3

I don't understand where the requirement for moving comes from. I can't find it in forward_range and sized_sentinel... Basic example:


    #include <ranges>
    #include <string>
    #include <iostream>
    
    class vrange: public std::ranges::view_interface<vrange>
    {
        public:
            vrange(std::string &d): data(d){;};
    
            vrange(const vrange &&) = delete;
    
            auto begin() const noexcept { return data.begin(); };
            auto end() const noexcept { return data.end(); };
    
        private:
            std::string data;
    };
    
    int main(){
        std::string h("Hello world");
        vrange r(h);
    
        std::cout << r.size() << std::endl;
        for (const auto &i: r){
            std::cout << i;
            }
        std::cout << std::endl;
        
    
    }

removing the call to r.size(), or defaulting the vrange move constructor and assignment operator makes it compile fine.

compiler message:

/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/bits/ranges_util.h: In instantiation of ‘constexpr _Derived& std::ranges::view_interface<_Derived>::_M_derived() [with _Derived = vrange]’:
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/bits/ranges_util.h:101:35:   required from ‘constexpr bool std::ranges::view_interface<_Derived>::empty() requires  forward_range<_Derived> [with _Derived = vrange]’
w.cpp:25:12:   required from here
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/bits/ranges_util.h:70:23: error: static assertion failed
   70 |         static_assert(view<_Derived>);
      |                       ^~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/bits/ranges_util.h:70:23: note: constraints not satisfied
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/ranges:37:
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/concepts:136:13:   required for the satisfaction of ‘constructible_from<_Tp, _Tp>’ [with _Tp = vrange]
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/concepts:150:13:   required for the satisfaction of ‘move_constructible<_Tp>’ [with _Tp = vrange]
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/concepts:247:13:   required for the satisfaction of ‘movable<_Tp>’ [with _Tp = vrange]
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/concepts:137:30: note: the expression ‘is_constructible_v<_Tp, _Args ...> [with _Tp = vrange; _Args = {vrange}]’ evaluated to ‘false’
  137 |       = destructible<_Tp> && is_constructible_v<_Tp, _Args...>;
Nicol Bolas
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serafean
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1 Answers1

5

This has nothing to do with size specifically.

view_interface is used to build a type that is a view. Well, the ranges::view concept requires that the type is at least moveable. And view_interface has a very specific requirement on the type given as its template argument:

Before any member of the resulting specialization of view_interface other than special member functions is referenced, D shall be complete, and model both derived_from<view_interface<D>> and view.

Well, your type does not model view because it is not moveable. So you broke the rules, and you therefore get undefined behavior. Which can include compile errors happening if you call certain members but not others.

Nicol Bolas
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  • It's unfortunate that the language design does not allow CRTP types to require a diagnostic once the derived type is complete. – Patrick Roberts Oct 31 '22 at 22:41
  • @PatrickRoberts: That would require that the language recognize that the CRTP is a thing. Which it doesn't. – Nicol Bolas Oct 31 '22 at 22:57
  • So it could be argued that it is too lenient in regard to the `view` concept when using only `begin()` and `end()` ? UB being UB, i know it could have blown up my PC, but for argument's sake... – serafean Nov 01 '22 at 09:44
  • @serafean: What is "too lenient"? Nothing in `view_interface` is able to apply the `view` concept to `D` because `D` is incomplete at that point. Only after `D` has been fully defined can you ask if it is a `view`; by then, it already has a `view_interface`. – Nicol Bolas Nov 01 '22 at 13:23
  • lenient - not strict enough. What you're saying implies to me that when using only the iterator getters the view concept doesn't even get applied. – serafean Nov 01 '22 at 14:07