Functions in C++

Introduction to Functions

A function in C++ contains a set of instructions that are executed when it is called.

A function declaration is composed of three parts:

Function return type
Function name
Function parameters

A function can be called by specifying its name followed by a pair of parentheses ().

#include <iostream>

void printTitle() {
  std::string msg = "Codecademy\n";
  std::cout << msg;
}

int main(){
  printTitle();

  return 0;
}

Function Parameters

When calling a function with multiple parameters, the number and order of the arguments must match with the parameters.

Default parameters initialize to a default value if an argument is not provided in the function call.

Pass by reference lets the function modify the arguments variables. Use the & operator to indicate that a parameter is passed by reference.

#include <iostream>

double totalPrice(int items, double price = 9.99) {
  return items * price;
}

// Pass by reference
void addOne(int &i) {
  i += 1;
}

int main() { 
  std::cout << totalPrice(10) << "\n";	// Output: 99.9
  
  int num = 2;
  addOne(num);
  std::cout << num;	// Output: 3

  return 0;
}

Function Overloading

With function overloading, C++ functions can have the same name but handle different input parameters.

At least one of the following criteria must be true in order for functions to be properly overloaded:

Each function has different types of parameters.
Each function has a different number of parameters.

The function return type is NOT used to differentiate overloaded functions.

#include <iostream>

int add(int a, int b) {
  return a + b;
}

double add(double a, double b) {
  return a + b;
}

int add(int a, int b, int c) {
  return a + b + c;
}

int main() {
  std::cout << add(3, 2); // Calls add(int, int)
  std::cout << "\n";
  std::cout << add(5.3, 1.4); // Calls add(double, double)
  std::cout << "\n";
  std::cout << add(2, 6, 9);  // Calls add(int, int, int)
}

Command Line Arguments

Command line arguments are optional arguments passed to the main() function of a C++ program.

Passing command line arguments is as easy as appending the arguments after the executable name. For example:

./greeting Hello World

In order to access command line arguments, the new form of main() takes two arguments:

argc: the number of command line arguments.
argv: an array containing the values of command line arguments.

#include <iostream>

int main(int argc, char* argv[]) {
  std::cout << argc << "\n";
  
  for(int i = 0; i < argc; i++) {
    std::cout << argv[i] << "\n";
  }
  return 0;
}