Function Default value for argument in C++

In C++, we can set default values for a function or method's arguments. This helps simplify function calls by allowing us to specify only the values that we need, and let the others default to a predefined value.

Here is an example of a function with default arguments:

void printMessage(std::string message, int numberOfTimes = 1) {
    for (int i = 0; i < numberOfTimes; i++) {
        std::cout << message << std::endl;
    }
}

In this example, we have defined a function called printMessage that takes two arguments: message and numberOfTimes. The second argument, numberOfTimes, has a default value of 1.

We can call this function in a couple of ways:

// Call with both arguments
printMessage("Hello World", 3);
// Call with only the first argument

printMessage("Hello World");
// Call with only the second argument

printMessage("Goodbye World", 5);

In the first example, we call the function and specify both arguments. In the second example, we call the function and don't specify the second argument, so it defaults to 1. In the third example, we call the function and specify only the second argument, so the first argument defaults to "Hello World".

Note that when specifying default values for arguments, we should start with the rightmost argument and move leftward. This is because once we specify a value for an argument, we must specify values for all arguments to the right of it. So, if we have a function with two arguments, we must specify a value for the second argument if we want to specify a value for the first argument with a default value.

// Invalid - cannot specify a default value for a leftward argument
void doSomething(int numberOfTimes = 2, std::string message) {
    // ...
}

// Valid - we can specify a default value for the rightward argument
void doSomething(std::string message, int numberOfTimes = 2) {
    // ...
}