cpp17 · medium

C++17: [[nodiscard]], [[maybe_unused]], [[fallthrough]]

C++17 standardized three widely useful attributes. [[nodiscard]], applied to a function or a type, makes the compiler warn when the return value is discarded — invaluable for error codes, allocations, and RAII handle/guard results; C++20 adds an explanatory string, [[nodiscard("reason")]]. [[maybe_unused]] suppresses unused-entity warnings for variables, parameters, functions, types, or enumerators that are used only in some build configurations — the textbook case being a variable referenced only inside assert(), which disappears under NDEBUG; it documents intent more clearly than the old (void)x; cast. [[fallthrough]], placed as the last statement of a switch case before the next label, declares that an implicit fall-through is deliberate and silences the implicit-fallthrough warning while documenting the intent for readers. Related standard attributes include [[deprecated]] and the C++20 [[likely]]/[[unlikely]] branch hints.

The code

[[nodiscard]] int compute(); // warn if the result is ignored
compute();                   // WARNING: discarded value

void f([[maybe_unused]] int debugOnly) { // no 'unused param' warning
    [[maybe_unused]] int x = check();    // used only in assert()
}

switch (n) {
case 1:
    setup();
    [[fallthrough]]; // 'I meant to fall through'
case 2:
    run();
    break;
}

What this lesson walks through

1. 01[[nodiscard]] — do not ignore this result
2. 02[[maybe_unused]] — intentionally unused
3. 03Gotcha — they only advise the compiler
4. 04[[fallthrough]] — deliberate switch fall-through

[[nodiscard]] on a function (or on a type) makes the compiler warn when the return value is discarded. It is for results that are dangerous to drop: error codes, allocations, RAII handles/guards, and any 'pure' query whose only purpose is its return value. C++20 lets you add a reason string: [[nodiscard("check the error")]].