C++ Functions

In programming, function refers to a segment that groups code to perform a specific task. Depending on whether a function is predefined or created by programmer; there are two types of function:

Library Function User-defined Function

Library Function

Library functions are the built-in function in C++ programming. Programmer can use library function by invoking function directly; they don't need to write it themselves.

Example 1: Library Function

# include <iostream>
#include <cmath>

using namespace std;

int main() {
    double number, squareRoot;
    cout<<"Enter a number: ";
    cin>>number;

/* sqrt() is a library function to calculate square root */
    squareRoot = sqrt(number);
    cout<<"Square root of "<<number<<" = "<<squareRoot;
    return 0;
}

Output

Enter a number: 26
Square root of 26 = 5.09902

In example above, sqrt() library function is invoked to calculate the square root of a number. Notice code #include <cmath> in the above program. Here, cmath is a header file. The function definition of sqrt()(body of that function) is written in that file. You can use all functions defined in cmath when you include the content of file cmath in this program using #include <cmath> .

Every valid C++ program has at least one function, that is, main() function.

User-defined Function

C++ allows programmer to define their own function. A user-defined function groups code to perform a specific task and that group of code is given a name(identifier). When that function is invoked from any part of program, it all executes the codes defined in the body of function.

How user-defined function works in C Programming?

Consider the figure above. When a program begins running, the system calls the main() function, that is, the system starts executing codes from main() function. When control of program reaches to function_name() inside main(), the control of program moves to void function_name(), all codes inside void function_name() is executed and control of program moves to code right after function_name() inside main() as shown in figure above.

Example 1: Function

C++ program to add two integers. Make a function add() to add integers and display sum in main() function.

# include <iostream>
using namespace std;

int add(int, int);            //Function prototype(declaration)

int main() {
    int num1, num2, sum;
    cout<<"Enters two numbers to add: ";
    cin>>num1>>num2;
    sum = add(num1,num2);         //Function call
    cout<<"Sum = "<<sum;
    return 0;
}

int add(int a,int b) {            //Function declarator
    int add;
    add = a+b;
    return add;                 //Return statement
}

Output

Enters two integers: 8
-4
Sum = 4

Function prototype(declaration)

If an user-defined function is defined after main() function, compiler will show error. It is because compiler is unaware of user-defined function, types of argument passed to function and return type.

In C++, function prototype is a declaration of function without function body to give compiler information about user-defined function. Function prototype in above example:

int add(int, int);

You can see that, there is no body of function in prototype. Also there are only return type of arguments but no arguments. You can also declare function prototype as below but it's not necessary to write arguments.

int add(int a, int b);

Note: It is not necessary to define prototype if user-defined function exists before main() function.

Function Call

To execute the codes of function body, the user-defined function needs to be invoked(called). In the above program, add(num1,num2); inside main() function calls the user-defined function. In the above program, user-defined function returns an integer which is stored in variable add.

Function Definition

The function itself is referred as function definition. Function definition in the above program:

/* Function definition  */

int add(int a,int b) {      // Function declarator
    int add;
    add = a+b;
    return add;             // Return statement
}

When the function is called, control is transferred to first statement of function body. Then, other statements in function body are executed sequentially. When all codes inside function definition is executed, control of program moves to the calling program.

Passing Arguments to Function

In programming, argument(parameter) refers to data this is passed to function(function definition) while calling function.

In above example, two variables, num1 and num2 are passed to function during function call. These arguments are known as actual arguments. The value of num1 and num2 are initialized to variables a and b respectively. These arguments a and b are called formal arguments. This is demonstrated in figure below:

Notes on passing arguments

• The numbers of actual arguments and formals argument should be same. (Exception: Function Overloading)
• The type of first actual argument should match the type of first formal argument. Similarly, type of second actual argument should match the type of second formal argument and so on.
• You may call function without passing any argument. The number(s) of argument passed to a function depends on how programmer want to solve the problem.
• In above program, both arguments are of int type. But it's not necessary to have both arguments of same type.

Return Statement

A function can return single value to the calling program using return statement. In the above program, the value of add is returned from user-defined function to the calling program using statement below:

return add;

The figure below demonstrates the working of return statement.

In the above program, the value of add inside user-defined function is returned to the calling function. The value is then stored to a variable sum. Notice that, the variable returned, that is, add is of type int and also sum is of int type. Also notice that, the return type of a function is defined in function declarator int add(int a,int b). The int before add(int a,int b) means that function should return a value of type int. If no value is returned to the calling function then, void should be used.