Defining functions

Learn how to define and call a function.

What is a function?

A function is a named, reusable block of code. You define it once and call it as many times as you like. The definition void greet() { std::cout << "hi"; } creates a function named greet.

Every function has a signature: return type, name and parameter list. The signature of void greet() reads 'greet, no parameters, returns nothing'. To call it you write its name followed by parentheses: greet();. This runs the code inside the function.

Definition vs call

The definition and the call of a function are different things. The definition says what the function does: void greet() { std::cout << "hi"; }. The call runs it: greet();.

In C++ the call can sit below or above the definition; as long as the compiler sees the definition, order does not matter. For readability it is common to place definitions before main. You can call a function many times; each call runs the same code again.

Exercise Define greet and call it

Define a void greet() function whose body is std::cout << "hi" << std::endl;. Call it from main.

Show example solution

#include <iostream>

void greet() {
    std::cout << "hi" << std::endl;
}

int main() {
    greet();
    return 0;
}

Test yourself

What do you write to call a function?
```
void greet() { std::cout << "hi"; }
```
- greet;
- greet();
- call greet();
- greet->();
Show answer

greet(); — greet(); runs the function.
Complete the function call
```
greet____;
```
Show answer
```
greet();
```
Match the function parts

void

Return type
greet

Function name
()

Parameter list
greet();

Call

Next lesson: Return values

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