Home - News ( United States | United Kingdom | Italy | Germany ) - Football scores

Showing content from https://cplusplus.github.io/LWG/issue3994 below:

adaptor(args...)(r) is not equivalent to std::bind_back(adaptor, args...)(r)

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

Section: 25.7.2 [range.adaptor.object] Status: New Submitter: Hewill Kang Opened: 2023-10-11 Last modified: 2023-11-03

Priority: 4

View other active issues in [range.adaptor.object].

View all other issues in [range.adaptor.object].

View all issues with New status.

Discussion:

25.7.2 [range.adaptor.object] p8 specifies that:

The expression adaptor(args...) produces a range adaptor closure object f that is a perfect forwarding call wrapper (22.10.4 [func.require]) with the following properties:

According to the subsequent description, it can be inferred that the behavior is similar to std::bind_back(adaptor, args...) which also returns a perfect forwarding call wrapper.

Every call wrapper (22.10.3 [func.def]) meets the Cpp17MoveConstructible and Cpp17Destructible requirements.

In order to conform with the specification, standard functions that return perfect forwarding call wrappers such as std::bind_front/back and std::not_fn all Mandates that (is_constructible_v<BoundArgs, Args> && ...) and (is_move_constructible_v<BoundArgs> && ...) are each true, the former condition corresponds to 25.7.2 [range.adaptor.object] p8:

The expression adaptor(args...) is well-formed if and only if the initialization of the bound argument entities of the result, as specified above, are all well-formed.

However, the latter does not have a corresponding description in <ranges>. In other words, range adaptor objects do not explicitly indicate that the bound argument must be move-constructible. This results in implementation divergence for some uncommon types (demo):

#include <ranges>
#include <string_view>

constexpr struct WeirdFive {
  WeirdFive() = default;
  WeirdFive(const WeirdFive&) = default;
  constexpr operator int() const { return 5; }

  WeirdFive(WeirdFive&&) = delete;
} five;

constexpr std::string_view sv{"hello"};
static_assert(sv == std::views::take(five)(sv)); // libstdc++/libc++ reject, MSVC-STL accepts

Above, libstdc++ always moves arguments into internal members, which leads to hard errors in the member initializer list; libc++ uses std::bind_back for argument binding, which also leads to hard errors in the function body as the former requires arguments to be move-constructible; MSVC-STL is the most compliant with current wording.

[2023-11-02; Reflector poll]

Set priority to 4 after reflector poll.

Proposed resolution:

RetroSearch is an open source project built by @garambo | Open a GitHub Issue

Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo

HTML: 3.2 | Encoding: UTF-8 | Version: 0.7.4