online C++ tutorial: Function Basics

Section 5: Functions

Section 5.2: Function Basics

Now that you know what a function is, let's look at function syntax. We've already seen that a function can take some inputs, do some stuff, and then produce an output.

The basic form of a function definition is this:

output function_name (input_1, input_2, input_3, input_...) {
   // code to execute inside function
}

It's called a function definition because we are defining the function. We are saying, "This is a function named function_name, whose inputs are input_1, input_2, etc., and whose output is output. When it is called, the function will execute the code in between its curly braces ({}).

At this point, let's refine our sample function definition. When programmers talk about functions, instead of the word input they usually use the word parameter. A parameter to a function is nothing more than an input to a function. At the same time, instead of using the word output, programmers generally refer to the return of a function. A particular function "returns" a value. So, here is our updated function definition:

return_type function_name (parameter_1, parameter_2, parameter_3, parameter_...) {
   // code to execute inside function
}

Notice that in place of output, the function definition says return_type. That's because when we are actually writing a function definition, we'll put the return type there, immediately preceding the name of the function. The return type is nothing more than a plain old variable type, such as int, or double, etc.

Similarly, parameters use variable types also. If the first input to a function is an int, then the first parameter will be something like int my_number. We'll see what my_number does in just a moment.

A Real Function!

Enough dilly-dally, let's see a real, working, C++ function that actually does something! Suppose we need a function that, converts a temperature from Celsius to Fahrenheit. Here it is: