[英]error in moving std::unique_ptr
I try to run this code to move the unique_ptr's from the first vector to another, in the copy constructor: 我尝试运行以下代码以将unique_ptr的值从第一个向量移动到副本构造函数中的另一个向量:
class Text
{
struct paragraph
{
int index;
string text;
};
vector<unique_ptr<paragraph>> paragraphs;
public:
Text()
{
paragraphs.push_back(unique_ptr<paragraph>(new paragraph));
}
Text(const Text & t)
{
for(int i = 0; i < (int)t.paragraphs.size(); i++)
{
paragraphs.push_back(move(t.paragraphs[i]));
}
}
};
and i got this error: 我得到了这个错误:
1>c:\program files\microsoft visual studio 10.0\vc\include\xmemory(208): error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'
1> with
1> [
1> _Ty=Text::paragraph
1> ]
// Etc.
There are several things wrong with your code. 您的代码有几处错误。
First, you cannot move from a const&
, and this applies to members of a const&
too. 首先,您不能从
const&
移开,这也适用于const&
成员。 Movement is destructive; 运动具有破坏性; you should only move from a
&&
. 您只能从
&&
移开。 Your copy constructor should copy ; 你的复制构造器应该复制 ; if you don't want a copy constructor, then
= delete
it or whatever your compiler allows. 如果您不想要复制构造函数,则
= delete
它或编译器允许的任何形式。
Second, assuming you took a Test &&
for a proper move constructor, you should not move each element like that. 其次,假设您为适当的move构造函数进行了
Test &&
,则不应该这样移动每个元素。 Instead, move the vector into the new one like this: 而是将向量移到新的向量中,如下所示:
Text(Text && t) : paragraphs(std::move(t.paragraphs)) {}
Third, you should only write this function assuming that your compiler of choice doesn't support automatically generated move constructors (ie: is Visual Studio). 第三,您只应在假设您选择的编译器不支持自动生成的move构造函数(即Visual Studio)的情况下编写此函数。 If it does support it, you shouldn't write one at all .
如果它支持它,那么您根本不应该编写一个 。 Let the compiler do it's job.
让编译器完成任务。
Text(const Text & t)
{
for(int i = 0; i < (int)t.paragraphs.size(); i++)
{
paragraphs.push_back(move(t.paragraphs[i]));
}
}
In this constructor t
is const, so t.paragraphs[i]
gives a const lvalue reference to a unique_ptr
. 在此构造函数中,
t
为const,因此t.paragraphs[i]
给出了对unique_ptr
的const左值引用。
move(t.paragraphs[i])
turns that into a const rvalue reference, but it's still const. move(t.paragraphs[i])
将其转换为const rvalue引用,但仍为const。 The unique_ptr
move constructor needs a non-const rvalue reference, so isn't viable, so the deleted copy constructor is the best match. unique_ptr
move构造函数需要一个非常量右值引用,因此不可行,因此删除的副本构造函数是最佳匹配。 you shouldn't be trying to move the contents of t
in a copy constructor, that's what move constructors are for. 您不应该尝试在副本构造函数中移动
t
的内容,这就是move构造函数的作用。
You should also say std::move
not just move
, to prevent ADL finding the wrong move. 您还应该说
std::move
不只是move
,以防止ADL找到错误的动作。
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