decltype Operator

decltype(expr) inspects the type of an expression at compile time without evaluating it.

Basics

int x{42};
decltype(x) a{0};               // int
decltype((x)) b{x};             // int&  (lvalue expression yields T&)
decltype(x + 0) c{0};           // int   (prvalue yields T)

Rules of Thumb

• decltype(id) → declared type of id.

• decltype(lvalue-expr) → T&.

• decltype(prvalue-expr) → T.

Use Cases

• Mirror another variable’s exact type (including refs/const).

• Trailing return type in templates: auto f(...) -> decltype(expr).