If you add a parameter to a function in C++, then an argument will be required when you call the function. What does “required” mean here? Well, you’ll get an error. But what if 9 times out of 10, you know you’ll use the same input value? It would be really annoying to have to enter the same value over and over again.

To make your code more flexible for situations like this, you can add default arguments to your function declarations. Default arguments are values assigned to parameters when the function is declared and defined:

// Declaration
void intro(std::string name, std::string lang = "C++");

// Definition
void intro(std::string name, std::string lang) {
  std::cout << "Hi, my name is "
            << name
            << " and I'm learning "
            << lang
            << ".\n";
}

Then, if you leave the argument blank in your function call, instead of an error, your function will run with the default value. Meanwhile, if you DO have an argument to add when you call the function, that argument will replace the default argument when your code executes.

Either of these will work for the function we defined:

intro("Mariel")
// "Hi, my name is Mariel and I'm learning C++."

intro("Mariel", "Python")
// "Hi, my name is Mariel and I'm learning Python."

Important: Your computer cannot read your mind; it determines which argument corresponds with which parameter based on order.

Parameters without default arguments come first. This will work for add_nums(3); because the computer knows 3 corresponds to num1:

int add_nums(int num1, int num2 = 0);

But the following does NOT work for add_num(3); the computer assumes that 3 still corresponds to num1:

int add_nums(int num1 = 0, int num2);

Similarly, when a function has two default arguments, you still need to call with both arguments — if BOTH of the following are true:

• The first argument IS the default value.
• The second argument is NOT the default value.