用 unique_pointers 填充矢量

Question

我必须 classes A 和 B.
B 派生自 A.
我还有矢量 std::vector<std::unique_ptr<A>> samples

这段代码有效：

std::vector<std::unique_ptr<A>> samples;
samples.push_back(std::make_unique<B>(param_1, param_2));

但是这个没有：

std::vector<std::unique_ptr<A>> samples = {std::make_unique<B>(param_1, param_2)};

并产生这样的错误：

/usr/include/c++/9/bits/stl_uninitialized.h: In instantiation of ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const std::unique_ptr<A>*; _ForwardIterator = std::unique_ptr<A>*]’:
/usr/include/c++/9/bits/stl_uninitialized.h:307:37:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = const std::unique_ptr<A>*; _ForwardIterator = std::unique_ptr<A>*; _Tp = std::unique_ptr<A>]’
/usr/include/c++/9/bits/stl_vector.h:1582:33:   required from ‘void std::vector<_Tp, _Alloc>::_M_range_initialize(_ForwardIterator, _ForwardIterator, std::forward_iterator_tag) [with _ForwardIterator = const std::unique_ptr<A>*; _Tp = std::unique_ptr<A>; _Alloc = std::allocator<std::unique_ptr<A> >]’
/usr/include/c++/9/bits/stl_vector.h:626:2:   required from ‘std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = std::unique_ptr<A>; _Alloc = std::allocator<std::unique_ptr<A> >; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<std::unique_ptr<A> >]’
my_file.cpp:88:113:   required from here
/usr/include/c++/9/bits/stl_uninitialized.h:127:72: error: static assertion failed: result type must be constructible from value type of input range
  127 |       static_assert(is_constructible<_ValueType2, decltype(*__first)>::value,
      |                                                                        ^~~~~
make: *** [makefile:15: cpp] Error 1

问题
1. 此错误是否与 vector (interator?) 或 A/B class?
中缺少适当的构造函数有关 2. 当我制作push_back时，我是否应该总是std:move和unique_pointer？我问是因为我怀疑如果没有 std::move 是否会隐式创建一些副本。另一方面，也许编译器做了一些优化，可以识别这样的 "short" 结构？

Answer 1

Is this error connected with lack of proper constructor in vector

这是一种解释。

(interator?)

没有

or in A/B class?

没有

问题是您使用的向量构造函数接受 std::initializer_list。 class 复制参数。唯一指针不可复制，所以这不起作用。

When I make push_back, should I do always std:move with unique_pointer or not?

给定一个左值表达式，如果你想把它移到向量中，那么你必须使用 std:move。您不需要也不应该将 std:move 与已经是右值的表达式一起使用。

Answer 2

Is this error connected with lack of proper constructor in vector (interator?) or in A/B class?

也不是，这是因为std::unique_ptr的复制构造函数被隐式删除了（由于用户声明的移动构造函数）；您无法复制 std::unique_ptr，您可以移动它。

When I make push_back, should I do always std:move with unique_pointer or not? I ask because I have doubts if maybe without std::move there are created implicitly some copies. In other hand maybe compiler does some optimalizations and can recognize such "short" construction?

是的，当在带有左值参数的 std::unique_ptr 元素的向量上调用 push_back 时，您需要使用 std::move 来调用 std::unique_ptr 的移动构造函数（而不是它隐式删除的复制构造函数）。

#include <memory>
#include <vector>

int main() {
    std::vector<std::unique_ptr<int>> v;
    auto value = std::make_unique<int>(1);

    // v.push_back(value);                  // error (attempts to copy)
    v.push_back(std::move(value));          // lvalue "std::to_xvalue" -> move ctor
    v.push_back(std::make_unique<int>(2));  // already an rvalue -> move ctor
    return 0;
}

用 unique_pointers 填充矢量

Filling of vector with unique_pointers

c++

initialization

vector

push-back

unique-ptr