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 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.

What happens when an object is destroyed?

In most cases, nothing. The destroyed object simply becomes invalid.

Any use of an object after it has been destroyed will result in undefined behavior.

At some point after destruction, the memory used by the object will be deallocated (freed up for reuse).

Local scope (block 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.

The identifier of a local variable has local scope. An identifier with local scope (technically called block scope) is usable from the point of definition to the end of the innermost pair of curly braces containing the identifier (or for function parameters, at the end of the function). This ensures local variables cannot be used before the point of definition (even if the compiler opts to create them before then) or after they are destroyed. 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. x goes out of scope at the end of the innermost pair of curly braces containing the identifier, which is the closing curly brace of function main(). 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 usable only within add()
    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 only within main()

    std::cout << add(a, b) << '\n'; // calls add(5, 6), where 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;
}

When to use function parameters vs Local variables

Because function parameters and local variable can both be used within the body of a function, new programmers sometimes struggle to understand when each should be used. A function parameter should be used when the caller will pass in the initialization value as an argument. A local variable should be used otherwise.

Using a function parameter when you should use a local variable leads to code looking like this:

#include <iostream>

int getValueFromUser(int val) // val is a function parameter
{
    std::cout << "Enter a value: ";
    std::cin >> val;
    return val;
}

int main()
{
    int x {};
    int num { getValueFromUser(x) }; // main must pass x as an argument

    std::cout << "You entered " << num << '\n';

    return 0;
}

In the above example, getValue() has defined val as a function parameter. Because of this, main() must define x so that it has something to pass as an argument. However, the actual value of x is never used, and the value that val is initialized with is never used. Making the caller define and pass a variable that is never used adds needless complexity.

The correct way to write this would be as follows:

#include <iostream>

int getValueFromUser()
{
    int val {}; // val is a local variable
    std::cout << "Enter a value: ";
    std::cin >> val;
    return val;
}

int main()
{
    int num { getValueFromUser() }; // main does not need to pass anything

    std::cout << "You entered " << num << '\n';

    return 0;
}

In this example, val is now a local variable. main() is now simpler because it does not need to define or pass a variable to call getValue().

Introduction to temporary objects

A temporary object (also sometimes called an anonymous object) is an unnamed object that is used to hold a value that is only needed for a short period of time. Temporary objects are generated by the compiler when they are needed.

There are many different ways that temporary values can be created, but here’s a common one:

#include <iostream>

int getValueFromUser()
{
 	std::cout << "Enter an integer: ";
	int input{};
	std::cin >> input;

	return input; // return the value of input back to the caller
}

int main()
{
	std::cout << getValueFromUser() << '\n'; // where does the returned value get stored?

	return 0;
}

In the above program, the function getValueFromUser() returns the value stored in local variable input back to the caller. Because input will be destroyed at the end of the function, the caller receives a copy of the value so that it has a value it can use even after input is destroyed.

But where is the value that is copied back to the caller stored? We haven’t defined any variables in main(). The answer is that the return value is stored in a temporary object. This temporary object is then passed to std::cout to be printed.

Temporary objects have no scope at all (this makes sense, since scope is a property of an identifier, and temporary objects have no identifier).

Temporary objects are destroyed at the end of the full expression in which they are created. This means temporary objects are always destroyed before the next statement executes.

In our example above, the temporary object created to hold the return value of getValueFromUser() is destroyed after std::cout << getValueFromUser() << '\n' executes.

In the case where a temporary object is used to initialize a variable, the initialization happens before the destruction of the temporary.

In modern C++ (especially since C++17), the compiler has many tricks to avoid generating temporaries where previously it would have needed to. For example, when we use a return value to initialize a variable, this would normally result in the creation of a temporary holding the return value, and then using the temporary to initialize the variable. However, in modern C++, the compiler will often skip creating the temporary and just initialize the variable directly with the return value.

Similarly, in the above example, since the return value of getValueFromUser() is immediately output, the compiler can skip creation and destruction of the temporary in main(), and use the return value of getValueFromUser() to directly initialize the parameter of operator<<.

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