Make std::invoke and std::reference_wrapper constexpr

[benni...@gmail.com]

I want to add constexpr to the following functions:

20.14.3 Function template invoke [func.invoke]

template <class F, class... Args>
  constexpr result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args);

20.14.4 Class template reference_wrapper [refwrap]

namespace std {
  template <class T> class reference_wrapper {
  public :
    // types
    using type = T;

    // construct/copy/destroy
    constexpr reference_wrapper(T&) noexcept;
    reference_wrapper(T&&) = delete; // do not bind to temporary objects
    constexpr reference_wrapper(const reference_wrapper& x) noexcept;

    // assignment
    constexpr reference_wrapper& operator=(const reference_wrapper& x) noexcept;

    // access
    constexpr operator T& () const noexcept;
    constexpr T& get() const noexcept;

    // invocation
    template <class... ArgTypes>
    constexpr result_of_t<T&(ArgTypes&&...)>
    operator() (ArgTypes&&...) const;
  };
}

20.14.4.5 reference_wrapper helper functions [refwrap.helpers]

template <class T> constexpr reference_wrapper<T> ref(T& t) noexcept;

template <class T> constexpr reference_wrapper<T> ref(reference_wrapper<T> t) noexcept;

template <class T> constexpr reference_wrapper<const T> cref(const T& t) noexcept;

template <class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;


invoke and reference_wrapper are very basic and powerful tools that might be used in constexpr functions. I'm working with C++17 for a while and need to implement constexpr versions by my self to use them in constexpr expressions.

[Richard Smith]

Yes please :)

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[benni...@gmail.com]

I did write a proposal, please comment ;-)

http://htmlpreview.github.io/?https://github.com/bebuch/proposals/blob/master/Make%20std::invoke%20and%20std::reference_wrapper%20constexpr.html
https://github.com/bebuch/proposals/blob/master/Make%20std::invoke%20and%20std::reference_wrapper%20constexpr.html

Yes please :)

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[benni...@gmail.com]

I renamed my proposal and added a constexpr wording for std::not_fn.

http://htmlpreview.github.io/?https://github.com/bebuch/proposals/blob/master/Add%20constexpr%20to%20functional%20header%20functions.html
https://github.com/bebuch/proposals/blob/master/Add%20constexpr%20to%20functional%20header%20functions.html

I will check if I can add constexpr to std::bind tomorrow.

[Tomasz]

W dniu wtorek, 27 września 2016 13:02:06 UTC+2 użytkownik benni...@gmail.com napisał:

I want to add constexpr to the following functions:

The problem you need to discuss is that per current resolution of the http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1581 adding constexpr to the invoke and other functional components, would break existing code. The actual examples may be found at:
https://llvm.org/bugs/show_bug.cgi?id=23141
https://llvm.org/bugs/show_bug.cgi?id=23135
I believe you paper need to discuss the scope of the issues and propose some resolutions workarounds.

[benni...@gmail.com]

Hi Tomasz,

thank you very much for the links, I will try to write a discussion about that. Interesting to learn that constexpr can break existing code. Any help is welcome of course. :-)