Conditionals (if / switch)

if/else branch on a boolean; switch does a multi-way jump on an integral or enum value, often compiled to a jump table for O(1) dispatch.

Why it matters

switch on a dense set of cases lowers to a jump table — constant-time regardless of case count — whereas a chain of ifs is linear; for hot dispatch (bytecode interpreters, state machines) this is measurable. C++17’s init-statement (if (auto x = f(); cond)) scopes a variable to the branch, a clean pattern for lock guards, std::optional unwrapping, and error checks.

How it works

• if (init; cond) and switch (init; cond) (C++17) declare a variable visible only inside the statement and its else — tighter scope, fewer leaks into the enclosing block.
• if constexpr (C++17) discards the untaken branch at compile time — essential in templates to avoid instantiating invalid code.
• switch requires integral/enum/constexpr-convertible conditions; non-matching falls to default if present.
• [[likely]] / [[unlikely]] (C++20) hint the branch predictor for codegen.

Construct	Dispatch	Compile-time	Typical use
if/else if	linear	no	few, complex conditions
switch	jump table	no	dense integral cases
if constexpr	none	yes	template branch pruning

Example

if (auto it = cache.find(key); it != cache.end())
    return it->second;             // it scoped to the if/else only
 
switch (token.kind) {
    case Plus:  emit_add(); break; // break: no fall-through
    case Minus: emit_sub(); break;
    default:    error();           // catch-all
}
 
template <class T> void f(T v) {
    if constexpr (std::is_pointer_v<T>) use(*v);  // other branch never compiled
    else use(v);
}

Pitfalls

• Missing break falls through to the next case — sometimes intended (mark with [[fallthrough]];), usually a bug.
• Declaring a variable in a case without braces can skip its initialization; wrap the case body in { }.
• if (x = 5) assigns then tests truthiness — meant ==. Enable -Wparentheses; or write if (5 == x).
• Runtime if, not if constexpr, on a type trait still compiles both branches and fails on the invalid one.

See also

• loops-for-while-do-while
• operators