9 Declarations [dcl.dcl]

9.5 Structured binding declarations [dcl.struct.bind]

A structured binding declaration introduces the identifiers , , of the identifier-list as names ([basic.scope.declarative]) of structured bindings.
Let cv denote the cv-qualifiers in the decl-specifier-seq and S consist of the storage-class-specifiers of the decl-specifier-seq (if any).
A cv that includes volatile is deprecated; see [depr.volatile.type].
First, a variable with a unique name e is introduced.
If the assignment-expression in the initializer has array type A and no ref-qualifier is present, e is defined by
attribute-specifier-seq S cv A e ;
and each element is copy-initialized or direct-initialized from the corresponding element of the assignment-expression as specified by the form of the initializer.
Otherwise, e is defined as-if by
attribute-specifier-seq decl-specifier-seq ref-qualifier e initializer ;
where the declaration is never interpreted as a function declaration and the parts of the declaration other than the declarator-id are taken from the corresponding structured binding declaration.
The type of the id-expression e is called E.
Note
:
E is never a reference type ([expr.prop]).
— end note
 ]
If the initializer refers to one of the names introduced by the structured binding declaration, the program is ill-formed.
If E is an array type with element type T, the number of elements in the identifier-list shall be equal to the number of elements of E.
Each v is the name of an lvalue that refers to the element i of the array and whose type is T; the referenced type is T.
Note
:
The top-level cv-qualifiers of T are cv.
— end note
 ]
Example
:
auto f() -> int(&)[2];
auto [ x, y ] = f();            // x and y refer to elements in a copy of the array return value
auto& [ xr, yr ] = f();         // xr and yr refer to elements in the array referred to by f's return value
— end example
 ]
Otherwise, if the qualified-id std​::​tuple_­size<E> names a complete class type with a member named value, the expression std​::​tuple_­size<E>​::​value shall be a well-formed integral constant expression and the number of elements in the identifier-list shall be equal to the value of that expression.
Let i be an index prvalue of type std​::​size_­t corresponding to .
The unqualified-id get is looked up in the scope of E by class member access lookup ([basic.lookup.classref]), and if that finds at least one declaration that is a function template whose first template parameter is a non-type parameter, the initializer is e.get<i>().
Otherwise, the initializer is get<i>(e), where get is looked up in the associated namespaces ([basic.lookup.argdep]).
In either case, get<i> is interpreted as a template-id.
Note
:
Ordinary unqualified lookup ([basic.lookup.unqual]) is not performed.
— end note
 ]
In either case, e is an lvalue if the type of the entity e is an lvalue reference and an xvalue otherwise.
Given the type designated by std​::​tuple_­element<i, E>​::​type and the type designated by either & or &&, where is an lvalue reference if the initializer is an lvalue and an rvalue reference otherwise, variables are introduced with unique names as follows:
S U r = initializer ;
Each is the name of an lvalue of type that refers to the object bound to ; the referenced type is .
Otherwise, all of E's non-static data members shall be direct members of E or of the same base class of E, well-formed when named as e.name in the context of the structured binding, E shall not have an anonymous union member, and the number of elements in the identifier-list shall be equal to the number of non-static data members of E.
Designating the non-static data members of E as , , (in declaration order), each v is the name of an lvalue that refers to the member m of e and whose type is cv , where is the declared type of that member; the referenced type is cv .
The lvalue is a bit-field if that member is a bit-field.
Example
:
struct S { int x1 : 2; volatile double y1; };
S f();
const auto [ x, y ] = f();
The type of the id-expression x is “const int”, the type of the id-expression y is “const volatile double.
— end example
 ]