why can't I construct an std::span from iterators? - c++

Question

Consider a large memory container. In this simple example an std::vector<int>:
std::vector v = { 0, 1, 2, 3, 4, 5 };
std::span allows me create a lightweight view over the memory. Now I want to simply print the span:
template<typename T>
void print(std::span<T> span) {
std::cout << '[';
if (span.size())
std::copy(span.begin(), span.end() - 1, std::ostream_iterator<int>(std::cout, ", "));
std::cout << span.back() << "]\n";
}
int main() {
std::vector v = { 0, 1, 2, 3, 4, 5 };
print(std::span{ v });
}
output:
[0, 1, 2, 3, 4, 5]
now I want to make subsets (which is where the std::span actually becomes useful as a view). I can use iterators to specify my range and call this constructor(3) from std::span
template< class It, class End >
explicit(extent != std::dynamic_extent)
constexpr span( It first, End last );
But that doesn't work:
print(std::span{ v.begin() + 2, v.end() }); //error E0289
C++ no instance of constructor matches the argument list argument types are: (std::_Vector_iterator<std::_Vector_val<std::conditional_t<true, std::_Simple_types, std::_Vec_iter_types<int, size_t, ptrdiff_t, int *, const int *, int &, const int &>>>>, std::_Vector_iterator<std::_Vector_val<std::conditional_t<true, std::_Simple_types, std::_Vec_iter_types<int, size_t, ptrdiff_t, int *, const int *, int &, const int &>>>>)
there is the possibility of using the constructor(2) which takes a pointer and size:
print(std::span{ v.data() + 1, 3 }); //-> prints [1, 2, 3]
But that defeats the purpose of iterators.
How can I construct an std::span using iterators? Am I missing something?
full code:
#include <iostream>
#include <vector>
#include <span>
#include <algorithm>
template<typename T>
void print(std::span<T> span) {
std::cout << '[';
if (span.size())
std::copy(span.begin(), span.end() - 1, std::ostream_iterator<int>(std::cout, ", "));
std::cout << span.back() << "]\n";
}
int main() {
std::vector v = { 0, 1, 2, 3, 4, 5 };
print(std::span{ v.begin() + 2, v.end() });
}
until MSVC has implemented the constructor I will be using this make_span function:
template<typename It>
constexpr auto make_span(It begin, It end) {
return std::span<std::remove_pointer_t<It::pointer>>(&(*begin), std::distance(begin, end));
}
Using Visual Studio Community 2019 Version 16.7.5. Configuration: x64, Release. C++ Language Standard = /std:c++latest

Answer 1

You can construct a span using iterators, it has such a constructor (as added by P1394, which you can see in [views.span]):
template< class It, class End >
explicit(extent != std::dynamic_extent)
constexpr span( It first, End last );
It's just that MSVC's standard library doesn't implement it. The program compiles fine on gcc, as expected.