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Return Values

A function that returns a value must have a return statement. The data type of the return value also must match the method’s declared return type.

On the other hand, a void function (one that does not return anything) does not require a return statement.

#include <iostream> int sum(int a, int b); int main() { int r = sum(10, 20); std::cout << r; } int sum(int a, int b) { return(a + b); }


Function parameters are placeholders for values passed to the function. They act as variables inside a function.

Here, x is a parameter that holds a value of 10 when it’s called.

#include <iostream> void print(int); int main() { print(10); } void print(int x) { std::cout << x; }


A function is a set of statements that are executed together when the function is called. Every function has a name, which is used to call the respective function.

#include <iostream> // Declaring a function void print(); int main() { print(); } // Defining a function void print() { std::cout << "Hello World!"; }

Built-in Functions

C++ has many built-in functions. In order to use them, we have to import the required library using #include.

#include <iostream> #include <cmath> int main() { // sqrt() is from cmath std::cout << sqrt(10); }

Calling a Function

In C++, when we define a function, it is not executed automatically. To execute it, we need to “call” the function by specifying its name followed by a pair of parentheses ().

// calling a function print();

void Functions

In C++, if we declare the type of a function as void, it does not return a value. These functions are useful for a set of statements that do not require returning a value.

#include <iostream> void print() { std::cout << "Hello World!"; } int main() { print(); }

Function Declaration & Definition

A C++ function has two parts:

  • Function declaration
  • Function definition

The declaration includes the function’s name, return type, and any parameters.

The definition is the actual body of the function which executes when a function is called. The body of a function is typically enclosed in curly braces.

#include <iostream> // function declaration void blah(); // main function int main() { blah(); } // function definition void blah() { std::cout << "Blah blah"; }

Function Arguments

In C++, the values passed to a function are known as arguments. They represent the actual input values.

#include <iostream> void print(int); int main() { print(10); // the argument 10 is received as input value } // parameter a is defined for the function print void print(int a) { std::cout << a; }

Scope of Code

The scope is the region of code that can access or view a given element:

  • Variables defined in global scope are accessible throughout the program.
  • Variables defined in a function have local scope and are only accessible inside the function.
#include <iostream> void print(); int i = 10; // global variable int main() { std::cout << i << "\n"; } void print() { int j = 0; // local variable i = 20; std::cout << i << "\n"; std::cout << j << "\n"; }

Function Declarations in Header file

C++ functions typically have two parts: declaration and definition.

Function declarations are generally stored in a header file (.hpp or .h) and function definitions (body of the function that defines how it is implemented) are written in the .cpp file.

// ~~~~~~ main.cpp ~~~~~~ #include <iostream> #include "functions.hpp" int main() { std::cout << say_hi("Sabaa"); } // ~~~~~~ functions.hpp ~~~~~~ // function declaration std::string say_hi(std::string name); // ~~~~~~ functions.cpp ~~~~~~ #include <string> #include "functions.hpp" // function defintion std::string say_hi(std::string name) { return "Hey there, " + name + "!\n"; }

Function Template

A function template is a C++ tool that allows programmers to add data types as parameters, enabling a function to behave the same with different types of parameters. The use of function templates and template parameters is a great C++ resource to produce cleaner code, as it prevents function duplication.

Default Arguments

In C++, default arguments can be added to function declarations so that it is possible to call the function without including those arguments. If those arguments are included the default value is overwritten. Function parameters are read from left to right, so default parameters should be placed from right to left.

Functions Definitions

In C++, it is common to store function definitions in a separate .cpp file from the main() function. This separation results in a more efficient implementation.

Note: If the file containing the main() function needs to be recompiled, it is not necessary to recompile the files containing the function definitions.

Function Overloading

In C++, function overloading enables functions to handle different types of input and return different types. It allows multiple definitions for the same function name, but all of these definitions must differ in their arguments.

Inline Functions

An inline function is a function definition, usually in a header file, qualified by the inline keyword, which advises the compiler to insert the function’s body where the function call is. If a modification is made in an inline function, it would require all files containing a call to that function to be recompiled.