20 General utilities library [utilities]

20.2 Utility components [utility]

20.2.4 Function template declval [declval]

The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands (Clause [expr]).

template <class T> typename add_rvalue_reference<T>::type declval() noexcept; // as unevaluated operand

Remarks: If this function is odr-used ([basic.def.odr]), the program is ill-formed.

Remarks: The template parameter T of declval may be an incomplete type.


template <class To, class From>
  decltype(static_cast<To>(declval<From>())) convert(From&&);

declares a function template convert which only participates in overloading if the type From can be explicitly converted to type To. For another example see class template common_type ([meta.trans.other]).  — end example ]