1 Revision History

R0 of this paper [P1061R0] was presented to EWGI in Kona 2019 [P1061R0.Minutes], who reviewed it favorably and thought this was a good investment of our time (4-3-4-1-0). The consensus in the room was that the restriction that the introduced pack need not be the trailing identifier.

2 Motivation

Function parameter packs and tuples are conceptually very similar. Both are heterogeneous sequences of objects. Some problems are easier to solve with a parameter pack, some are easier to solve with a tuple. Today, it’s trivial to convert a pack to a tuple, but it’s somewhat more involved to convert a tuple to a pack. You have to go through std::apply() [N3915]:

std::tuple<A, B, C> tup = ...;
std::apply([&](auto&&... elems){
    // now I have a pack
}, tup);

This is great for cases where we just need to call a [non-overloaded] function or function object, but rapidly becomes much more awkward as we dial up the complexity. Not to mention if I want to return from the outer scope based on what these elements have to be.

How do we compute the dot product of two tuples? It’s a choose your own adventure of awkward choices:

template <class P, class Q>
auto dot_product(P p, Q q) {
    return std::apply([&](auto... p_elems){
        return std::apply([&](auto... q_elems){
            return (... + (p_elems * q_elems));
        }, q)
    }, p);
}

template <size_t... Is, class P, class Q>
auto dot_product(std::index_sequence<Is...>, P p, Q, q) {
    return (... + (std::get<Is>(p) * std::get<Is>(q)));
}

template <class P, class Q>
auto dot_product(P p, Q q) {
    return dot_product(
        std::make_index_sequence<std::tuple_size<P>::value>{},
        p, q);
}

Regardless of which option you dislike the least, both are limited to only std::tuples. We don’t have the ability to do this at all for any of the other kinds of types that can be used in a structured binding declaration [P0144R2] - because we need to explicit list the correct number of identifiers, and we might not know how many there are.

3 Proposal

We propose to extend the structured bindings syntax to allow the user to introduce a pack as (at most) one of the identifiers:

std::tuple<X, Y, Z> f();

auto [x,y,z] = f();          // OK today
auto [...xs] = f();          // proposed: xs is a pack of length three containing an X, Y, and a Z
auto [x, ...rest] = f();     // proposed: x is an X, rest is a pack of length two (Y and Z)
auto [x,y,z, ...rest] = f(); // proposed: rest is an empty pack
auto [x, ...rest, z] = f();  // proposed: x is an X, rest is a pack of length one
                             //   consisting of the Y, z is a Z
auto [...a, ...b] = f();     // ill-formed: multiple packs

If we additionally add the structured binding customization machinery to std::integer_sequence, this could greatly simplify generic code:

namespace detail {
    template <class F, class Tuple, std::size_t... I>
    constexpr decltype(auto) apply_impl(F &&f, Tuple &&t,
        std::index_sequence<I...>) 
    {
        return std::invoke(std::forward<F>(f),
            std::get<I>(std::forward<Tuple>(t))...);
    }
}
 
template <class F, class Tuple>
constexpr decltype(auto) apply(F &&f, Tuple &&t) 
{
    return detail::apply_impl(
        std::forward<F>(f), std::forward<Tuple>(t),
        std::make_index_sequence<std::tuple_size_v<
            std::decay_t<Tuple>>>{});
}

template <class F, class Tuple>
constexpr decltype(auto) apply(F &&f, Tuple &&t)
{
    auto&& [...elems] = t;
    return std::invoke(std::forward<F>(f),
        forward_like<Tuple, decltype(elems)>(elems)...);
}

template <class P, class Q>
auto dot_product(P p, Q q) {
    return std::apply([&](auto... p_elems){
        return std::apply([&](auto... q_elems){
            return (... + (p_elems * q_elems));
        }, q)
    }, p);
}

template <class P, class Q>
auto dot_product(P p, Q q) {
    // no indirection!
    auto&& [...p_elems] = p;
    auto&& [...q_elems] = q;
    return (... + (p_elems * q_elems));
}

template <size_t... Is, class P, class Q>
auto dot_product(std::index_sequence<Is...>, P p, Q, q) {
    return (... + (std::get<Is>(p) * std::get<Is>(q)));
}

template <class P, class Q>
auto dot_product(P p, Q q) {
    return dot_product(
        std::make_index_sequence<std::tuple_size_v<P>>{},
        p, q);
}

template <class P, class Q>
auto dot_product(P p, Q q) {
    // no helper function necessary!
    auto [...Is] = std::make_index_sequence<
        std::tuple_size_v<P>>{};
    return (... + (std::get<Is>(p) * std::get<Is>(q)));
}

Not only are these implementations more concise, but they are also more functional. I can just as easily use apply() with user-defined types as I can with std::tuple:

struct Point {
    int x, y, z;
};

Point getPoint();
double calc(int, int, int);

double result = std::apply(calc, getPoint()); // ill-formed today, ok with proposed implementation

3.1 Other Languages

Python 2 had always allowed for a syntax similar to C++17 structured bindings, where you have to provide all the identifiers:

>>> a, b, c, d, e = range(5) # ok
>>> a, *b = range(3)
  File "<stdin>", line 1
    a, *b = range(3)
       ^
SyntaxError: invalid syntax       

But you could not do any more than that. Python 3 went one step further by way of PEP-3132 [PEP.3132]. That proposal allowed for a single starred identifier to be used, which would bind to all the elements as necessary:

>>> a, *b, c = range(5)
>>> a
0
>>> c
4
>>> b
[1, 2, 3]

The Python 3 behavior is synonymous with what is being proposed here. Notably, from that PEP:

Possible changes discussed were:

• Only allow a starred expression as the last item in the exprlist. This would simplify the unpacking code a bit and allow for the starred expression to be assigned an iterator. This behavior was rejected because it would be too surprising.

R0 of this proposal only allowed a pack to be introduced as the last item, which was changed in R1.

4 Wording

Add a new grammar option for simple-declaration to 9 [dcl.dcl]:

sb-identifier:
    ... identifier

sb-identifier-list:
    identifier
    sb-identifier
    sb-identifier-list , identifier
    sb-identifier-list , sb-identifier

simple-declaration:
    decl-specifier-seq init-declarator-list_opt ;
    attribute-specifier-seq decl-specifier-seq init-declarator-list ;
    attribute-specifier-seq_opt decl-specifier-seq ref-qualifier_opt [ identifier-list sb-identifier-list ] initializer ;

Change 9 [dcl.dcl] paragraph 8:

A simple-declaration with an identifier-list sb-identifier-list is called a structured binding declaration ( [dcl.struct.bind]). The decl-specifier-seq shall contain only the type-specifier auto and cv-qualifiers. The initializer shall be of the form “= assignment-expression”, of the form “{ assignment-expression }”, or of the form “( assignment-expression )”, where the assignment-expression is of array or non-union class type.

Change 9.5 [dcl.struct.bind] paragraph 1:

A structured binding declaration introduces the identifiers v₀, v₁, v₂, … of the identifier-list sb-identifier-list as names ([basic.scope.declarative]) of structured bindings. The declaration shall contain at most one sb-identifier. If the declaration contains an sb-identifier, the declaration introduces a structured binding pack ([temp.variadic]). Let cv denote the cv-qualifiers in the decl-specifier-seq.

Introduce a new paragraph after 9.5 [dcl.struct.bind] paragraph 1, introducing the term “structured binding size”:

The structured binding size of a type E is the required number of names that need to be introduced by the structured binding declaration, as defined below. If there is no structured binding pack, then the number of elements in the sb-identifier-list shall be equal to the structured binding size. Otherwise, the number of elements of the structured binding pack is the structured binding size less the number of elements in the sb-identifier-list.

Change 9.5 [dcl.struct.bind] paragraph 3 to define a structured binding size:

If E is an array type with element type T, the number of elements in the identifier-list the structured binding size of E shall be equal to the number of elements of E. Each v_i The i^th identifier 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.

Change 9.5 [dcl.struct.bind] paragraph 3 to define a structured binding size:

Otherwise, if the qualified-id std::tuple_size<E> names a complete type, the expression std::tuple_size<E>::value shall be a well-formed integral constant expression and the number of elements in the identifier-list structured binding size of E shall be equal to the value of that expression. […] Each v_i The i^th identifier is the name of an lvalue of type Ti that refers to the object bound to ri; the referenced type is Ti.

Change 9.5 [dcl.struct.bind] paragraph 5 to define a structured binding size:

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 structured binding size of E shall be equal to the number of non-static data members of E. Designating the non-static data members of E as m0, m1, m2, . . . (in declaration order), each vi the i^th identifier is the name of an lvalue that refers to the member mi of e and whose type is cv Ti, where Ti is the declared type of that member; the referenced type is cv Ti. The lvalue is a bit-field if that member is a bit-field.

Add a new clause to 13.6.3 [temp.variadic], after paragraph 3:

A structured binding pack is an identifier that introduces zero or more structured bindings ([dcl.struct.bind]). [ Example

auto foo() -> int(&)[2];
auto [...a] = foo();          // a is a structured binding pack containing 2 elements
auto [b, c, ...d] = foo();    // d is a structured binding pack containing 0 elements
auto [e, f, g, ...h] = foo(); // error: too many identifiers

• end example]

In 13.6.3 [temp.variadic], change paragraph 4:

A pack is a template parameter pack, a function parameter pack, or an init-capture pack, or a structured binding pack. The number of elements of a template parameter pack or a function parameter pack is the number of arguments provided for the parameter pack. The number of elements of an init-capture pack is the number of elements in the pack expansion of its initializer.

In 13.6.3 [temp.variadic], paragraph 5 (describing pack expansions) remains unchanged.

In 13.6.3 [temp.variadic], add a bullet to paragraph 8:

Such an element, in the context of the instantiation, is interpreted as follows:

• if the pack is a template parameter pack, the element is a template parameter ([temp.param]) of the corresponding kind (type or non-type) designating the i^th corresponding type or value template argument;
• if the pack is a function parameter pack, the element is an id-expression designating the i^th function parameter that resulted from instantiation of the function parameter pack declaration; otherwise
• if the pack is an init-capture pack, the element is an id-expression designating the variable introduced by the i^thth init-capture that resulted from instantiation of the init-capture pack. ; otherwise
• if the pack is a structured binding pack, the element is an id-expression designating the i^th structured binding that resulted from the structured binding declaration.

5 Acknowledgements

Thanks to Michael Park and Tomasz Kamiński for their helpful feedback.

6 References