template argument deduction compile error with boost's `static_vector`

April 19, 2023

The following piece of code works fine:

#include <span>
#include <boost/container/static_vector.hpp>
#include <iostream>
#include <vector>

int main(){
    boost::container::static_vector<int, 10> x{1,2,3};
    std::vector y{1,2,3};
    for(const auto& i : std::span(y.begin()+1, y.end()))
        std::cout << i << '\n';
}

but will fail at compile if i try to use x in place of y in the ranged for loop: https://godbolt.org/z/ahWPv7rfe

I think this is a minor bug with static_vector as the above works fine with vector… Still, what can I do with my code to resolve the compiler error? Thanks!

>Solution :

Here is most important error line:

/opt/compiler-explorer/gcc-12.2.0/include/c++/12.2.0/span:170:9: note: candidate: 'template<class _Type, long unsigned int _Extent, class _It, class _End>  requires (contiguous_iterator<_It>) && (sized_sentinel_for<_End, _It>) && ((std::span<_Type, _Extent>::__is_compatible_ref::value) && !(is_convertible_v<_End, std::size_t>)) span(_It, _End)-> std::span<_Type, _Extent>'
  170 |         span(_It __first, _End __last)

As you can see contiguous_iterator concept is not meet in this version of boost.

Basically boost support for C++20 was not introduced for this version.

It works since boost 1.77: https://godbolt.org/z/cfMxTrGsn
So you need to update boost library.