C ++来自同一原始指针的多个唯一指针 [英] C++ multiple unique pointers from same raw pointer

问题描述

在下面考虑我的代码.我对唯一指针的理解是，只能使用一个唯一指针来引用一个变量或对象.在我的代码中，我有多个unique_ptr访问同一个变量.

Consider my code below. My understanding of unique pointers was that only one unique pointer can be used to reference one variable or object. In my code I have more than one unique_ptr accessing the same variable.

我知道，使用智能指针显然不是正确的方法，因为指针应具有创建后的完全所有权.但是，为什么这是有效的却没有编译错误呢?谢谢.

It's obviously not the correct way to use smart pointers i know, in that the pointer should have complete ownership from creation. But still, why is this valid and not having a compilation error? Thanks.

#include <iostream>
#include <memory>

using namespace std;

int main()
{
    int val = 0;
    int* valPtr = &val;

    unique_ptr <int> uniquePtr1(valPtr);
    unique_ptr <int> uniquePtr2(valPtr);

    *uniquePtr1 = 10;
    *uniquePtr2 = 20;

    return 0;
}

推荐答案

但是，为什么这仍然有效

But still, why is this valid

它无效有效！这是不确定的行为，因为std::unique_ptr的析构函数将释放具有自动存储持续时间的对象.

It is not valid! It's undefined behaviour, because the destructor of std::unique_ptr will free an object with automatic storage duration.

实际上，您的程序尝试破坏int对象三次.首先通过uniquePtr2，然后通过uniquePtr1，然后通过val本身.

Practically, your program tries to destroy the int object three times. First through uniquePtr2, then through uniquePtr1, and then through val itself.

是否没有编译错误?

and not having a compilation error?

因为通常在编译时无法检测到此类错误:

Because such errors are not generally detectable at compile time:

unique_ptr <int> uniquePtr1(valPtr);
unique_ptr <int> uniquePtr2(function_with_runtime_input());

在此示例中，function_with_runtime_input()可能执行许多复杂的运行时操作，最终会返回指向valPtr所指向的同一对象的指针.

In this example, function_with_runtime_input() may perform a lot of complicated runtime operations which eventually return a pointer to the same object valPtr points to.

如果正确使用std::unique_ptr，则几乎总是使用std::make_unique，这样可以防止出现此类错误.

If you use std::unique_ptr correctly, then you will almost always use std::make_unique, which prevents such errors.

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