• C++ allows you to declare functions without defining them
• Function declarations (prototypes) have the function heading without the function body
• The general syntax for declaring a function is:

  returnType functionName(parameter1, ..., parametern);
  

• Where:
  • returnType: the type of the value returned
  • functionName: the name you make up for the function
  • parameter_x: the input values, if any
• As an example, we can declare a function to calculate the square of a number like this:

  double square(double number);

• By declaring a function, the compiler can resolve a function call made inside main()
• Thus, we can reorganize our programs to place function definitions after main()
• For now the use of function prototypes is optional
• However, there are times in C++ when you need to use function prototypes
• Note that if you use function prototypes, you place the block comments before the prototypes and not the definitions
• You can see this new function organization in the following example

Example Program with Function Prototypes

#include <iostream>
using namespace std;

int square(int number);

void printSquare(int length);

int main() {
    cout << "Enter a number to square: ";
    int side;
    cin >> side;
    cout << "The square of the number is " << square(side) << endl;
    cout << "As you can see for yourself!\n";
    printSquare(side);
}

int square(int number) {
    int result = number * number;
    return result;
}

void printSquare(int length) {
    for (int i = 0; i < length; i++) 
    {
        for (int j = 0; j < length; j++)
        {
            cout << "*";
        }
        cout << endl;
}

Note that the function signatures must match in regards to data types of the parameters and return values!

Programming Style Requirements for Functions

Commenting Functions

• Good programming style dictates that each function have a comment, stating what it does.
• There is no universal standard for comment layout.
• Often in C++, you will see a comment written underneath each function prototype
  
• The following example has commented functions

Example Program with Commented Functions

#include <iostream>
using namespace std;

double square(double number);
//Multiplies a number by itself

void printDate(int month, int day, int year);
//Prints a date in the m/d/y format

int main() {
    double number = 5;
    double result = square(number);
    cout << "The square of 5: " << result << endl;
    cout << "The square of 3: " << square(3) << endl;
    int month = 4;
    int day = 2;
    int year = 1845;
    printDate(month, day, year);
    printDate(3, 26, 2015);
    return 0;
}

double square(double number) {
    double result = number * number;
    return result;
}

void printDate(int month, int day, int year) {
    cout << "The date: " << month << "/" << day << "/" << year << endl;
    return;
}