Re: [Boost-users] Fwd: Boost.Range: 'Use of deleted function' GCC error when using lambdas

Subject: Re: [Boost-users] Fwd: Boost.Range: 'Use of deleted function' GCC error when using lambdas
From: Daniel Hofmann (daniel_at_[hidden])
Date: 2016-09-05 04:59:49

On 09/01/2016 05:48 PM, Remko Tronçon wrote:
> Hi,
> 
> I have the following piece of code:
> 
>     std::vector<std::string> list1;
>     std::vector<std::string> list2; 
>     push_back(list2, list1
>         | transformed([](const auto& w) { return w; })
>         | sliced(0, 1));
> 
> This compiles fine with CLang, but with GCC I get the error:
> 
>     /usr/include/boost/optional/optional.hpp:546:64: error: use of
> deleted function 'main(int, const char**)::<lambda(const auto:1&)>&
> main(int, const char**)::<lambda(const auto:1&)>::operator=(const
> main(int, const char**)::<lambda(const auto:1&)>&)'
>      void assign_value ( rval_reference_type val ) { get_impl() =
> static_cast<rval_reference_type>(val); }
> 
>                                                    
>  ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>    main.cpp:46:19: note: a lambda closure type has a deleted copy
> assignment operator
>     | transformed([](const auto& w) { return w; })
> 
> 
> When I replace the lambda by a regular function, the code compiles; when
> I don't use 'sliced' (but pipe it to e.g. 'transformed' instead), the
> code also compiles.
> 
> Is there a way to make this code compile with a lambda?

You can work around that using the unary plus in order to explicitely
trigger the implicit convertion from your capture-less lambda to a
function pointer.

  list1
    | transformed(+[](const std::string& w) { return w; })
    | sliced(0, 1);

This is a workaround, though. I still think your original problem is an
issue in the Boost.Range library and should be properly fixed there.

> thanks!
> Remko
> 
> PS: I'm using GCC 6.2.0 with Boost 1.61.
> 
> 
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