1

The following code does not compile.

#include <iostream>
#include <memory>
#include <ranges>
#include <vector>


class Base
{
    public:
        virtual ~Base() = default;
        virtual auto say() const -> void = 0;
};

class Derived : public Base
{
    public:
        auto say() const -> void override
        {
            std::cout << "Hi!\n";
        }
};

auto say_all(std::ranges::view auto bases) -> void
{
    for (auto const & base : bases)
    {
        base.say();
    }
}

int main()
{
    std::vector<std::unique_ptr<Derived>> bases;
    bases.push_back(std::make_unique<Derived>());
    bases.push_back(std::make_unique<Derived>());

    say_all(bases | std::views::transform([](auto const & up) { return static_cast<Base &>(*(up.get())); }));
}

The error under gcc is quite lengthy:

<source>: In instantiation of 'main()::<lambda(const auto:11&)> [with auto:11 = std::unique_ptr<Derived>]':
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/type_traits:3302:30:   required from 'struct std::is_invocable<main()::<lambda(const auto:11&)>&, std::unique_ptr<Derived, std::default_delete<Derived> >&>'
 3302 |     : public __bool_constant<__is_invocable(_Fn, _ArgTypes...)>
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/type_traits:3707:71:   required from 'constexpr const bool std::is_invocable_v<main()::<lambda(const auto:11&)>&, std::unique_ptr<Derived, std::default_delete<Derived> >&>'
 3707 |   inline constexpr bool is_invocable_v = is_invocable<_Fn, _Args...>::value;
      |                                                                       ^~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/concepts:362:25:   required by substitution of 'template<class _Vp, class _Fp>  requires (input_range<_Vp>) && (copy_constructible<_Fp>) && ((view<_Vp>) && (is_object_v<_Fp>) && (regular_invocable<_Fp&, decltype(*(declval<decltype(std::ranges::__access::__begin((declval<_Container&>)()))&>)())>) && (__can_reference<typename std::invoke_result<_Fp&, decltype(*(declval<decltype(std::ranges::__access::__begin((declval<_Container&>)()))&>)())>::type>)) class std::ranges::transform_view [with _Vp = std::ranges::ref_view<std::vector<std::unique_ptr<Derived> > >; _Fp = main()::<lambda(const auto:11&)>]'
  362 |     concept invocable = is_invocable_v<_Fn, _Args...>;
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:2244:5:   required by substitution of 'template<class _Range, class _Fp> std::ranges::transform_view(_Range&&, _Fp) -> transform_view<views::all_t<_Range>, _Fp> [with _Range = std::vector<std::unique_ptr<Derived> >&; _Fp = main()::<lambda(const auto:11&)>]'
 2244 |     transform_view(_Range&&, _Fp) -> transform_view<views::all_t<_Range>, _Fp>;
      |     ^~~~~~~~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:933:44:   recursively required by substitution of 'template<class _Range>  requires  __adaptor_invocable<_Adaptor, _Range, const _Arg&> constexpr auto std::ranges::views::__adaptor::_Partial<std::ranges::views::_Transform, main()::<lambda(const auto:11&)> >::operator()(_Range&&) const [with _Range = std::vector<std::unique_ptr<Derived> >&]'
  933 |       = requires { std::declval<_Adaptor>()(declval<_Args>()...); };
      |                    ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:933:44:   required by substitution of 'template<class _Self, class _Range>  requires (__is_range_adaptor_closure<_Self>) && (__adaptor_invocable<_Self, _Range>) constexpr auto std::ranges::views::__adaptor::operator|(_Range&&, _Self&&) [with _Self = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Transform, main()::<lambda(const auto:11&)> >; _Range = std::vector<std::unique_ptr<Derived> >&]'
<source>:37:107:   required from here
   37 |     say_all(bases | std::views::transform([](auto const & up) { return static_cast<Base &>(*(up.get())); }));
      |                                                                                                           ^
<source>:37:72: error: cannot allocate an object of abstract type 'Base'
   37 |     say_all(bases | std::views::transform([](auto const & up) { return static_cast<Base &>(*(up.get())); }));
      |                                                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:7:7: note:   because the following virtual functions are pure within 'Base':
    7 | class Base
      |       ^~~~
<source>:11:22: note:     'virtual void Base::say() const'
   11 |         virtual auto say() const -> void = 0;
      |                      ^~~
<source>: In function 'int main()':
<source>:37:19: error: no match for 'operator|' (operand types are 'std::vector<std::unique_ptr<Derived> >' and 'std::ranges::views::__adaptor::_Partial<std::ranges::views::_Transform, main()::<lambda(const auto:11&)> >')
   37 |     say_all(bases | std::views::transform([](auto const & up) { return static_cast<Base &>(*(up.get())); }));
      |             ~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |             |                            |
      |             |                            std::ranges::views::__adaptor::_Partial<std::ranges::views::_Transform, main()::<lambda(const auto:11&)> >
      |             std::vector<std::unique_ptr<Derived> >
<source>:37:19: note: there are 7 candidates
   37 |     say_all(bases | std::views::transform([](auto const & up) { return static_cast<Base &>(*(up.get())); }));
      |             ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from <source>:3:
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:981:5: note: candidate 1: 'template<class _Lhs, class _Rhs>  requires (__is_range_adaptor_closure<_Lhs>) && (__is_range_adaptor_closure<_Rhs>) constexpr auto std::ranges::views::__adaptor::operator|(_Lhs&&, _Rhs&&)'
  981 |     operator|(_Lhs&& __lhs, _Rhs&& __rhs)
      |     ^~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:981:5: note: template argument deduction/substitution failed:
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:981:5: note: constraints not satisfied
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges: In substitution of 'template<class _Lhs, class _Rhs>  requires (__is_range_adaptor_closure<_Lhs>) && (__is_range_adaptor_closure<_Rhs>) constexpr auto std::ranges::views::__adaptor::operator|(_Lhs&&, _Rhs&&) [with _Lhs = std::vector<std::unique_ptr<Derived> >&; _Rhs = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Transform, main()::<lambda(const auto:11&)> >]':
<source>:37:107:   required from here
   37 |     say_all(bases | std::views::transform([](auto const & up) { return static_cast<Base &>(*(up.get())); }));
      |                                                                                                           ^
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:962:13:   required for the satisfaction of '__is_range_adaptor_closure<_Lhs>' [with _Lhs = std::vector<std::unique_ptr<Derived, std::default_delete<Derived> >, std::allocator<std::unique_ptr<Derived, std::default_delete<Derived> > > >&]
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:963:9:   in requirements with '_Tp __t' [with _Tp = std::vector<std::unique_ptr<Derived, std::default_delete<Derived> >, std::allocator<std::unique_ptr<Derived, std::default_delete<Derived> > > >&]
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:963:70: note: the required expression 'std::ranges::views::__adaptor::__is_range_adaptor_closure_fn(__t, __t)' is invalid
  963 |       = requires (_Tp __t) { __adaptor::__is_range_adaptor_closure_fn(__t, __t); };
      |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
cc1plus: note: set '-fconcepts-diagnostics-depth=' to at least 2 for more detail
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:972:5: note: candidate 2: 'template<class _Self, class _Range>  requires (__is_range_adaptor_closure<_Self>) && (__adaptor_invocable<_Self, _Range>) constexpr auto std::ranges::views::__adaptor::operator|(_Range&&, _Self&&)'
  972 |     operator|(_Range&& __r, _Self&& __self)
      |     ^~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ranges:972:5: note: substitution of deduced template arguments resulted in errors seen above
In file included from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/memory_resource.h:40,
                 from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/string:72,
                 from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/locale_classes.h:42,
                 from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/ios_base.h:43,
                 from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ios:46,
                 from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/ostream.h:43,
                 from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/ostream:42,
                 from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/iostream:43,
                 from <source>:1:
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/cstddef:141:3: note: candidate 3: 'constexpr std::byte std::operator|(byte, byte)'
  141 |   operator|(byte __l, byte __r) noexcept
      |   ^~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/cstddef:141:18: note: no known conversion for argument 1 from 'std::vector<std::unique_ptr<Derived> >' to 'std::byte'
  141 |   operator|(byte __l, byte __r) noexcept
      |             ~~~~~^~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/ios_base.h:91:3: note: candidate 4: 'constexpr std::_Ios_Fmtflags std::operator|(_Ios_Fmtflags, _Ios_Fmtflags)'
   91 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b) _GLIBCXX_NOTHROW
      |   ^~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/ios_base.h:91:27: note: no known conversion for argument 1 from 'std::vector<std::unique_ptr<Derived> >' to 'std::_Ios_Fmtflags'
   91 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b) _GLIBCXX_NOTHROW
      |             ~~~~~~~~~~~~~~^~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/ios_base.h:150:3: note: candidate 5: 'constexpr std::_Ios_Openmode std::operator|(_Ios_Openmode, _Ios_Openmode)'
  150 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b) _GLIBCXX_NOTHROW
      |   ^~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/ios_base.h:150:27: note: no known conversion for argument 1 from 'std::vector<std::unique_ptr<Derived> >' to 'std::_Ios_Openmode'
  150 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b) _GLIBCXX_NOTHROW
      |             ~~~~~~~~~~~~~~^~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/ios_base.h:197:3: note: candidate 6: 'constexpr std::_Ios_Iostate std::operator|(_Ios_Iostate, _Ios_Iostate)'
  197 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b) _GLIBCXX_NOTHROW
      |   ^~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/ios_base.h:197:26: note: no known conversion for argument 1 from 'std::vector<std::unique_ptr<Derived> >' to 'std::_Ios_Iostate'
  197 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b) _GLIBCXX_NOTHROW
      |             ~~~~~~~~~~~~~^~~
In file included from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/shared_ptr_atomic.h:33,
                 from /cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/memory:83,
                 from <source>:2:
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/atomic_base.h:104:3: note: candidate 7: 'constexpr std::memory_order std::operator|(memory_order, __memory_order_modifier)'
  104 |   operator|(memory_order __m, __memory_order_modifier __mod) noexcept
      |   ^~~~~~~~
/cefs/22/22e6cdc013c8541ce3d1548e_consolidated/compilers_c++_x86_gcc_15.2.0/include/c++/15.2.0/bits/atomic_base.h:104:26: note: no known conversion for argument 1 from 'std::vector<std::unique_ptr<Derived> >' to 'std::memory_order'
  104 |   operator|(memory_order __m, __memory_order_modifier __mod) noexcept
      |             ~~~~~~~~~~~~~^~~
Compiler returned: 1

Basically, it complains it cannot create an object of type Base as it is an abstract class. But why does it try to?

All I am trying to do is to create a view of references to base class and pass it to a function. The code here may not make much sense, but it is distilled from a larger example that uses multiple inheritance and just serves the purpose of illustrating the problem.

Improve this question
1
  • At a guess, because the type deduction on the lambda is dropping the reference and returning a copy. Does [](auto const & up) -> decltype(auto){ return static_cast<Base &>(*(up.get())); }) help? I will be honest, I love ranges, but the template machinery that drives them is pretty opaque to me. Commented Sep 15 at 21:43

1 Answer 1

Reset to default
14

The issue is that the lambda in your transform call is automatically deducing the return type. By default a lambda that doesn't call out a return type uses auto rules for deduction which drops references. See What is the return type of a lambda expression if an item of a vector is returned?

In your case, you can salvage things by explicitly calling out the return type as Base &

#include <iostream>
#include <memory>
#include <ranges>
#include <vector>


class Base
{
    public:
        virtual ~Base() = default;
        virtual auto say() const -> void = 0;
};

class Derived : public Base
{
    public:
        auto say() const -> void override
        {
            std::cout << "Hi!\n";
        }
};

auto say_all(std::ranges::view auto bases) -> void
{
    for (auto const & base : bases)
    {
        base.say();
    }
}

int main()
{
    std::vector<std::unique_ptr<Derived>> bases;
    bases.push_back(std::make_unique<Derived>());
    bases.push_back(std::make_unique<Derived>());

    say_all(bases | std::views::transform([](auto const & up) -> Base & { return *up; }));
}

https://godbolt.org/z/GWfYoa3vx

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2 Comments

Since the return type is now specified, there's no longer any use for static_cast<Base &>. That can be dropped, which conveniently makes the working code shorter than the failed attempt. While you're at it, the question's *(up.get()) is gratuitously complex; it could be simply *up.
Ahh, yes and yes. I did ditch the static_cast and the silly *(up.get()) was a leftover from code that used pointers to base class (and of course compiled fine).

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