c++ - 从shared_ptr 派生

Question

我有一个无法理解的奇怪编译问题。

//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>){ });

编译日志:

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&&’

如果我更改像 SharedClass (T2 t2)这样的SharedClass类的构造函数，它将进行编译。

Answer 1

当您的lambda按值获取其参数时，它需要能够创建这些参数的副本，因此可以通过调用move构造函数来做到这一点。当您定义了具有与move构造函数相同签名的模板化构造函数时，编译器会尝试将其用作move构造函数。添加默认副本(出于完整性)并移动构造函数可解决以下问题:

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

您现有的构造函数无法用作move构造函数，因为没有 A的构造函数，该构造函数需要 SharedClass<A>参数，因此传递 make_shared<A>参数时 SharedClass<A>不会编译。