1. This revision
This revision contains only wording for the changes approved by EWG:
By value capture of structured bindings
static and thread_local for structured binding declarations
and various proposed wording fixes by Core.
Better wording for Core
[[ maybe_unused ]]
Wording as reviewed by Core for what EWG approved:
by-value capture of SBs
static and thread_local
There are a lot of restriction on structured bindings compared to variable declarations, like not being able to mark them
or them not having unclear linkage. This proposal’s aim is to fix this by for example making the underlying structured binding object (and tuple binding variables) have external linkage and by allowing various specifiers (
) on structured bindings.
Structured bindings, although very useful, are actually pretty magical. They don’t introduce variables in the normal sense for each binding, rather, they are names that refer to specific objects. As such, there are problems including what it means for a binding to be
and how it would work and what linkage do those bindings even have.
Those problems could not be resolved during the discussion of the paper and afterwards, a paper was requested to analyse the possible design impact that such additions to structured bindings would have after two NB comments proposing this were rejected. This paper attempts to do so.
One motivation to do so is to bring structured bindings closer to actual variable declarations, so consistency. This will also make structured bindings more useful, as they are currently lacking for example
, which is becoming every more important for various features of the language.
As a consequence this paper also fixes some DRs that were filed and are under consideration or going to be eventually by either Evolution or Core.
As per [basic.link]p8, bindings do not have any linkage because they’re just names. Currently, in both gcc and clang, tuple binding have linkage, as they were specified as variable declarations in the standard. This is no longer the case though due to the resolution of DR23131.
But the underlying structured binding object is an actual variable, which can have either internal or external linkage depending on the declaration of the structured binding, as gcc and clang do it. There is no way to refer to that object though without making the program IF-NDR as the object has a name like
[dcl.struct.bind]p1 has the following to say about the object:
First, a variable with a unique name e is introduced.
It follows that the variable cannot be referenced in a conforming program anyways, and as such, it doesn’t make much sense to give it external linkage. Nonetheless, to be consistent with the rest of the declarations and being able to use just
(see below) without an extra
to give the structured binding external linkage, the underlying object should have external linkage.
As discussed in the previous section, there is no way to reference either the bindings or the underlying object within a conforming program, so allowing
on such a structured binding would not make much sense.
However, this would prohibit
on structured bindings that have been declared
, which might be desirable in some cases. For this reason, it should be allowed. Note that
would have no effect on the individual bindings, except for the tuple case.
7. Static and thread_local
on a structured binding make sense and are actually useful. The way to make this work nicely in the standard is to only apply them on the underlying object, and not on the bindings (which wouldn’t make sense and can’t work today without major changes to the specification of bindings anyways). For the tuple case on the additional variable declarations too.
Because bindings refer to certain objects (depending on the initializer of the structured binding), they would implicitly get the desired semantics of
, as they refer to either objects within the underlying object or to separate variables (in the tuple case) which are marked with the desired specifiers.
is also useful and will be consistent with inline variables. It will work just as with
: The underlying object is marked
and any additional variables introduced as part of tuple bindings.
were applied just like
and co. are, then there would be a problem, because the current language rules make the following code ill-formed:
// at block scope constexpr auto [ a ] = std :: tuple < int > ( 1 ); // "equivalent" to constexpr auto __sb = std :: tuple < int > ( 1 ); constexpr const int & __a = std :: get < 0 > ( __sb ); // ill-formed today
A reference must be initialized by a constant expression to be a core constant expression ([expr.const]p2.11], but
is not a constant expression due to [expr.const]p6.
Richard Smith on the core reflector2 suggested to relax the restriction on what constitutes a core constant expression of a reference by just requiring that the reference must be initialized by a core constant expression instead (see below to what this change entails).
This would mean that to make structured bindings
, it is necessary to apply
to the underlying object, and apply
to any other variable introduced by tuple bindings.
Of course, one thing to note is that it is important to guarantee that the call to
is a constant expression, because or else
will act like
, which is only maybe a core constant expression.
10. Lambda captures
Lambda captures aren’t currently allowed to refer to a structured binding. There seems to be no technical reason to disallow this, and indeed, the wording for allowing this just removes the restriction on capturing structured bindings.
cannot be applied to a structured binding declaration. There doesn’t seem to be a good reason to disallow this; EDG, clang and gcc all already support
on structured bindings.
The following code is ill-formed as of C++20:
template < auto Var > constexpr auto [ X , Y ] = Var ;
As it uses 1) a structured binding declaration template and 2) it is constexpr. The latter is already handled. Should a structured binding be a valid template-declaration? The author argues that yes, it should be. It allows for code that can decompose any non-type template parameter (which now can be any class type, thanks to P0732r2).
constexpr std :: pair Position ( 1 , 2 ); constexpr std :: pair Flag ( 4 , 5 ); if ( X < Position > == Y < Flag > ) ; // almost there!
Note: The author doesn’t know how to change the standard to allow this completely.
This proposal only makes ill-formed or code with unspecified behavior well-formed in relation to structured bindings.
tuple binding variables requiring a change to what constitutes a core constant expression, ill-formed code today will become well-formed:
// at block scope const int var = 1 ; const int & ref = var ; static_assert ( ref == 1 ); // ill-formed today, well-formed with this proposal
14. Proposed wording
Change [expr.prim.lambda.capture]p8 (126.96.36.199) as follows:
If a lambda-expression explicitly captures an entity that is not odr-usable
or captures a structured binding (explicitly or implicitly), the program is ill-formed.
Change [expr.prim.lambda.capture]p12 (188.8.131.52) as follows:
A bit-field or a member of an anonymous union shall not be captured by reference.
Change [dcl.struct.bind]p1 (9.5) as follows:
Let cv denote the cv-qualifiers in the decl-specifier-seq . [...] If the assignment-expression in the initializer has array type
and no ref-qualifier is present,
e has type cv A
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.
Change [dcl.struct.bind]p4 (9.5) as follows:
Given the type Ti designated by
, variables are introduced with unique names ri
std :: tuple_element < i , E >:: type
of type "reference to Ti" initialized with the initializer, where the referernce is an lvalue reference if the initializer is an lvalue and an rvalue reference otherwise.
Change [dcl.dcl]p8 (10) completely as follows:
shallcontain only the type-specifier
Change [dcl.stc]p3 (10.1.1) as follows:
specifier indicates that the named entity has thread storage duration. It shall be applied only to the names of variables of namespace or block scope and to the names of static data members.
Change [dcl.stc]p4 (10.1.1) as follows:
specifier can be applied only to names of variables and functions and to anonymous unions.
Thanks to Jens for reviewing the first draft of the proposed wording.