decltype(expr) yields the type of its operand expression without evaluating it. For an identifier or class member access, it gives the declared type. For other expressions, it produces the type including value category: lvalue expressions yield a reference type, xvalues yield an rvalue reference, and prvalues yield the non-reference type.

Before decltype, expressing the return type of a function in terms of its parameters or capturing the type of a complex expression required manual specification or type traits. decltype allows the compiler to deduce the exact type from an expression, which is essential for generic code and trailing return types.

#include <print>
#include <type_traits>

template <typename A, typename B>
auto multiply(A a, B b) -> decltype(a * b) {
    return a * b;
}

int main() {
    auto x = 123;
    decltype(x) y = 10;       // int (declared type of identifier)
    decltype((x)) z = x;      // int& (lvalue expression)

    auto result = multiply(3, 2.5);

    std::println("y = {}", y);
    std::println("result type is double: {}", std::is_same_v<decltype(result), double>);
    std::println("result = {}", result);
}