为什么不能使用initializer_list初始化unique_ptr的向量? [英] Why it is not possible to use an initializer_list to initialize a vector of unique_ptr's?

问题描述

我想知道为什么initializer_list无法与unique_ptr一起使用:

I'm wondering why initializer_list doesn't work with unique_ptr:

std::vector<std::unique_ptr<int>> vptr = {std::make_unique<int>(1), std::make_unique<int>(2)};

不编译.

但是:

std::vector<std::unique_ptr<int>> vptr(2);
vptr[0] =std::make_unique<int>(1);
vptr[1] =std::make_unique<int>(2);

编译.

和

std::vector<int*>vint={new int{1},new int{2}};

或

 std::vector<std::shared_ptr<int>> vptr= {std::make_shared<int>(1), std::make_shared<int>(2)};

编译.

unique_ptr 变体在clang中给出此错误消息:

The unique_ptr variant is giving this error message in clang:

In file included from main.cpp:2:
In file included from /usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/vector:62:
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/stl_construct.h:75:38: error: call to deleted constructor of 'std::unique_ptr<int, std::default_delete<int> >'
    { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
                                     ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/stl_uninitialized.h:75:8: note: in instantiation of function template specialization 'std::_Construct<std::unique_ptr<int, std::default_delete<int> >, const std::unique_ptr<int, std::default_delete<int> > &>' requested here
                std::_Construct(std::__addressof(*__cur), *__first);
                     ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/stl_uninitialized.h:125:2: note: in instantiation of function template specialization 'std::__uninitialized_copy<false>::__uninit_copy<const std::unique_ptr<int, std::default_delete<int> > *, std::unique_ptr<int, std::default_delete<int> > *>' requested here
        __uninit_copy(__first, __last, __result);
        ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/stl_uninitialized.h:278:19: note: in instantiation of function template specialization 'std::uninitialized_copy<const std::unique_ptr<int, std::default_delete<int> > *, std::unique_ptr<int, std::default_delete<int> > *>' requested here
    { return std::uninitialized_copy(__first, __last, __result); }
                  ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/stl_vector.h:1284:11: note: in instantiation of function template specialization 'std::__uninitialized_copy_a<const std::unique_ptr<int, std::default_delete<int> > *, std::unique_ptr<int, std::default_delete<int> > *, std::unique_ptr<int, std::default_delete<int> > >' requested here
            std::__uninitialized_copy_a(__first, __last,
                 ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/stl_vector.h:377:2: note: in instantiation of function template specialization 'std::vector<std::unique_ptr<int, std::default_delete<int> >, std::allocator<std::unique_ptr<int, std::default_delete<int> > > >::_M_range_initialize<const std::unique_ptr<int, std::default_delete<int> > *>' requested here
        _M_range_initialize(__l.begin(), __l.end(),
        ^
main.cpp:10:42: note: in instantiation of member function 'std::vector<std::unique_ptr<int, std::default_delete<int> >, std::allocator<std::unique_ptr<int, std::default_delete<int> > > >::vector' requested here
std::vector<std::unique_ptr<int>> vptr = {std::make_unique<int>(1), std::make_unique<int>(2)};
                                         ^
/usr/local/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.9.0/../../../../include/c++/4.9.0/bits/unique_ptr.h:356:7: note: 'unique_ptr' has been explicitly marked deleted here
      unique_ptr(const unique_ptr&) = delete;
      ^
1 error generated.

推荐答案

vector< T> 的构造函数采用 initializer_list< T> 进行复制列出向量.当您处理不可复制的类型时，这将不起作用: unique_ptr< T> 的设计可防止进行复制.它确实允许移动，这就是赋值起作用的原因: make_unique 返回一个右值，编译器知道该右值可以安全移动.

The constructor for vector<T> taking an initializer_list<T> copies the elements from the initializer list into the vector. This does not work when you're dealing with an uncopyable type: the design of unique_ptr<T> prevents copies from being made. It does allow moving, which is why assignment works: make_unique returns an rvalue, for which the compiler knows it's safe to move.

您需要某种采用 initializer_list< T&> 的构造函数，但是C ++没有这样的构造函数(确实，如dyp所指出的那样，类型 initializer_list< T&> 甚至函数).

You'd need some sort of constructor taking an initializer_list<T &&>, but C++ doesn't have such a constructor (nor indeed, as dyp pointed out, does the type initializer_list<T &&> even function).

这篇关于为什么不能使用initializer_list初始化unique_ptr的向量?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！