Functions in C++: Declaration, Types and Function Call

C++ Functions: An Overview

What is a Function in C++?

There are two types of functions in C++:

1. Standard Library Functions: Inbuilt functions in C++
2. User-defined Functions: Created by users

C++ User-defined Functions

There are some aspects of a user-defined function in C++. They are

1. Function Declaration
2. Function Call
3. Function Definition
4. Function Parameters
5. Function Prototype
6. return statement

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Function Declaration in C++

Syntax

returnType functionName (parameter1, parameter2,...) {
 // function body 
}

• returnType signifies the type of value returned by the function. e.g. int, void.
• functionName specifies the name of the function. You must give a function name relevant to the task the function performs. It increases understandability. e.g. area()
• The parentheses () after functionName contains parameters. They are like values given by the programmer to be used by the function to perform its task. The () can be empty i.e. no parameters.
• function body in the {} contains the code to perform the given task.

Example of Function Declaration in C++

// function declaration
void welcomeFunction() {
 cout << "Welcome to ScholarHat";
}

Function Call in C++

Syntax

functionName (parameter1, parameter2,...);

Example of Function Call in C++

int main() {
 
 // function call 
 welcomeFunction(); 

}

• You can even call a function as many times as you want.

  Example of Multiple Function Calls in C++ Online Compiler

  
  #include <iostream>
  using namespace std;
  void welcomeFunction() {
   cout << "Welcome to ScholarHat";
  }
  int main() {
   
   // function call 
   welcomeFunction(); 
   welcomeFunction(); 
   welcomeFunction(); 
   return 0;
  } 

  Run Code >>

  Output

  Welcome to ScholarHat
  Welcome to ScholarHat
  Welcome to ScholarHat
  

Function Definition in C++

Example of Function Definition in C++

// function declaration
void welcomeFunction() {
 cout << "Welcome to ScholarHat"; //function definition
}

Function Parameters in C++

Syntax

returnType functionName (parameter1, parameter2,...) {
 // function body 
}

There can be as many parameters in the () as you want in the function declaration. They must be separated by commas ,. The values of the function parameters are passed during a function call.

• The parameters in the function call are known as arguments or actual parameters.
• The parameters in the function declaration are known as parameters or formal parameters

Example of Function Parameters in C++ Compiler

#include <iostream>
#include <string>
using namespace std;

void welcomeFunction(string value) //formal parameter{
 cout << "Welcome \t to \t" << value << endl;
}

int main() {
 welcomeFunction("ScholarHat"); //actual parameter
 welcomeFunction("ScholarHat"); 
 welcomeFunction("ScholarHat"); 

 return 0;
}
 

In the above C++ code, the welcomeFunction() takes a string parameter. In the main() function, we have called the welcomeFunction() three times with the string parameter value or argumentScholarHat. Therefore the function executes three times.

Output

Welcome to ScholarHat
Welcome to ScholarHat
Welcome to ScholarHat

Remember: The arguments in the function call must always match with the corresponding parameters in the function declaration.

Function Prototype in C++

In C++, if a user-defined function, such as welcomeFunction() is declared after the function call, an error will occur. Therefore, we always have to declare the function in C++ above the function call. But, we can separate the function declaration and function definition in C++. Here comes the role of function prototype in C++.

Function Prototype in C++ is just the function declaration without the function definition above the function call.

Syntax

returnType functionName(dataType1, dataType2, ...);

Example of Function Prototype in C++

#include <iostream>
#include <string>
using namespace std;
void welcomeFunction(string); //function prototype

int main() {
 welcomeFunction("ScholarHat"); //function call above function declaration
 return 0;
}

void welcomeFunction(string value) { //function definition
 cout << "Welcome \t to \t" << value << endl;
}

Here, the function prototype, void welcomeFunction(string); informs the compiler about the function name, welcomeFunction(), and its parameter of type string, value. Therefore, we can call the welcomeFunction() from the main() function before its definition.

Output

Welcome to ScholarHat

return statement in C++

We saw returnType in the function declaration syntax. We read that it signifies the type of the value returned by the function. In the above examples we have given the return type as void which means the function doesn't return any value.

If you want your function to return a value, you must mention the required returnType in the function declaration and use the return statement in the function definition. The return keyword denotes that the function has ended. Any code after return inside the function is not executed.

Example of return statement in C++ Online Editor

#include <iostream>
using namespace std;

int returnFunction(int a, int b) {
 return a + b;
}

int main() {
 cout << returnFunction(20, 40);
 return 0;
}

In the above C++ code, the returnFunction() adds the values of a and b and returns it to the main() function from where it is called. As the returnFunction() returns an integer value the returnType of the function is int.

Output

60

Summary