std::make_shared inside std::apply

Question

Consider the following code example.

It creates class A(with template parameters T and Args...) and saves its constructor arguments into tuple args_. Later on it uses args_ to create an instance of T inside get method.

All works fine, except, as you can see I do use my_own_make_shared function inside std::apply just because if I replace it with std::make_shared I can not compile this code. Anybody have any idea what the problem is?

#include <iostream>
#include <charconv>
#include <array>
#include <cstring>
#include <iostream>
#include <tuple>
#include <utility>
#include <memory>
#include <algorithm>

struct Base
{
    virtual void foo() = 0;
};

struct Test : Base
{
    Test(int, int) {}
    void foo() override {std::cout << "Test\n";}
};

struct Test2 : Base
{
    Test2(int, int, const std::string&) {}
    void foo() override {std::cout << "Test2\n";}
};

template<typename T, typename... Args>
std::shared_ptr<T> my_own_make_shared(Args... args)
{
    return std::make_shared<T>(args...);
}

template <typename T, typename... Args>
struct A
{
    A(Args... args) : args_(std::make_tuple(args...)) {}

    std::shared_ptr<Base> get()
    {
        return std::apply(my_own_make_shared<T, Args...>, args_);
    }

    std::tuple<Args...> args_;
};

template <typename T, typename... Args>
auto make_A(Args... args)
{
    return A<T, Args...>(args...);
}

int main(int argc, char** argv)
{
    make_A<Test>(1, 2).get()->foo();
    make_A<Test2>(1, 2, "").get()->foo();
}

Compiler error (GCC-9). Assuming we replaced my_own_make_shared with std::make_shared

    In file included from test.cpp:6:
/usr/include/c++/9/tuple: In instantiation of ‘constexpr decltype(auto) std::__apply_impl(_Fn&&, _Tuple&&, std::index_sequence<_Idx ...>) [with _Fn = std::shared_ptr<Test> (&)(int&&, int&&); _Tuple = std::tuple<int, int>&; long unsigned int ..._Idx = {0, 1}; std::index_sequence<_Idx ...> = std::integer_sequence<long unsigned int, 0, 1>]’:
/usr/include/c++/9/tuple:1694:31:   required from ‘constexpr decltype(auto) std::apply(_Fn&&, _Tuple&&) [with _Fn = std::shared_ptr<Test> (&)(int&&, int&&); _Tuple = std::tuple<int, int>&]’
test.cpp:41:26:   required from ‘std::shared_ptr<Base> A<T, Args>::get() [with T = Test; Args = {int, int}]’
test.cpp:55:28:   required from here
/usr/include/c++/9/tuple:1684:27: error: no matching function for call to ‘__invoke(std::shared_ptr<Test> (&)(int&&, int&&), std::__tuple_element_t<0, std::tuple<int, int> >&, std::__tuple_element_t<1, std::tuple<int, int> >&)’
 1684 |       return std::__invoke(std::forward<_Fn>(__f),
      |              ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
 1685 |       std::get<_Idx>(std::forward<_Tuple>(__t))...);
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/9/tuple:41,
                 from test.cpp:6:
/usr/include/c++/9/bits/invoke.h:89:5: note: candidate: ‘template<class _Callable, class ... _Args> constexpr typename std::__invoke_result<_Functor, _ArgTypes>::type std::__invoke(_Callable&&, _Args&& ...)’
   89 |     __invoke(_Callable&& __fn, _Args&&... __args)
      |     ^~~~~~~~
/usr/include/c++/9/bits/invoke.h:89:5: note:   template argument deduction/substitution failed:
/usr/include/c++/9/bits/invoke.h: In substitution of ‘template<class _Callable, class ... _Args> constexpr typename std::__invoke_result<_Functor, _ArgTypes>::type std::__invoke(_Callable&&, _Args&& ...) [with _Callable = std::shared_ptr<Test> (&)(int&&, int&&); _Args = {int&, int&}]’:
/usr/include/c++/9/tuple:1684:27:   required from ‘constexpr decltype(auto) std::__apply_impl(_Fn&&, _Tuple&&, std::index_sequence<_Idx ...>) [with _Fn = std::shared_ptr<Test> (&)(int&&, int&&); _Tuple = std::tuple<int, int>&; long unsigned int ..._Idx = {0, 1}; std::index_sequence<_Idx ...> = std::integer_sequence<long unsigned int, 0, 1>]’
/usr/include/c++/9/tuple:1694:31:   required from ‘constexpr decltype(auto) std::apply(_Fn&&, _Tuple&&) [with _Fn = std::shared_ptr<Test> (&)(int&&, int&&); _Tuple = std::tuple<int, int>&]’
test.cpp:41:26:   required from ‘std::shared_ptr<Base> A<T, Args>::get() [with T = Test; Args = {int, int}]’
test.cpp:55:28:   required from here
/usr/include/c++/9/bits/invoke.h:89:5: error: no type named ‘type’ in ‘struct std::__invoke_result<std::shared_ptr<Test> (&)(int&&, int&&), int&, int&>’
test.cpp: In instantiation of ‘std::shared_ptr<Base> A<T, Args>::get() [with T = Test; Args = {int, int}]’:
test.cpp:55:28:   required from here
test.cpp:41:26: error: could not convert ‘std::apply<std::shared_ptr<Test> (&)(int&&, int&&), std::tuple<int, int>&>(std::make_shared<Test, int, int>, ((A<Test, int, int>*)this)->A<Test, int, int>::args_)’ from ‘void’ to ‘std::shared_ptr<Base>’
   41 |         return std::apply(std::make_shared<T, Args...>, args_);
      |                ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                          |
      |                          void
In file included from test.cpp:6:
/usr/include/c++/9/tuple: In instantiation of ‘constexpr decltype(auto) std::__apply_impl(_Fn&&, _Tuple&&, std::index_sequence<_Idx ...>) [with _Fn = std::shared_ptr<Test2> (&)(int&&, int&&, const char*&&); _Tuple = std::tuple<int, int, const char*>&; long unsigned int ..._Idx = {0, 1, 2}; std::index_sequence<_Idx ...> = std::integer_sequence<long unsigned int, 0, 1, 2>]’:
/usr/include/c++/9/tuple:1694:31:   required from ‘constexpr decltype(auto) std::apply(_Fn&&, _Tuple&&) [with _Fn = std::shared_ptr<Test2> (&)(int&&, int&&, const char*&&); _Tuple = std::tuple<int, int, const char*>&]’
test.cpp:41:26:   required from ‘std::shared_ptr<Base> A<T, Args>::get() [with T = Test2; Args = {int, int, const char*}]’
test.cpp:56:33:   required from here
/usr/include/c++/9/tuple:1684:27: error: no matching function for call to ‘__invoke(std::shared_ptr<Test2> (&)(int&&, int&&, const char*&&), std::__tuple_element_t<0, std::tuple<int, int, const char*> >&, std::__tuple_element_t<1, std::tuple<int, int, const char*> >&, const char*&)’
 1684 |       return std::__invoke(std::forward<_Fn>(__f),
      |              ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
 1685 |       std::get<_Idx>(std::forward<_Tuple>(__t))...);
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/9/tuple:41,
                 from test.cpp:6:
/usr/include/c++/9/bits/invoke.h:89:5: note: candidate: ‘template<class _Callable, class ... _Args> constexpr typename std::__invoke_result<_Functor, _ArgTypes>::type std::__invoke(_Callable&&, _Args&& ...)’
   89 |     __invoke(_Callable&& __fn, _Args&&... __args)
      |     ^~~~~~~~
/usr/include/c++/9/bits/invoke.h:89:5: note:   template argument deduction/substitution failed:
/usr/include/c++/9/bits/invoke.h: In substitution of ‘template<class _Callable, class ... _Args> constexpr typename std::__invoke_result<_Functor, _ArgTypes>::type std::__invoke(_Callable&&, _Args&& ...) [with _Callable = std::shared_ptr<Test2> (&)(int&&, int&&, const char*&&); _Args = {int&, int&, const char*&}]’:
/usr/include/c++/9/tuple:1684:27:   required from ‘constexpr decltype(auto) std::__apply_impl(_Fn&&, _Tuple&&, std::index_sequence<_Idx ...>) [with _Fn = std::shared_ptr<Test2> (&)(int&&, int&&, const char*&&); _Tuple = std::tuple<int, int, const char*>&; long unsigned int ..._Idx = {0, 1, 2}; std::index_sequence<_Idx ...> = std::integer_sequence<long unsigned int, 0, 1, 2>]’
/usr/include/c++/9/tuple:1694:31:   required from ‘constexpr decltype(auto) std::apply(_Fn&&, _Tuple&&) [with _Fn = std::shared_ptr<Test2> (&)(int&&, int&&, const char*&&); _Tuple = std::tuple<int, int, const char*>&]’
test.cpp:41:26:   required from ‘std::shared_ptr<Base> A<T, Args>::get() [with T = Test2; Args = {int, int, const char*}]’
test.cpp:56:33:   required from here
/usr/include/c++/9/bits/invoke.h:89:5: error: no type named ‘type’ in ‘struct std::__invoke_result<std::shared_ptr<Test2> (&)(int&&, int&&, const char*&&), int&, int&, const char*&>’
test.cpp: In instantiation of ‘std::shared_ptr<Base> A<T, Args>::get() [with T = Test2; Args = {int, int, const char*}]’:
test.cpp:56:33:   required from here
test.cpp:41:26: error: could not convert ‘std::apply<std::shared_ptr<Test2> (&)(int&&, int&&, const char*&&), std::tuple<int, int, const char*>&>(std::make_shared<Test2, int, int, const char*>, ((A<Test2, int, int, const char*>*)this)->A<Test2, int, int, const char*>::args_)’ from ‘void’ to ‘std::shared_ptr<Base>’
   41 |         return std::apply(std::make_shared<T, Args...>, args_);
      |                ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                          |
      |                          void

Answer 1

Below is implementation of std::apply taken from reference

namespace detail {
template <class F, class Tuple, std::size_t... I>
constexpr decltype(auto) apply_impl(F&& f, Tuple&& t, std::index_sequence<I...>)
{
    // This implementation is valid since C++20 (via P1065R2)
    // In C++17, a constexpr counterpart of std::invoke is actually needed here
    return std::invoke(std::forward<F>(f), std::get<I>(std::forward<Tuple>(t))...);
}
}  // namespace detail

as we can see functor and its arguments as element of tuples are forwarded.

In your case args_ is Lvaue. So by std::get<I>(std::forward<Tuple>(t))... you are getting Lvalue reference to particular tuple's elements.

For

make_A<Test>(1, 2).get()->foo();

parameter pack Args... is deduced to be: {int,int} and the following

return std::apply(std::make_shared<T, Args...>, args_);

cannot compile because R-value references cannot be bound to Lvalues. R-value references appear on make_shared parameter lists. You use this overload:

template< class T, class... Args >
shared_ptr<T> make_shared( Args&&... args );

where Args... is {int,int}, so parameters list is int&&, int&&.

If you want to use elements of tuple as arguments of apply as Lvalue just cast Args to Lvalues:

return std::apply(std::make_shared<T, std::add_lvalue_reference_t<Args>...>, args_);

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Demo