2.5 — Introduction to local scope

Local variables

Variables defined inside the body of a function are called local variables (as opposed to global variables, which we’ll discuss in a future chapter):

int add(int x, int y)
{
    int z{ x + y }; // z is a local variable

    return z;
}

Function parameters are also generally considered to be local variables, and we will include them as such:

int add(int x, int y) // function parameters x and y are local variables
{
    int z{ x + y };

    return z;
}

In this lesson, we’ll take a look at some properties of local variables in more detail.

Local variable lifetime

In lesson 1.3 -- Introduction to objects and variables, we discussed how a variable definition such as int x; causes the variable to be instantiated (created) when this statement is executed. Function parameters are created and initialized when the function is entered, and variables within the function body are created and initialized at the point of definition.

For example:

int add(int x, int y) // x and y created and initialized here
{ 
    int z{ x + y };   // z created and initialized here

    return z;
}

The natural follow-up question is, “so when is an instantiated variable destroyed?”. Local variables are destroyed in the opposite order of creation at the end of the set of curly braces in which it is defined (or for a function parameter, at the end of the function).

int add(int x, int y)
{ 
    int z{ x + y };

    return z;
} // z, y, and x destroyed here

Much like a person’s lifetime is defined to be the time between their birth and death, an object’s lifetime is defined to be the time between its creation and destruction. Note that variable creation and destruction happen when the program is running (called runtime), not at compile time. Therefore, lifetime is a runtime property.

Here’s a slightly more complex program demonstrating the lifetime of a variable named x:

#include <iostream>

void doSomething()
{
    std::cout << "Hello!\n";
}

int main()
{
    int x{ 0 };    // x's lifetime begins here

    doSomething(); // x is still alive during this function call

    return 0;
} // x's lifetime ends here

In the above program, the lifetime of x runs from the point of definition to the end of function main. This includes the time spent during the execution of function doSomething.

Local scope

An identifier’s scope determines where the identifier can be seen and used within the source code. When an identifier can be seen and used, we say it is in scope. When an identifier can not be seen, we can not use it, and we say it is out of scope. Scope is a compile-time property, and trying to use an identifier when it is not in scope will result in a compile error.

A local variable’s scope begins at the point of variable definition, and stops at the end of the set of curly braces in which it is defined (or for function parameters, at the end of the function). This ensures variables can not be used before the point of definition (even if the compiler opts to create them before then). Local variables defined in one function are also not in scope in other functions that are called.

Here’s a program demonstrating the scope of a variable named x:

#include <iostream>

// x is not in scope anywhere in this function
void doSomething()
{
    std::cout << "Hello!\n";
}

int main()
{
    // x can not be used here because it's not in scope yet

    int x{ 0 }; // x enters scope here and can now be used within this function

    doSomething();

    return 0;
} // x goes out of scope here and can no longer be used

In the above program, variable x enters scope at the point of definition and goes out of scope at the end of the main function. Note that variable x is not in scope anywhere inside of function doSomething. The fact that function main calls function doSomething is irrelevant in this context.

“Out of scope” vs “going out of scope”

The terms “out of scope” and “going out of scope” can be confusing to new programmers.

An identifier is out of scope anywhere it cannot be accessed within the code. In the example above, the identifier x is in scope from its point of definition to the end of the main function. The identifier x is out of scope outside of that code region.

The term “going out of scope” is typically applied to objects rather than identifiers. We say an object goes out of scope at the end of the scope (the end curly brace) in which the object was instantiated. In the example above, the object named x goes out of scope at the end of the function main.

A local variable’s lifetime ends at the point where it goes out of scope, so local variables are destroyed at this point.

Note that not all types of variables are destroyed when they go out of scope. We’ll see examples of these in future lessons.

Another example

Here’s a slightly more complex example. Remember, lifetime is a runtime property, and scope is a compile-time property, so although we are talking about both in the same program, they are enforced at different points.

#include <iostream>

int add(int x, int y) // x and y are created and enter scope here
{
    // x and y are visible/usable within this function only
    return x + y;
} // y and x go out of scope and are destroyed here

int main()
{
    int a{ 5 }; // a is created, initialized, and enters scope here
    int b{ 6 }; // b is created, initialized, and enters scope here

    // a and b are usable within this function only
    std::cout << add(a, b) << '\n'; // calls function add() with x=5 and y=6

    return 0;
} // b and a go out of scope and are destroyed here

Parameters x and y are created when the add function is called, can only be seen/used within function add, and are destroyed at the end of add. Variables a and b are created within function main, can only be seen/used within function main, and are destroyed at the end of main.

To enhance your understanding of how all this fits together, let’s trace through this program in a little more detail. The following happens, in order:

• Execution starts at the top of main.
• main variable a is created and given value 5.
• main variable b is created and given value 6.
• Function add is called with argument values 5 and 6.
• add parameters x and y are created and initialized with values 5 and 6 respectively.
• The expression x + y is evaluated to produce the value 11.
• add copies the value 11 back to caller main.
• add parameters y and x are destroyed.
• main prints 11 to the console.
• main returns 0 to the operating system.
• main variables b and a are destroyed.

And we’re done.

Note that if function add were to be called twice, parameters x and y would be created and destroyed twice -- once for each call. In a program with lots of functions and function calls, variables are created and destroyed often.

Functional separation

In the above example, it’s easy to see that variables a and b are different variables from x and y.

Now consider the following similar program:

#include <iostream>

int add(int x, int y) // add's x and y are created and enter scope here
{
    // add's x and y are visible/usable within this function only
    return x + y;
} // add's y and x go out of scope and are destroyed here

int main()
{
    int x{ 5 }; // main's x is created, initialized, and enters scope here
    int y{ 6 }; // main's y is created, initialized, and enters scope here

    // main's x and y are usable within this function only
    std::cout << add(x, y) << '\n'; // calls function add() with x=5 and y=6

    return 0;
} // main's y and x go out of scope and are destroyed here

In this example, all we’ve done is change the names of variables a and b inside of function main to x and y. This program compiles and runs identically, even though functions main and add both have variables named x and y. Why does this work?

First, we need to recognize that even though functions main and add both have variables named x and y, these variables are distinct. The x and y in function main have nothing to do with the x and y in function add -- they just happen to share the same names.

Second, when inside of function main, the names x and y refer to main’s locally scoped variables x and y. Those variables can only be seen (and used) inside of main. Similarly, when inside function add, the names x and y refer to function parameters x and y, which can only be seen (and used) inside of add.

In short, neither add nor main know that the other function has variables with the same names. Because the scopes don’t overlap, it’s always clear to the compiler which x and y are being referred to at any time.

We’ll talk more about local scope, and other kinds of scope, in a future chapter.

Where to define local variables

In modern C++, the best practice is that local variables inside the function body should be defined as close to their first use as reasonable:

#include <iostream>

int main()
{
	std::cout << "Enter an integer: ";
	int x{};       // x defined here
	std::cin >> x; // and used here

	std::cout << "Enter another integer: ";
	int y{};       // y defined here
	std::cin >> y; // and used here

	int sum{ x + y }; // sum can be initialized with intended value
	std::cout << "The sum is: " << sum << '\n';

	return 0;
}

In the above example, each variable is defined just before it is first used. There’s no need to be strict about this -- if you prefer to swap lines 5 and 6, that’s fine.

#include <iostream>

int main()
{
	int x{}, y{}, sum{}; // how are these used?

	std::cout << "Enter an integer: ";
	std::cin >> x;

	std::cout << "Enter another integer: ";
	std::cin >> y;

	sum = x + y;
	std::cout << "The sum is: " << sum << '\n';

	return 0;
}

Quiz time

#include <iostream>

void doIt(int x)
{
    int y{ 4 };
    std::cout << "doIt: x = " << x << " y = " << y << '\n';

    x = 3;
    std::cout << "doIt: x = " << x << " y = " << y << '\n';
}

int main()
{
    int x{ 1 };
    int y{ 2 };

    std::cout << "main: x = " << x << " y = " << y << '\n';

    doIt(x);

    std::cout << "main: x = " << x << " y = " << y << '\n';

    return 0;
}

main: x = 1 y = 2
doIt: x = 1 y = 4
doIt: x = 3 y = 4
main: x = 1 y = 2