7.5 — Switch fallthrough and scoping

This lesson continues our exploration of switch statements that we started in the prior lesson 7.4 -- Switch statement basics. In the prior lesson, we mentioned that each set of statements underneath a label should end in a break statement or a return statement.

In this lesson, we’ll explore why, and talk about some switch scoping issues that sometimes trip up new programmers.

Fallthrough

When a switch expression matches a case label or optional default label, execution begins at the first statement following the matching label. Execution will then continue sequentially until one of the following termination conditions happens:

1. The end of the switch block is reached.
2. Another control flow statement (typically a break or return) causes the switch block or function to exit.
3. Something else interrupts the normal flow of the program (e.g. the OS shuts the program down, the universe implodes, etc…)

Note that the presence of another case label is not one of these terminating conditions -- thus, without a break or return, execution will overflow into subsequent cases.

Here is a program that exhibits this behavior:

#include <iostream>

int main()
{
    switch (2)
    {
    case 1: // Does not match
        std::cout << 1 << '\n'; // Skipped
    case 2: // Match!
        std::cout << 2 << '\n'; // Execution begins here
    case 3:
        std::cout << 3 << '\n'; // This is also executed
    case 4:
        std::cout << 4 << '\n'; // This is also executed
    default:
        std::cout << 5 << '\n'; // This is also executed
    }

    return 0;
}

This program outputs the following:

2
3
4
5

This is probably not what we wanted! When execution flows from a statement underneath a label into statements underneath a subsequent label, this is called fallthrough.

Since fallthrough is rarely desired or intentional, many compilers and code analysis tools will flag fallthrough as a warning.

The [[fallthrough]] attribute

Commenting intentional fallthrough is a common convention to tell other developers that fallthrough is intended. While this works for other developers, the compiler and code analysis tools don’t know how to interpret comments, so it won’t get rid of the warnings.

To help address this, C++17 adds a new attribute called [[fallthrough]].

Attributes are a modern C++ feature that allows the programmer to provide the compiler with some additional data about the code. To specify an attribute, the attribute name is placed between double hard braces. Attributes are not statements -- rather, they can be used almost anywhere where they are contextually relevant.

The [[fallthrough]] attribute modifies a null statement to indicate that fallthrough is intentional (and no warnings should be triggered):

#include <iostream>

int main()
{
    switch (2)
    {
    case 1:
        std::cout << 1 << '\n';
        break;
    case 2:
        std::cout << 2 << '\n'; // Execution begins here
        [[fallthrough]]; // intentional fallthrough -- note the semicolon to indicate the null statement
    case 3:
        std::cout << 3 << '\n'; // This is also executed
        break;
    }

    return 0;
}

This program prints:

2
3

And it should not generate any warnings about the fallthrough.

Sequential case labels

You can use the logical OR operator to combine multiple tests into a single statement:

bool isVowel(char c)
{
    return (c=='a' || c=='e' || c=='i' || c=='o' || c=='u' ||
        c=='A' || c=='E' || c=='I' || c=='O' || c=='U');
}

This suffers from the same challenges that we presented in the introduction to switch statements: c gets evaluated multiple times and the reader has to make sure it is c that is being evaluated each time.

You can do something similar using switch statements by placing multiple case labels in sequence:

bool isVowel(char c)
{
    switch (c)
    {
        case 'a': // if c is 'a'
        case 'e': // or if c is 'e'
        case 'i': // or if c is 'i'
        case 'o': // or if c is 'o'
        case 'u': // or if c is 'u'
        case 'A': // or if c is 'A'
        case 'E': // or if c is 'E'
        case 'I': // or if c is 'I'
        case 'O': // or if c is 'O'
        case 'U': // or if c is 'U'
            return true;
        default:
            return false;
    }
}

Remember, execution begins at the first statement after a matching case label. Case labels aren’t statements (they’re labels), so they don’t count.

The first statement after all of the case statements in the above program is return true, so if any case labels match, the function will return true.

Thus, we can “stack” case labels to make all of those case labels share the same set of statements afterward. This is not considered fallthrough behavior, so use of comments or [[fallthrough]] is not needed here.

Switch case scoping

With if statements, you can only have a single statement after the if-condition, and that statement is considered to be implicitly inside a block:

if (x > 10)
    std::cout << x << " is greater than 10\n"; // this line implicitly considered to be inside a block

However, with switch statements, the statements after labels are all scoped to the the switch block. No implicit blocks are created.

switch (1)
{
    case 1: // does not create an implicit block
        foo(); // this is part of the switch scope, not an implicit block to case 1
        break; // this is part of the switch scope, not an implicit block to case 1
    default:
        std::cout << "default case\n";
        break;
}

In the above example, the 2 statements between the case 1 and the default label are scoped as part of the switch block, not a block implicit to case 1.

Variable declaration and initialization inside case statements

You can declare (but not initialize) variables inside the switch, both before and after the case labels:

switch (1)
{
    int a; // okay: declaration is allowed before the case labels
    int b{ 5 }; // illegal: initialization is not allowed before the case labels

    case 1:
        int y; // okay but bad practice: declaration is allowed within a case
        y = 4; // okay: assignment is allowed
        break;

    case 2:
        int z{ 4 }; // illegal: initialization is not allowed if subsequent cases exist
        y = 5; // okay: y was declared above, so we can use it here too
        break;

    case 3:
        break;
}

Although variable y was defined in case 1, it was used in case 2 as well. Because the statements under each case are not inside an implicit block, that means all statements inside the switch are part of the same scope. Thus, a variable defined in one case can be used in a later case, even if the case in which the variable is defined is never executed!

Put another way, defining a variable without an initializer is just telling the compiler that the variable is now in scope from that point on. This happens at compile time, and doesn’t require the definition to actually be executed at runtime.

However, initialization of variables does require execution at runtime. Initialization of variables is disallowed in any case that is not the last case (because the initializer could be jumped over, which would leave the variable uninitialized). Initialization is also disallowed before the first case, as those statements will never be executed, as there is no way for the switch to reach them.

If a case needs to define and/or initialize a new variable, best practice is to do so inside an explicit block underneath the case statement:

switch (1)
{
    case 1:
    { // note addition of explicit block here
        int x{ 4 }; // okay, variables can be initialized inside a block inside a case
        std::cout << x;
        break;
    }
    default:
        std::cout << "default case\n";
        break;
}

Quiz time

#include <iostream>

int calculate(int x, int y, char op)
{
    switch (op)
    {
        case '+':
            return x + y;
        case '-':
            return x - y;
        case '*':
            return x * y;
        case '/':
            return x / y;
        case '%':
            return x % y;
        default:
            std::cerr << "calculate(): Unhandled case\n";
            return 0;
    }
}

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

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

    std::cout << "Enter a mathematical operator (+, -, *, /, or %): ";
    char op{};
    std::cin >> op;

    std::cout << x << ' ' << op << ' ' << y << " is " << calculate(x, y, op) << '\n';

    return 0;
}