23 General utilities library [utilities]

23.2 Utility components [utility]

23.2.7 Function template declval [declval]

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