Difference between revisions of "cpp/language/switch"

#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 fall through
        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 doesn't have to be a compound statement
    switch (0)
        std::cout << "this does nothing\n";
 
    // labels don't require a compound statement either
    switch (int n = 1)
    {
        case 0:
        case 1:
            std::cout << n << '\n';
    }
}