23 General utilities library [utilities]

23.2 Utility components [utility]

23.2.7 Function template declval [declval]

The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands.
template <class T> add_rvalue_reference_t<T> declval() noexcept; // as unevaluated operand
Remarks: If this function is odr-used, the program is ill-formed.
Remarks: The template parameter T of declval may be an incomplete type.
[Example
:
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
]