Jens Maurer <Jens.Maurer@gmx.net>
Audience: Evolution and Core Working Groups

P0217R0: Proposed wording for structured bindings


This paper presents the proposed wording to implement structured bindings as described by Herb Sutter, Bjarne Stroustrup, and Gabriel dos Reis in P0144R1.


In section 6.5 [stmt.iter] paragraph 1, change the grammar to allow a decomposition declaration:
      attribute-specifier-seqopt decl-specifier-seq declarator
      attribute-specifier-seqopt decl-specifier-seq &opt { identifier-list }
In section 7 [dcl.dcl] paragraph 1, change the grammar to allow a decomposition declaration:
      decl-specifier-seq init-declarator-listopt ;
      attribute-specifier-seq decl-specifier-seq init-declarator-list ;
      attribute-specifier-seqopt decl-specifier-seq &opt { identifier-list } brace-or-equal-initializer ;
Add a new paragraph after 7 [dcl.dcl] paragraph 8:
A simple-declaration with an identifier-list is called a decomposition declaration. The decl-specifier-seq shall contain only the type-specifier auto ( [dcl.spec.auto]) and cv-qualifiers. The brace-or-equal-initializer shall be of the form "= assignment-expression" or of the form "{ assignment-expression }". If the assignment-expression is a prvalue, the decl-specifier-seq does not contain the cv-qualifier const, and & follows the decl-specifier-seq, the program is ill-formed.
Add after [dcl.spec.auto] paragraph 8:
A decomposition declaration introduces the identifiers of the identifier-list as variable names in the order of appearance, with types deduced from the initializer. If the assignment-expression of the decomposition declaration has scalar type, the identifier-list shall contain a single identifier and a single variable is introduced as if by
  attribute-specifier-seqopt decl-specifier-seq &opt identifier brace-or-equal-initializer ;
If the assignment-expression has union type, the program is ill-formed.

Otherwise, i.e. the assignment-expression has non-union class or array type E, let e be a glvalue of type E.

The expression getn is defined as follows: If E is an array type, the number of identifiers shall be equal to the number of elements of E. Otherwise, if the expression get0 is not well-formed, all of E's non-static data members and bit-fields shall be public direct members of E or of the same public base class of E, E shall not have an anonymous union member, and the number of identifiers in the identifier-list shall be equal to the number of non-static data members of E.

For the i-th identifier vi in the identifier-list, where numbering starts at 1, a variable is declared using the attribute-specifier-seq and decl-specifier-seq from the decomposition declaration, and a declarator &opt vi , where & is present if and only if it is present in the decomposition declaration. If the expression get0 is well-formed, the variable is initialized with the expression geti-1, otherwise it is initialized with the expression e.mi , where mi designates the i-th non-static data member of E. The variable is copy-initialized or direct-initialized as indicated by the form of the brace-or-equal-initializer (8.5 [dcl.init]). [ Note: The type of vi is deduced from its initializer as described in the remainder of this section. -- end note ] [ Example:

struct S { int x; volatile double y; };
const S f();
auto { x, y } = f();
The type of x is "int", the type of y is "double". -- end example ]