Default parameters

Learn to define parameters with default values.

Default parameters

You can give a parameter a default value; if the call skips it, the default is used. In void greet(std::string name = "guest") the default of name is "guest". The call greet() is the same as greet("guest").

The default is written once in the definition; if the declaration and definition are separate, the default goes only on the declaration. Key habit: default parameters must come after non-default ones. void f(int a, int b = 0) is valid, void f(int b = 0, int a) is not.

Which parameter gets a default?

The order of default parameters matters. void f(int a, int b = 0, int c = 0) is valid: defaults at the end. void f(int a = 0, int b, int c = 0) is invalid because b has no default while a parameter after it does.

The logic is that arguments bind left to right at the call. When you skip one parameter, you must skip every parameter to its right. That is why defaults cluster at the end. Putting optional parameters last makes life easier for both the compiler and the reader.

Exercise Write greet with a default parameter

Define void greet(std::string name = "guest") whose body is std::cout << "hi " << name << std::endl;. In main, first call greet(), then greet("Alex").

Show example solution

#include <iostream>
#include <string>

void greet(std::string name = "guest") {
    std::cout << "hi " << name << std::endl;
}

int main() {
    greet();
    greet("Alex");
    return 0;
}

Test yourself

Match default parameter behavior

greet()

Call with default
greet("Alex")

Call with argument
name = "guest"

Default value
int a, int b = 0

Valid order
For void f(int a, int b = 0), what is b in the call f(5)?
```
void f(int a, int b = 0);
f(5);
```
- 5
- 0
- Undefined
- Compile error
Show answer

0 — when b is skipped, the default 0 is used.

Complete the default value

void greet(std::string name ____ "guest") { std::cout << name; }

Show answer

void greet(std::string name = "guest") { std::cout << name; }

Next lesson: Summary - Functions

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