CODEWITHSUNDEEP
c++inheritancelambdashared-ptrsmart-pointers
Parminder Singh
Parminder Singh

Reputation: 352

Derive from shared_ptr<T>

I have a strange compilation problem that I can't understand.

//I know, you should never derive from the STL Library
template<class T>
class SharedClass : private shared_ptr<T>
{
public:
    template<class T2>
    SharedClass(T2&& t2) :
    shared_ptr<T>(make_shared<T>(move(t2)))
    {}

    virtual ~SharedClass() 
    {}
};

class A
{
public:
    typedef function<void(SharedClass<A>)> Callback;

    A(Callback callback)
    { }
};

main.cpp

SharedClass<A> shared([](SharedClass<A>){ });

Compile Log:

In file included from /usr/include/x86_64-linux-gnu/c++/7/bits/c++allocator.h:33:0,
                 from /usr/include/c++/7/bits/allocator.h:46,
                 from /usr/include/c++/7/string:41,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from main.cpp:9:
/usr/include/c++/7/ext/new_allocator.h: In instantiation of ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = A; _Args = {SharedClass<A>}; _Tp = A]’:
/usr/include/c++/7/bits/alloc_traits.h:475:4:   required from ‘static void std::allocator_traits<std::allocator<_CharT>>::construct(std::allocator_traits<std::allocator<_CharT> >::allocator_type&, _Up*, _Args&& ...) [with _Up = A; _Args = {SharedClass<A>}; _Tp = A; std::allocator_traits<std::allocator<_CharT>>::allocator_type = std::allocator<A>]’
/usr/include/c++/7/bits/shared_ptr_base.h:526:39: required from ‘std::_Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp>::_Sp_counted_ptr_inplace(_Alloc, _Args&& ...) [with _Args = {SharedClass<A>}; _Tp = A; _Alloc = std::allocator<A>; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2]’ /usr/include/c++/7
/bits/shared_ptr_base.h:637:4:   required from ‘std::__shared_count<_Lp>::__shared_count(std::_Sp_make_shared_tag, _Tp*, const _Alloc&, _Args&& ...) [with _Tp = A; _Alloc = std::allocator<A>; _Args = {SharedClass<A>}; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2]’
/usr/include/c++/7/bits/shared_ptr_base.h:1295:35:   required from ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...) [with _Alloc = std::allocator<A>; _Args = {SharedClass<A>}; _Tp = A; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2]’
/usr/include/c++/7/bits/shared_ptr.h:344:64:   required from ‘std::shared_ptr<_Tp>::shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...) [with _Alloc = std::allocator<A>; _Args = {SharedClass<A>}; _Tp = A]’
/usr/include/c++/7/bits/shared_ptr.h:690:14:   [ skipping 9 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/usr/include/c++/7/bits/shared_ptr_base.h:1295:35:   required from ‘std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...) [with _Alloc = std::allocator<A>; _Args = {main()::<lambda(SharedClass<A>)>}; _Tp = A; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2]’
/usr/include/c++/7/bits/shared_ptr.h:344:64:   required from ‘std::shared_ptr<_Tp>::shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...) [with _Alloc = std::allocator<A>; _Args = {main()::<lambda(SharedClass<A>)>}; _Tp = A]’ /usr/include/c++/7/bits/shared_ptr.h:690:14:   required from ‘std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = A; _Alloc = std::allocator<A>; _Args = {main()::<lambda(SharedClass<A>)>}]’
/usr/include/c++/7/bits/shared_ptr.h:706:39:   required from ‘std::shared_ptr<_Tp> std::make_shared(_Args&& ...) [with _Tp = A; _Args = {main()::<lambda(SharedClass<A>)>}]’
main.cpp:21:33:   required from ‘SharedClass<T>::SharedClass(T2&&) [with T2 = main()::<lambda(SharedClass<A>)>; T = A]’
main.cpp:39:48: required from here
/usr/include/c++/7/ext/new_allocator.h:136:4: error: no matching function for call to ‘A::A(SharedClass)’
   { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:33:5: note: candidate: A::A(A::Callback)
      A(Callback callback)
      ^ main.cpp:33:5: note:   no known conversion for argument 1 from ‘SharedClass’ to ‘A::Callback {aka std::function)>}’
main.cpp:28:7: note: candidate: constexpr A::A(const A&)
  class A
        ^ main.cpp:28:7: note:   no known conversion for argument 1 from ‘SharedClass’ to ‘const A&’
main.cpp:28:7: note: candidate: constexpr A::A(A&&)
main.cpp:28:7: note:   no known conversion for argument 1 from ‘SharedClass’ to ‘A&&’

If I change the constructor of the SharedClass class like this SharedClass (T2 t2) it compiles.

Upvotes: 1

Views: 436

Answers (1)

Alan Birtles
Alan Birtles

Reputation: 36419

As your lambda takes its parameters by value it needs to be able to create copies of those parameters, it does this by calling the move constructor. As you have defined a templated constructor with the same signature as the move constructor the compiler attempts to use it as a move constructor. Adding default copy (for completeness) and move constructors fixes the issue:

SharedClass(const SharedClass&) = default;
SharedClass(SharedClass&&) = default;

Your existing constructor doesn't work as a move constructor because there is no constructor for A which takes a SharedClass<A> parameter so make_shared<A> doesn't compile when passed a SharedClass<A> parameter.

Upvotes: 2

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