switch: Choosing Among Fixed Cases

switch exists because comparing one value against many fixed constants is common enough — menu dispatch, state machines, day-of-week tables, instruction decoding — that the language gives it dedicated syntax. The payoff is real: the compiler can turn a switch into a single jump-table lookup, one indexed jump instead of testing each else if in turn.

switch

When you are comparing one integer (or char) against a list of fixed values, switch reads better than a ladder of else-ifs:

int day = 3;
switch (day) {
    case 1: std::cout << "Mon\n"; break;
    case 2: std::cout << "Tue\n"; break;
    case 3: std::cout << "Wed\n"; break;
    default: std::cout << "unknown\n"; break;
}

Rules that matter:

• Each case must be a compile-time constant (a literal or const int). You cannot switch on a std::string or on ranges.
• break is mandatory at the end of each case. Without it, execution falls through into the next case's code. Occasionally useful, usually a bug.
• default runs when nothing matched, like a final else.

Python only recently got match. C++'s switch is older and dumber: it is a fast jump on a number, nothing more.

Worked example: fall-through as a feature

Because execution falls through until it hits a break, several case labels can deliberately share one body. This is the one situation where omitting break between cases is idiomatic rather than a bug:

char c = 'e';
switch (c) {
    case 'a': case 'e': case 'i': case 'o': case 'u':
        std::cout << "vowel\n";
        break;
    default:
        std::cout << "consonant\n";
        break;
}

All five vowel labels funnel into the same two lines. Anything else lands on default.