declval – Tarik Billa

Is there a reason declval returns add_rvalue_reference instead of add_lvalue_reference

November 22, 2023 by Tarik

With add_rvalue_reference: declval<Foo>() is of type Foo&&. declval<Foo&>() is of type Foo& (reference collapsing: “Foo& &&” collapses to Foo&). declval<Foo&&>() is of type Foo&& (reference collapsing: “Foo&& &&” collapses to Foo&&). With add_lvalue_reference: declval<Foo>() would be of type Foo&. declval<Foo&>() would be of type Foo& (reference collapsing: “Foo& &” collapses to Foo&). declval<Foo&&>() would be … Read more

Why does the implementation of declval in libstdc++-v3 look so complicated?

May 7, 2023 by Tarik

std::declval is actually: template<class T> typename std::add_rvalue_reference<T>::type declval() noexcept; Where std::add_rvalue_reference<T> is usually T&&, except in cases where that is invalid (Like if T = void or T = int() const), where it is just T. The main difference is that functions cannot return arrays, but can return array references like U(&&)[] or U(&&)[N]. The … Read more

Why does std::declval add a reference?

May 3, 2023 by Tarik

The “no temporary is introduced for function returning prvalue of object type in decltype” rule applies only if the function call itself is either the operand of decltype or the right operand of a comma operator that’s the operand of decltype (§5.2.2 [expr.call]/p11), which means that given declprval in the OP, template< typename t > … Read more