6.3 — Local variables

In lesson 2.5 -- Introduction to local scope, we introduced local variables, which are variables that are defined inside a function (including function parameters).

It turns out that C++ actually doesn’t have a single attribute that defines a variable as being a local variable. Instead, local variables have several different properties that differentiate how local variables behave from other kinds of (non-local) variables. We’ll explore these properties in this and upcoming lessons.

In lesson 2.5 -- Introduction to local scope, we also introduced the concept of scope. An identifier’s scope determines where an identifier can be accessed within the source code. When an identifier can be accessed, we say it is in scope. When an identifier can not be accessed, we say it is out of scope. Scope is a compile-time property, and trying to use an identifier when it is out of scope will result in a compile error.

Local variables have block scope

Local variables have block scope, which means they are in scope from their point of definition to the end of the block they are defined within.

int main()
{
    int i { 5 }; // i enters scope here
    double d { 4.0 }; // d enters scope here

    return 0;
} // d and i go out of scope here

Although function parameters are not defined inside the function body, for typical functions they can be considered to be part of the scope of the function body block.

int max(int x, int y) // x and y enter scope here
{
    // assign the greater of x or y to max
    int max{ (x > y) ? x : y }; // max enters scope here

    return max;
} // max, y, and x leave scope here

All variable names within a scope must be unique

Variable names must be unique within a given scope, otherwise any reference to the name will be ambiguous. Consider the following program:

void someFunction(int x)
{
    int x{}; // compilation failure due to name collision with function parameter
}

int main()
{
    return 0;
}

The above program doesn’t compile because the variable x defined inside the function body and the function parameter x have the same name and both are in the same block scope.

Local variables have automatic storage duration

A variable’s storage duration (usually just called duration) determines what rules govern when and how a variable will be created and destroyed. In most cases, a variable’s storage duration directly determines its lifetime.

For example, local variables have automatic storage duration, which means they are created at the point of definition and destroyed at the end of the block they are defined in. For example:

int main()
{
    int i { 5 }; // i created and initialized here
    double d { 4.0 }; // d created and initialized here

    return 0;
} // d and i are destroyed here

For this reason, local variables are sometimes called automatic variables.

Local variables in nested blocks

Local variables can be defined inside nested blocks. This works identically to local variables in function body blocks:

int main() // outer block
{
    int x { 5 }; // x enters scope and is created here

    { // nested block
        int y { 7 }; // y enters scope and is created here
    } // y goes out of scope and is destroyed here

    // y can not be used here because it is out of scope in this block

    return 0;
} // x goes out of scope and is destroyed here

In the above example, variable y is defined inside a nested block. Its scope is limited from its point of definition to the end of the nested block, and its lifetime is the same. Because the scope of variable y is limited to the inner block in which it is defined, it’s not accessible anywhere in the outer block.

Note that nested blocks are considered part of the scope of the outer block in which they are defined. Consequently, variables defined in the outer block can be seen inside a nested block:

#include <iostream>

int main()
{ // outer block

    int x { 5 }; // x enters scope and is created here

    { // nested block
        int y { 7 }; // y enters scope and is created here

        // x and y are both in scope here
        std::cout << x << " + " << y << " = " << x + y << '\n';
    } // y goes out of scope and is destroyed here

    // y can not be used here because it is out of scope in this block

    return 0;
} // x goes out of scope and is destroyed here

Local variables have no linkage

Identifiers have another property named linkage. An identifier’s linkage determines whether other declarations of that name refer to the same object or not.

Local variables have no linkage, which means that each declaration refers to a unique object. For example:

int main()
{
    int x { 2 }; // local variable, no linkage

    {
        int x { 3 }; // this identifier x refers to a different object than the previous x
    }

    return 0;
}

Scope and linkage may seem somewhat similar. However, scope defines where a single declaration can be seen and used. Linkage defines whether multiple declarations refer to the same object or not.

Linkage isn’t very interesting in the context of local variables, but we’ll talk about it more in the next few lessons.

Variables should be defined in the most limited scope

If a variable is only used within a nested block, it should be defined inside that nested block:

#include <iostream>

int main()
{
    // do not define y here

    {
        // y is only used inside this block, so define it here
        int y { 5 };
        std::cout << y << '\n';
    }

    // otherwise y could still be used here, where it's not needed

    return 0;
}

By limiting the scope of a variable, you reduce the complexity of the program because the number of active variables is reduced. Further, it makes it easier to see where variables are used (or aren’t used). A variable defined inside a block can only be used within that block (or nested blocks). This can make the program easier to understand.

If a variable is needed in an outer block, it needs to be declared in the outer block:

#include <iostream>

int main()
{
    int y { 5 }; // we're declaring y here because we need it in this outer block later

    {
        int x{};
        std::cin >> x;

        // if we declared y here, immediately before its actual first use...
        if (x == 4)
            y = 4;
    } // ... it would be destroyed here

    std::cout << y; // and we need y to exist here

    return 0;
}

The above example shows one of the rare cases where you may need to declare a variable well before its first use.

New developers sometimes wonder whether it’s worth creating a nested block just to intentionally limit a variable’s scope (and force it to go out of scope / be destroyed early). Doing so makes that variable simpler, but the overall function becomes longer and more complex as a result. The tradeoff generally isn’t worth it. If creating a nested block seems useful to intentionally limit the scope of a chunk of code, that code might be better to put in a separate function instead.

Quiz time

Enter an integer: 4
Enter a larger integer: 2
Swapping the values
The smaller value is 2
The larger value is 4

#include <iostream>

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

    std::cout << "Enter a larger integer: ";
    int larger{};
    std::cin >> larger;

    // if user did it wrong
    if (smaller > larger)
    {
        // swap values of smaller and larger
        std::cout << "Swapping the values\n";

        int temp{ larger };
        larger = smaller;
        smaller = temp;
    } // temp dies here

    std::cout << "The smaller value is: " << smaller << '\n';
    std::cout << "The larger value is: " << larger << '\n';

    return 0;
} // smaller and larger die here

int temp{ larger };
larger = smaller;
smaller = temp;

// is the same as
std::swap(larger, smaller);