[declval]

# 22 General utilities library [[utilities]](./#utilities)

## 22.2 Utility components [[utility]](utility#declval)

### 22.2.6 Function template declval [declval]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L508)

The library provides the function template declval to simplify the definition of
expressions which occur as [unevaluated operands](expr.context#def:unevaluated_operand "7.2.3 Context dependence [expr.context]")[.](#1.sentence-1)

[🔗](#lib:declval)

`template<class T> add_rvalue_reference_t<T> declval() noexcept;    // as unevaluated operand
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L518)

*Mandates*: This function is not odr-used ([[basic.def.odr]](basic.def.odr#term.odr.use "6.3 One-definition rule"))[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L522)

*Remarks*: The template parameter T of declval may be an incomplete type[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L526)

[*Example [1](#example-1)*:

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
type From can be explicitly converted to type To[.](#4.sentence-1)

For another example see class
template common_type ([[meta.trans.other]](meta.trans.other "21.3.9.7 Other transformations"))[.](#4.sentence-2)

— *end example*]