A function is a block of code which only runs when it is called.
You can pass data, known as parameters, into a function.
Functions are used to perform certain actions, and they are important for reusing code: Define the code once, and use it many times.
Create a Function
C++ provides some pre-defined functions, such as main() , which is used to execute code. But you can also create your own functions to perform certain actions.
To create (often referred to as declare ) a function, specify the name of the function, followed by parentheses () :
Syntax
void
myFunction
() {
// code to be executed
}- myFunction() is the name of the function
- void means that the function does not have a return value. You will learn more about return values later in the next chapter
- inside the function (the body), add code that defines what the function should do
Call a Function
Declared functions are not executed immediately. They are "saved for later use", and will be executed later, when they are called.
To call a function, write the function's name followed by two parentheses () and a semicolon ;
In the following example, myFunction() is used to print a text (the action), when it is called:
mainA function can be called multiple times:
Example
void myFunction() {
cout << "I just got executed!\n";
}
int main() {
myFunction();
myFunction();
myFunction();
return 0;
}
// I just got executed! // I just got executed! // I just got executed!Function Declaration and Definition
A C++ function consist of two parts:
- Declaration: the return type, the name of the function, and parameters (if any)
- Definition: the body of the function (code to be executed)
void
myFunction() { // declaration
// the body of the function ( definition )
}Note
If a user-defined function, such as myFunction() is declared after the main() function, an error will occur :
Example
int main() {
myFunction();
return 0;
}
void myFunction() {
cout << "I just got executed!";
}
// ErrorHowever, it is possible to separate the declaration and the definition of the function - for code optimization.
You will often see C++ programs that have function declaration above main() , and function definition below main() . This will make the code better organized and easier to read:
Example
// Function declaration void myFunction(); // The main method int main() { myFunction(); // call the function
return 0;
}
// Function definition void myFunction() { cout << "I just got executed!";
}