Difference between revisions of "cpp/language/switch"

Latest revision as of 18:04, 11 August 2024

Transfers control to one of several statements, depending on the value of a condition.

[edit] Syntax

[edit] Condition

A condition can either be an expression or a simple declaration.

• If it can be syntactically resolved as an expression, it is treated as an expression. Otherwise, it is treated as a declaration that is not a structured binding declaration(since C++26).

When control reaches condition, the condition will yield a value, which is used to determine which label the control will go to.

[edit] Expression

If condition is an expression, the value it yields is the the value of the expression.

[edit] Declaration

If condition is a simple declaration, the value it yields is the value of the decision variable (see below).

[edit] Non-structured binding declaration

The declaration has the following restrictions:

• Syntactically conforms to the following form:

• The declarator cannot specify a function or an array.
• The type specifier sequence(until C++11)declaration specifier sequence can only contain type specifiers and constexpr, and it(since C++11) cannot define a class or enumeration.

The decision varaiable of the declaration is the declared variable.

[edit] Type

condition can only yield the following types:

• integral types
• enumeration types
• class types

If the yielded value is of a class type, it is contextually implicitly converted to an integral or enumeration type.

If the (possibly converted) type is subject to integral promotions , the yielded value is converted to the promoted type.

[edit] Labels

Any statement within the switch statement can be labeled with one or more following labels:

A case or default label is associated with the innermost switch statement enclosing it.

If any of the following conditions is satisfied, the program is ill-formed:

• A switch statement is associated with multiple case labels whose constant-expression s have the same value after conversions.
• A switch statement is associated with multiple default labels.

[edit] Control flow transfer

When the condition of a switch statement yields a (possibly converted) value:

• If one of the associated case label constants has the same value, control is passed to the statement labeled by the matched case label.
• Otherwise, if there is an associated default label, control is passed to the statement labeled by the default label.
• Otherwise, none of the statements in the switch statement will be executed.

case and default labels in themselves do not alter the flow of control. To exit from a switch statement from the middle, see break statements.

Compilers may issue warnings on fallthrough (reaching the next case or default label without a break) unless the attribute [[fallthrough]] appears immediately before the case label to indicate that the fallthrough is intentional(since C++17).

switch (1)
{
    case 1:
        std::cout << '1'; // prints "1",
    case 2:
        std::cout << '2'; // then prints "2"
}

switch (1)
{
    case 1:
        std::cout << '1'; // prints "1"
        break;            // and exits the switch
    case 2:
        std::cout << '2';
        break;
}

[edit] Notes

Because transfer of control is not permitted to enter the scope of a variable, if a declaration statement is encountered inside the statement, it has to be scoped in its own compound statement:

switch (1)
{
    case 1:
        int x = 0; // initialization
        std::cout << x << '\n';
        break;
    default:
        // compilation error: jump to default:
        // would enter the scope of 'x' without initializing it
        std::cout << "default\n";
        break;
}

switch (1)
{
    case 1:
        {
            int x = 0;
            std::cout << x << '\n';
            break;
        } // scope of 'x' ends here
    default:
        std::cout << "default\n"; // no error
        break;
}

[edit] Keywords

switch, case, default

[edit] Example

#include <iostream>
 
int main()
{
    const int i = 2;
    switch (i)
    {
        case 1:
            std::cout << '1';
        case 2:              // execution starts at this case label
            std::cout << '2';
        case 3:
            std::cout << '3';
            [[fallthrough]]; // C++17 attribute to silent the warning on fallthrough
        case 5:
            std::cout << "45";
            break;           // execution of subsequent statements is terminated
        case 6:
            std::cout << '6';
    }
 
    std::cout << '\n';
 
    switch (i)
    {
        case 4:
            std::cout << 'a';
        default:
            std::cout << 'd'; // there are no applicable constant expressions 
                              // therefore default is executed
    }
 
    std::cout << '\n';
 
    switch (i)
    {
        case 4:
            std::cout << 'a'; // nothing is executed
    }
 
    // when enumerations are used in a switch statement, many compilers
    // issue warnings if one of the enumerators is not handled
    enum color { RED, GREEN, BLUE };
    switch (RED)
    {
        case RED:
            std::cout << "red\n";
            break;
        case GREEN:
            std::cout << "green\n";
            break;
        case BLUE:
            std::cout << "blue\n";
            break;
    }
 
    // the C++17 init-statement syntax can be helpful when there is
    // no implicit conversion to integral or enumeration type
    struct Device
    {
        enum State { SLEEP, READY, BAD };
        auto state() const { return m_state; }
 
        /* ... */
 
    private:
        State m_state{};
    };
 
    switch (auto dev = Device{}; dev.state())
    {
        case Device::SLEEP:
            /* ... */
            break;
        case Device::READY:
            /* ... */
            break;
        case Device::BAD:
            /* ... */
            break;
    }
 
    // pathological examples
 
    // the statement does not have to be a compound statement
    switch (0)
        std::cout << "this does nothing\n";
 
    // labels do not require a compound statement either
    switch (int n = 1)
        case 0:
        case 1:
            std::cout << n << '\n';
}

2345
d
red
1

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also

[edit] External links