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cppdraft/meta/reflection/extract.md

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+[meta.reflection.extract]
+
+# 21 Metaprogramming library [[meta]](./#meta)
+
+## 21.4 Reflection [[meta.reflection]](meta.reflection#extract)
+
+### 21.4.12 Value extraction [meta.reflection.extract]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5837)
+
+The extract function template may be used
+to extract a value out of a reflection when its type is known[.](#1.sentence-1)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5841)
+
+The following are defined for exposition only
+to aid in the specification of extract[.](#2.sentence-1)
+
+[🔗](#itemdecl:1)
+
+`template<class T>
+  consteval T extract-ref(info r);      // exposition only
+`
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5851)
+
+[*Note [1](#note-1)*:
+
+T is a reference type[.](#3.sentence-1)
+
+— *end note*]
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5856)
+
+*Returns*: If r represents an object O,
+then a reference to O[.](#4.sentence-1)
+
+Otherwise, a reference to the object declared, or referred to,
+by the variable represented by r[.](#4.sentence-2)
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5863)
+
+*Throws*: meta​::​exception unless
+
+- [(5.1)](#5.1)
+
+r represents a variable or object of type U,
+
+- [(5.2)](#5.2)
+
+is_convertible_v<remove_reference_t<U>(*)[],​ remove_reference_t<​T>(​*)[]> is true,  
+and
+  [*Note [2](#note-2)*:
+  The intent is to allow only qualification conversion from U to T[.](#5.2.sentence-1)
+ — *end note*]
+
+- [(5.3)](#5.3)
+
+If r represents a variable,
+ then either that variable is usable in constant expressions
+ or its lifetime began within the core constant expression currently under evaluation[.](#5.sentence-1)
+
+[🔗](#itemdecl:2)
+
+`template<class T>
+  consteval T extract-member-or-function(info r);       // exposition only
+`
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5888)
+
+*Returns*: 
+
+- [(6.1)](#6.1)
+
+  If T is a pointer type,
+ then a pointer value pointing to the function represented by r[.](#6.1.sentence-1)
+
+- [(6.2)](#6.2)
+
+  Otherwise, a pointer-to-member value
+ designating the non-static data member or function represented by r[.](#6.2.sentence-1)
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5899)
+
+*Throws*: meta​::​exception unless
+
+- [(7.1)](#7.1)
+
+r represents a non-static data member with type X,
+ that is not a bit-field,
+ that is a direct member of class C, T and X C​::​* are similar types ([[conv.qual]](conv.qual "7.3.6 Qualification conversions")), and is_convertible_v<X C​::​*, T> is true;
+
+- [(7.2)](#7.2)
+
+r represents an implicit object member function
+ with type F or F noexcept that is a direct member of a class C,
+ and T is F C​::​*; or
+
+- [(7.3)](#7.3)
+
+r represents a non-member function,
+ static member function, or
+ explicit object member function
+ of function type F or F noexcept,
+ and T is F*[.](#7.sentence-1)
+
+[🔗](#itemdecl:3)
+
+`template<class T>
+  consteval T extract-value(info r);    // exposition only
+`
+
+[8](#8)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5929)
+
+Let U be the type of the value or object that r represents[.](#8.sentence-1)
+
+[9](#9)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5932)
+
+*Returns*: static_cast<T>([:R:]),
+where R is a constant expression of type info such that R == r is true[.](#9.sentence-1)
+
+[10](#10)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5938)
+
+*Throws*: meta​::​exception unless
+
+- [(10.1)](#10.1)
+
+U is a pointer type, T and U are either similar ([[conv.qual]](conv.qual "7.3.6 Qualification conversions"))
+ or both function pointer types, and is_convertible_v<U, T> is true,
+
+- [(10.2)](#10.2)
+
+U is not a pointer type
+ and the cv-unqualified types of T and U are the same,
+
+- [(10.3)](#10.3)
+
+U is an array type, T is a pointer type, and
+ the value r represents is convertible to T, or
+
+- [(10.4)](#10.4)
+
+U is a closure type, T is a function pointer type, and
+ the value that r represents is convertible to T[.](#10.sentence-1)
+
+[🔗](#lib:extract)
+
+`template<class T>
+  consteval T extract(info r);
+`
+
+[11](#11)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5968)
+
+Let U be remove_cv_t<T>[.](#11.sentence-1)
+
+[12](#12)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5971)
+
+*Effects*: Equivalent to:if constexpr (is_reference_type(^^T)) {return *extract-ref*<T>(r);} else if constexpr (is_nonstatic_data_member(r) || is_function(r)) {return *extract-member-or-function*<U>(r);} else {return *extract-value*<U>(constant_of(r));}