Accordingly to the <a href="https://en.cppreference.com/w/cpp/container/span/deduction

with this <div class="highlight highlight-source-c++ notranslate position-relative

<div class="highlight highlight-source-c++ notranslate position-relative overflow-auto"

Yes, the relevant error is <div class="snippet-clipboard-content notranslate posit

I am not sure I understood what you meant in <p dir="aut

This implementation seems able to perform the deduction. <a href="https://github.com/m

The following code works with span-line <div clas

I tried to edit the <a href="https://github.com/ami-iit/matio-cpp/blob/master/test/Spa

See <a class="commit-link" data-hovercard-type="commit" data-hovercard-url="https://gi

Missing deduction guides for Span object about matio-cpp HOT 10 CLOSED

ami-iit commented on June 19, 2024

Missing deduction guides for Span object

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Comments (10)

GiulioRomualdi commented on June 19, 2024 1

with this

std::vector<int32_t> v{2};
matioCpp::Span<int32_t> s{v};

everything works fine

It is nice to have this deduction guide since it will allow to write the following code

void foo(Span<int> s);

std::vector<int> v{1};
foo(v);

right now what we should do is

void foo(Span<int> s);

std::vector<int> v{1};
foo(make_span(v));

Notice that make_span is not defined by the standard

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S-Dafarra commented on June 19, 2024 1

std::ranges::range_reference_t

Ah I see. The problem is that std::ranges is C++20 (https://en.cppreference.com/w/cpp/ranges)

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S-Dafarra commented on June 19, 2024 1

I would say that before moving to a completely new implementation, I would like to understand if it is possible to fix the current implementation.

Moving to a new implementation of span might be potentially a lot of work (also for those applications depending on this). I would rather do this step only once, when std::span will be more accessible

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S-Dafarra commented on June 19, 2024

Yes, the relevant error is

In file included from /home/gromualdi/robot-code/yarp-telemetry/src/libYARP_telemetry/src/yarp/telemetry/experimental/Record.h:12,
                 from /home/gromualdi/robot-code/yarp-telemetry/src/examples/circular_buffer_record_example.cpp:14:
/home/gromualdi/robot-code/robotology-superbuild/build/install/include/matioCpp/Span.h:411:24: note: candidate: ‘template<class ElementType, long int Extent, class Container, class, class> Span(const Container&)-> matioCpp::Span<ElementType, Extent>’
  411 |     MATIOCPP_CONSTEXPR Span(const Container& cont) : Span(cont.data(), static_cast<index_type>(cont.size()))
      |                        ^~~~
/home/gromualdi/robot-code/robotology-superbuild/build/install/include/matioCpp/Span.h:411:24: note:   template argument deduction/substitution failed:
/home/gromualdi/robot-code/yarp-telemetry/src/examples/circular_buffer_record_example.cpp:44:27: note:   couldn’t deduce template parameter ‘ElementType’
   44 |         matioCpp::Span s{v};
      |                           ^
In file included from /home/gromualdi/robot-code/yarp-telemetry/src/libYARP_telemetry/src/yarp/telemetry/experimental/Record.h:12,
                 from /home/gromualdi/robot-code/yarp-telemetry/src/examples/circular_buffer_record_example.cpp:14:
/home/gromualdi/robot-code/robotology-superbuild/build/install/include/matioCpp/Span.h:402:24: note: candidate: ‘template<class ElementType, long int Extent, class Container, class, class> Span(Container&)-> matioCpp::Span<ElementType, Extent>’
  402 |     MATIOCPP_CONSTEXPR Span(Container& cont) : Span(cont.data(), static_cast<index_type>(cont.size()))
      |                        ^~~~
/home/gromualdi/robot-code/robotology-superbuild/build/install/include/matioCpp/Span.h:402:24: note:   template argument deduction/substitution failed:
/home/gromualdi/robot-code/yarp-telemetry/src/examples/circular_buffer_record_example.cpp:44:27: note:   couldn’t deduce template parameter ‘ElementType’

As you noticed, it fails to deduce the ElementType.

I am not sure I understood what you meant in

In this case the used deduction guides used should be:
template<class R>
span(R&&) -> span<std::remove_reference_t<std::ranges::range_reference_t<R>>>;

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GiulioRomualdi commented on June 19, 2024

I am not sure I understood what you meant in
In this case the used deduction guides used should be:
template<class R>
span(R&&) -> span<std::remove_reference_t<std::ranges::range_reference_t<R>>>;

Accordingly to what is written here
the deduction guide used for std::vector is

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GiulioRomualdi commented on June 19, 2024

This implementation seems able to perform the deduction. https://github.com/martinmoene/span-lite

What do you think to port it matio-cpp?

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GiulioRomualdi commented on June 19, 2024

Before taking any decision let me try in a toy problem 😃

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GiulioRomualdi commented on June 19, 2024

The following code works with span-line

#include "nonstd/span.hpp"
#include <array>
#include <vector>
#include <iostream>

std::ptrdiff_t size( nonstd::span<const int> spn )
{
    return spn.size();
}

int main()
{
    int arr[] = { 1, };

    std::cout <<
        "C-array:" << size( arr ) <<
        " array:"  << size( std::array <int, 2>{ 1, 2, } ) <<
        " vector:" << size( std::vector<int   >{ 1, 2, 3, } );
}

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S-Dafarra commented on June 19, 2024

I tried to edit the SpanUnitTest as follows

But I am able to compile 🤔

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S-Dafarra commented on June 19, 2024

See 708827b

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Missing deduction guides for Span object about matio-cpp HOT 10 CLOSED

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