Parameters

Learn to pass parameters to a function.

Parameter and argument

A function can take values from outside. In the definition you write a parameter: name in void greet(std::string name) is a parameter. The value you pass at the call is an argument: "Alex" in greet("Alex") is an argument.

The parameter behaves like a local variable inside the function. At call time the argument's value is copied into the parameter; this is called call by value. So even if you change the parameter inside the function, the caller's original variable is unaffected.

Call by value

The default behavior in C++ is call by value. When you call void addTen(int x) with addTen(n), the value of n is copied into x. Even if you write x = x + 10;, n stays the same.

This limits side effects and makes code easier to follow. If you want the function to modify the caller, you need a reference parameter; we cover that in a later level. For now, knowing parameters are copies is enough.

Exercise Write parameterized greet and call it

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

Show example solution

#include <iostream>
#include <string>

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

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

Test yourself

Match parameter and argument

void greet(std::string name)

Definition
greet("Alex")

Call
name

Parameter
"Alex"

Argument
After calling void inc(int x) { x++; }, what is n? (n = 5)
```
int n = 5;
inc(n);
```
- Compile error
- n becomes undefined
- n becomes 6
- n stays 5
Show answer

n stays 5 — in call by value the copy of n changes, n itself stays 5.

Complete the parameterized function

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

Show answer

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

Next lesson: Function overloading

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