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This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 118f. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-11-07

2867. Order of initialization for structured bindings

[Accepted as a DR at the June, 2024 meeting.]

Consider:

  auto [a, b] = f(X{});

IfX is a tuple-like type, this is transformed toapproximately the following:

  auto e = f(X{});  T1 &a = get<0>(std::move(e));  T2 &b = get<1>(std::move(e));

However, the sequencing of the initializations of e, a, and b isnot specified. Further, the temporaryX{} should bedestroyed after the initializations of a and b.

Possible resolution [SUPERSEDED]:

1. Change in 6.10.1 [intro.execution] paragraph 5 as follows:

   Afull-expression is

   • an unevaluated operand (7.2.3 [expr.context]),
   • aconstant-expression (7.7 [expr.const]),
   • an immediate invocation (7.7 [expr.const]),
   • aninit-declarator (9.3 [dcl.decl]) oramem-initializer (11.9.3 [class.base.init]), including theconstituent expressions of the initializer,
   • theinitializers for all uniquely-namedvariables introduced by a structured binding declaration(9.7 [dcl.struct.bind]), including the constituent expressions ofall initializers,
   • an invocation of a destructor generated atthe end of the lifetime of an object other than a temporary object(6.8.7 [class.temporary]) whose lifetime has not been extended, or
   • an expression that is not a subexpression of another expressionand that is not otherwise part of a full-expression.

   ...

2. Change in 9.7 [dcl.struct.bind] paragraph 4 as follows:

   ... Each v_i is the name of an lvalue of type T_ithat refers to the object bound to r_i; the referenced typeis T_i.The initialization of e is sequenced before the initialization ofany r_i. The initialization of r_i is sequencedbefore the initialization of r_j if i < j.

CWG 2024-05-03

CWG tentatively agreed that all temporaries in a structured binding(including those from default arguments ofget invocations)should persist until the semicolon on the declaration as written.

Possible resolution [SUPERSEDED]:

1. Change in 6.10.1 [intro.execution] paragraph 5 as follows:

   Afull-expression is

   • an unevaluated operand (7.2.3 [expr.context]),
   • aconstant-expression (7.7 [expr.const]),
   • an immediate invocation (7.7 [expr.const]),
   • aninit-declarator (9.3 [dcl.decl]), aninitializer of a structuredbinding declaration (9.1 [dcl.pre]), oramem-initializer (11.9.3 [class.base.init]), including theconstituent expressions of the initializer,
   • an invocation of a destructor generated atthe end of the lifetime of an object other than a temporary object(6.8.7 [class.temporary]) whose lifetime has not been extended, or
   • an expression that is not a subexpression of another expressionand that is not otherwise part of a full-expression.

   ...

2. Change in 9.7 [dcl.struct.bind] paragraph 4 as follows:

   ... Each v_i is the name of an lvalue of type T_ithat refers to the object bound to r_i; the referenced typeis T_i.The initialization of e is sequenced before the initialization ofany r_i. The initialization of each r_i is sequencedbefore the initialization of any r_j where i < j.

3. Append a new paragraph at the end of 9.7 [dcl.struct.bind] as follows:

   ... [ Example: ...The type of the id-expression x is "int", the type of the id-expression y is "const volatile double". -- end example ]

   The initialization ofe and of any r_i are sequencedbefore the destruction of any temporary object introduced bytheinitializer fore or by theinitializers forthe r_i. The temporary objects are destroyed in thereverse order of their construction.

CWG 2024-06-14

The specification for the lifetime of temporaries should be movedto 6.8.7 [class.temporary].)

Proposed resolution (approved by CWG 2024-06-28):

1. Change in 6.8.7 [class.temporary] paragraph 5 as follows:

   There arefourfive contexts in whichtemporaries are destroyed at a different point than the end of thefull-expression. ...

2. Insert a new paragraph after 6.8.7 [class.temporary] paragraph 7 as follows:

   The fourth context is when a temporary object is created inthefor-range-initializer of a range-based for statement. Ifsuch a temporary object would otherwise be destroyed at the end ofthefor-range-initializer full-expression, the object persistsfor the lifetime of the reference initialized bythefor-range-initializer.

   The fifth context is when a temporary object is created in astructured binding declaration (9.7 [dcl.struct.bind]). Anytemporary objects introduced by theinitializers for thevariables with unique names are destroyed at the end ofthe structured binding declaration.

3. Change in 6.10.1 [intro.execution] paragraph 5 as follows:

   Afull-expression is

   • an unevaluated operand (7.2.3 [expr.context]),
   • aconstant-expression (7.7 [expr.const]),
   • an immediate invocation (7.7 [expr.const]),
   • aninit-declarator (9.3 [dcl.decl])(including such introduced by a structured binding (9.7 [dcl.struct.bind])) oramem-initializer (11.9.3 [class.base.init]), including theconstituent expressions of the initializer,
   • an invocation of a destructor generated atthe end of the lifetime of an object other than a temporary object(6.8.7 [class.temporary]) whose lifetime has not been extended, or
   • an expression that is not a subexpression of another expressionand that is not otherwise part of a full-expression.

   ...

4. Change in 9.7 [dcl.struct.bind] paragraph 4 as follows:

   ... Each v_i is the name of an lvalue of type T_ithat refers to the object bound to r_i; the referenced typeis T_i.The initialization of e is sequenced before the initialization ofany r_i. The initialization of each r_i is sequencedbefore the initialization of any r_j where i < j.