C++ std::unique_ptr 用法

std::unique_ptr usage

提问by Guillaume07

std::unique_ptr<int> p1(new int);
std::unique_ptr<int> p2(new int);
p2=p1;

It seems here that p1 is no longer "unique" since p2 refer to it also

这里似乎 p1 不再是“唯一的”，因为 p2 也指代它

It is legal c++ ? Does unique_ptr have copy_semantics ? If no, and if it has only move semantics, is p1 set to NULL after assign it to p2 ?

它是合法的 c++ 吗？unique_ptr 有 copy_semantics 吗？如果没有，并且只有移动语义，那么在将 p1 分配给 p2 后是否将其设置为 NULL？

EDIT:

编辑：

ok so the correct version is

好的，所以正确的版本是

 p2=std::move(p1)

According to that, after this assign, p1 is not valid ? And the difference with auto_ptr is here? it is more safe to explictly specfiy transfer of ownership than implicitly as it is the case with auto_ptr I guess

据此，在此分配之后，p1 无效？和 auto_ptr 的区别就在这里？显式指定所有权转移比隐式指定更安全，因为我猜 auto_ptr 就是这种情况

回答by Tony The Lion

std::unique_ptr is non-assignable and non-copyable. You need to use std::move();

std::unique_ptr 是不可赋值和不可复制的。你需要使用 std::move();

so

所以

p1 = std::move(p2);

Have a look herefor more info.

看看这里了解更多信息。

回答by Howard Hinnant

Here is an article I wrote which answers your questions. I originally wrote this article to present an emulation of unique_ptr. However you can ignore the first few paragraphs dealing with the emulation and just start reading at "Basic Examples".

这是我写的一篇文章，可以回答您的问题。我最初写这篇文章是为了展示 unique_ptr 的模拟。但是，您可以忽略处理仿真的前几段，直接从“基本示例”开始阅读。

http://howardhinnant.github.io/unique_ptr03.html

http://howardhinnant.github.io/unique_ptr03.html

Edit:

编辑：

I had trouble distilling the above linked article down to something small enough to make a practical answer in this format. However here is my best shot:

我无法将上面链接的文章提炼成足够小的内容，无法以这种格式做出实用的答案。然而，这是我最好的镜头：

The reason: Safety in generic code. One can not really make copies of either auto_ptror unique_ptr. Consider:

template <class T>
void foo(T t)
{
    T copy_of_t = t;  // line 4
    assert(copy_of_t == t);
}

It is not unusual at all for generic code to look like fooabove. The assertis probably not actually there, but the assumption that the assertwould hold often is there ... implicitly. Indeed, a popular implementation of std::sorthad exactly this logic in 1996, which is exactly what prompted the second auto_ptrredesign (which helped, but didn't completely fix the problem).

原因：通用代码的安全性。一个人不能真正制作auto_ptr或 的副本unique_ptr。考虑：

template <class T>
void foo(T t)
{
    T copy_of_t = t;  // line 4
    assert(copy_of_t == t);
}

通用代码看起来像foo上面那样并不罕见。The assert可能实际上并不存在，但assert通常会成立的假设 存在...... 隐含地。事实上，std::sort1996 年的一个流行实现正是这种逻辑，这正是促使第二次 auto_ptr重新设计的原因（这有帮助，但并没有完全解决问题）。

回答by amit

As per this, p2=p1is a compilation error.

按照这个，p2=p1是一个编译错误。