
A function declaration tells the compiler about the number of parameters function takes, data-types of parameters and return type of function. Putting parameter names in function declaration is optional in the function declaration, but it is necessary to put them in the definition. Below are an example of function declarations. (parameter names are not there in below declarations)

A function declaration tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately.

A function declaration has the following parts −

return_type function_name( parameter list );

For the above defined function max(), following is the function declaration −

int max(int num1, int num2);

Parameter names are not important in function declaration only their type is required, so following is also valid declaration −

int max(int, int);

Parameter Passing to functions

The parameters passed to function are called actual parameters. For example, in the above program 10 and 20 are actual parameters.


The parameters received by function are called formal parameters. For example, in the above program x and y are formal parameters.


There are  most popular ways to pass parameters.

Call by ValueThis method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument. In this parameter passing method, values of actual parameters are copied to function’s formal parameters
	Call by PointerThis method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.
	Call by ReferenceThis method copies the reference of an argument into the formal parameter. Inside the function, the reference is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument. Both actual and formal parameters refer to same locations, so any changes made inside the function are actually reflected in actual parameters of caller.

By default, C++ uses call by value to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function and above mentioned example while calling max() function used the same method.

ex--

#include

using namespace std;

  

void fun(int x) {

    x = 30;

}

  

int main() {

    int x = 20;

    fun(x);

    cout << "x = " << x;

    return 0;

}

Output:

x = 20

#include

using namespace std;

  

void fun(int *ptr)

{

    *ptr = 30;

}

  

int main() {

    int x = 20;

    fun(&x);

    cout << "x = " << x;

      

    return 0;

}

Output:

x = 30




Default Values for Parameters

When you define a function, you can specify a default value for each of the last parameters. This value will be used if the corresponding argument is left blank when calling to the function.

This is done by using the assignment operator and assigning values for the arguments in the function definition. If a value for that parameter is not passed when the function is called, the default given value is used, but if a value is specified, this default value is ignored and the passed value is used instead. Consider the following example −

#include 
using namespace std;
 
int sum(int a, int b = 20) {
   int result;
   result = a + b;
  
   return (result);
}
int main () {
   // local variable declaration:
   int a = 100;
   int b = 200;
   int result;
 
   // calling a function to add the values.
   result = sum(a, b);
   cout << "Total value is :" << result << endl;

   // calling a function again as follows.
   result = sum(a);
   cout << "Total value is :" << result << endl;
 
   return 0;
}

 output

Total value is :300
Total value is :120