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Difference between revisions of "cpp/ranges/to"

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< cpp‎ | ranges
m (add reference)
(use container's element type instead of range's element type)
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{{dcl|1=
 
{{dcl|1=
 
to<C>(r {{!}} views::transform([](auto&& elem) {
 
to<C>(r {{!}} views::transform([](auto&& elem) {
−
   return to<ranges::range_value_t<R>>(std::forward<decltype(elem)>(elem));
+
   return to<ranges::range_value_t<C>>(std::forward<decltype(elem)>(elem));
 
}, std::forward<Args>(args)...)
 
}, std::forward<Args>(args)...)
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
−
Which allows nested range constructions within the range if {{c|ranges::input_range<ranges::range_reference_t<R>>}} is {{c|true}}.  
+
Which allows nested range constructions within the range if {{c|ranges::input_range<ranges::range_reference_t<C>>}} is {{c|true}}.  
 
Otherwise, the program is ill-formed.
 
Otherwise, the program is ill-formed.
  

Revision as of 12:58, 13 February 2023

 
C++
 
Ranges library
Range adaptors
 
Defined in header <ranges>
template< class C, ranges::input_range R, class... Args >

  requires (!ranges::view<C>)

constexpr C to( R&& r, Args&&... args );
 (1) (since C++23)
template< template< class... > class C, ranges::input_range R, class... Args >
constexpr auto to( R&& r, Args&&... args );
 (2) (since C++23)
template< class C, class... Args >

  requires (!ranges::view<C>)

constexpr /*range adaptor closure*/ to( Args&&... args );
 (3) (since C++23)
template< template< class... > class C, class... Args >
constexpr /*range adaptor closure*/ to( Args&&... args );
 (4) (since C++23)
Helper templates
template< class Container >

constexpr bool /*reservable-container*/ =
  ranges::sized_range<Container> &&
  requires (Container& c, ranges::range_size_t<Container> n) {
    c.reserve(n);
    { c.capacity() } -> std::same_as<decltype(n)>;
    { c.max_size() } -> std::same_as<decltype(n)>;

  };
 (5) (since C++23)
template< class Container, class Reference >

constexpr bool /*container-insertable*/ = requires (Container& c, Reference&& ref) {
  requires (requires { c.push_back(std::forward<Reference>(ref)); } ||
            requires { c.insert(c.end(), std::forward<Reference>(ref)); });

};
 (6) (since C++23)
template< class Reference, class C >

constexpr auto /*container-inserter*/(C& c) {
  if constexpr (requires { c.push_back(std::declval<Reference>()); })
    return std::back_inserter(c);
  else
    return std::inserter(c, c.end());

}
 (7) (since C++23)
template< class R, class T >

concept /*container-compatible-range*/ =
  ranges::input_range<R> &&

  std::convertible_to<ranges::range_reference_t<R>, T>;
 (8) (since C++23)

The overloads of the range conversion function construct a new non-view object from a source range as its first argument by calling a constructor taking a range, a std::from_range_t tagged ranged constructor, a constructor taking an iterator-sentinel pair, or by back inserting each element of the source range into the arguments-constructed object.

1) Constructs an object of type C from the elements of r in the following:
a) If C does not satisfy input_range or std::convertible_to<ranges::range_reference_t<R>, ranges::range_value_t<C>> is true:
1) Constructing a non-view object as if direct-initializing (but not direct-list-initializing) an object of type C from the source range std::forward<R>(r) and the rest of the functional arguments std::forward<Args>(args)... if std::constructible_from<C, R, Args...> is true.
2) Otherwise, constructing a non-view object as if direct-initializing (but not direct-list-initializing) an object of type C from additional disambiguation tag std::from_range, the source range std::forward<R>(r) and the rest of the functional arguments std::forward<Args>(args)... if std::constructible_from<C, std::from_range_t, R, Args...> is true.
3) Otherwise, constructing a non-view object as if direct-initializing (but not direct-list-initializing) an object of type C from the iterator-sentinel pair (ranges::begin(r) as an iterator and ranges::end(r) as sentinel where iterator and sentinel have the same type. In other words, the source range must be a common range), and the rest of function arguments std::forward<Args>(args)... if all of the conditions below are true:
4) Otherwise, constructing a non-view range object as if direct-initializing (but not direct-list-initializing) an object of type C from the rest of the function arguments std::forward<Args>(args)... with the following equivalent call below after the construction:
if constexpr (ranges::sized_range<R> && /*reservable-container*/<C>)

  c.reserve(static_cast<ranges::range_size_t<C>>(ranges::size(r)));

ranges::copy(r, /*container-inserter*/<ranges::range_reference_t<R>>(c));

If the R satisfies sized_range and C satisfies /*reservable-container*/, the constructed object c of type C is able to reserve storage with the initial storage size ranges::size(r) to prevent additional allocations during inserting new elements. Each range reference element of r is back inserted to c through ranges::copy with back inserter adaptor. The operations above are valid if both of the conditions below are true:

b) Otherwise, the return expression is equivalent to:
to<C>(r | views::transform([](auto&& elem) {

  return to<ranges::range_value_t<C>>(std::forward<decltype(elem)>(elem));

}, std::forward<Args>(args)...)

Which allows nested range constructions within the range if ranges::input_range<ranges::range_reference_t<C>> is true.

Otherwise, the program is ill-formed.
2) Constructs an object of deduced type from the elements of r.

Let /*input-iterator*/ be an exposition only type that satisfies LegacyInputIterator:

struct /*input-iterator*/ {                         // exposition only

  using iterator_category = std::input_iterator_tag;
  using value_type = ranges::range_value_t<R>;
  using difference_type = std::ptrdiff_t;
  using pointer = std::add_pointer_t<ranges::range_reference_t<R>>;
  using reference = ranges::range_reference_t<R>;
  reference operator*() const;                      // not defined
  pointer operator->() const;                       // not defined
  /*input-iterator*/& operator++();                 // not defined
  /*input-iterator*/ operator++(int);               // not defined
  bool operator==(const /*input-iterator*/&) const; // not defined

};

Let /*DEDUCE-EXPR*/ be defined as follows:

The call is equivalent to to<decltype(/*DEDUCE-EXPR*/)>(std::forward<R>(r), std::forward<Args>(args)...).
3-4) Returns a perfect forwarding call wrapper that is also a RangeAdaptorClosureObject.
5) The exposition-only variable template /*reservable-container*/<Container> is true if it satisfies ranges::sized_range and is eligible to be reservable.
6) The exposition-only variable template /*container-insertable*/<Container, Reference> is true if Container is back insertable by a member function call push_back or insert.
7) The exposition-only function template /*container-inserter*/ returns an output iterator of type std::back_insert_iterator if member function push_back is available, otherwise the type is std::insert_iterator.
8) The exposition-only concept /*container-compatible-range*/ is used in the definition of containers in constructing an input range R and its range reference type must be convertible to T.

Contents

Parameters

r - a source range object
args - list of the arguments to (1-2) construct a range or (3-4) bind to the last parameters of range adaptor closure object.

Return value

1-2) a constructed non-view object
3-4) a range adaptor closure object of unspecified type, with the following properties:

ranges::to return type

The return type is derived from ranges::range_adaptor_closure</*return-type*/>.

Member objects

The returned object behaves as if it has no target object, and an std::tuple object tup constructed with std::tuple<std::decay_t<Args>...>(std::forward<Args>(args)...), except that the returned object's assignment behavior is unspecified and the names are for exposition only.

Constructors

The return type of ranges::to (3-4) behaves as if its copy/move constructors perform a memberwise copy/move. It is CopyConstructible if all of its member objects (specified above) are CopyConstructible, and is MoveConstructible otherwise.

Member function operator()

Given an object G obtained from an earlier call to range::to</* see below */>(args...), when a glvalue g designating G is invoked in a function call expression g(r), an invocation of the stored object takes place, as if by

  • ranges::to</* see below */>(r, std::get<Ns>(g.tup)...), where
  • r is a source range object that must satisfy input_range
  • Ns is an integer pack 0, 1, ..., (sizeof...(Args) - 1)
  • g is an lvalue in the call expression if it is an lvalue in the call expression, and is an rvalue otherwise. Thus std::move(g)(r) can move the bound arguments into the call, where g(r) would copy.
  • The specified template argument is (3) C or (4) the deduced type from a class template C that must not satisfy view.

The program is ill-formed if g has volatile-qualified type.

Exceptions

Only throws if construction of a range object throws.

Notes

The insertion of elements into the container may involve copy which can be more inefficient than move because lvalue references are produced during the indirection call. Users can opt-in to use views::as_rvalue to adapt the range in order for their elements to always produce an rvalue reference during the indirection call which implies move.

Feature-test macro Value Std Feature
__cpp_lib_ranges_to_container 202202L (C++23) std::ranges::to

Example

A link to test Compiler Explorer msvc.latest

Run this code
#include <algorithm>
#include <concepts>
#include <iostream>
#include <ranges>
#include <vector>
 
int main()
{
    auto vec = std::views::iota(1, 5)
             | std::views::transform([](auto const v){ return v * 2; })
             | std::ranges::to<std::vector>();
 
    static_assert(std::same_as<decltype(vec), std::vector<int>>);
 
    std::ranges::for_each(vec, [](auto const v){ std::cout << v << ' '; });
}

Output: 

2 4 6 8

References

  • C++23 standard (ISO/IEC 14882:2024):
  • 26.5.7 Range conversions [range.utility.conv]
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