46 lines
1.6 KiB
Markdown
46 lines
1.6 KiB
Markdown
[declval]
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# 22 General utilities library [[utilities]](./#utilities)
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## 22.2 Utility components [[utility]](utility#declval)
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### 22.2.6 Function template declval [declval]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L508)
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The library provides the function template declval to simplify the definition of
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expressions which occur as [unevaluated operands](expr.context#def:unevaluated_operand "7.2.3 Context dependence [expr.context]")[.](#1.sentence-1)
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[ð](#lib:declval)
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`template<class T> add_rvalue_reference_t<T> declval() noexcept; // as unevaluated operand
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`
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L518)
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*Mandates*: This function is not odr-used ([[basic.def.odr]](basic.def.odr#term.odr.use "6.3 One-definition rule"))[.](#2.sentence-1)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L522)
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*Remarks*: The template parameter T of declval may be an incomplete type[.](#3.sentence-1)
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L526)
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[*Example [1](#example-1)*:
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template<class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&); declares a function template convert which only participates in overload resolution if the
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type From can be explicitly converted to type To[.](#4.sentence-1)
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For another example see class
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template common_type ([[meta.trans.other]](meta.trans.other "21.3.9.7 Other transformations"))[.](#4.sentence-2)
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â *end example*]
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