在将unique

在将unique_ptr分配给引用时，析构函数在成员函数之前被调用(Destructor gets called before member function when assigning unique_ptr to a reference)

我正在使用unique_ptr，我得到了一些奇怪的结果。 这是代码：

class Sniffer { public: Sniffer() { cout << "Sniffer()" << endl; s = "String!"; } void operator()() { cout << "operator()(): " << s << endl; } ~Sniffer() { cout << "~Sniffer()" << endl; } private: string s; }; int main() { cout << "Begin scope!" << endl; { Sniffer& h = *std::unique_ptr<Sniffer>(new Sniffer()); // 1 //std::unique_ptr<Sniffer> p(new Sniffer()); // 2 //Sniffer& h = *p; // 2 h(); } cout << "End scope!" << endl; return 0; }

我不明白为什么使用代码“1”，析构函数在operator（）（）之前调用，而“End Scope！”根本不打印。 代码不会崩溃，它会执行到最后一行，我得到的输出是：

Begin scope! Sniffer() ~Sniffer() operator()(): Press any key to continue . . .

另一方面，代码“2”表现如预期：

Begin scope! Sniffer() operator()(): String! ~Sniffer() End scope! Press any key to continue . . .

我使用引用的原因仅仅是因为我感觉h()感觉比(*p)()或p->operator()()更自然。 谢谢。

I'm using unique_ptr, and I'm getting some odd results. This is the code:

class Sniffer { public: Sniffer() { cout << "Sniffer()" << endl; s = "String!"; } void operator()() { cout << "operator()(): " << s << endl; } ~Sniffer() { cout << "~Sniffer()" << endl; } private: string s; }; int main() { cout << "Begin scope!" << endl; { Sniffer& h = *std::unique_ptr<Sniffer>(new Sniffer()); // 1 //std::unique_ptr<Sniffer> p(new Sniffer()); // 2 //Sniffer& h = *p; // 2 h(); } cout << "End scope!" << endl; return 0; }

I don't understand why with code "1", the destructor gets called before operator()(), and "End Scope!", is not printed at all. Code doesn't crash, it executes till the last line, the output I get is:

Begin scope! Sniffer() ~Sniffer() operator()(): Press any key to continue . . .

On the other side code "2" behaves as expected:

Begin scope! Sniffer() operator()(): String! ~Sniffer() End scope! Press any key to continue . . .

The reason I'm using a reference is only because I felt h() felt more natural than (*p)() or p->operator()(). Thanks.

最满意答案

在这个声明中

Sniffer& h = *std::unique_ptr<Sniffer>(new Sniffer());

创建一个std::unique_ptr<Sniffer>类型的临时对象，该对象在语句执行结束时被销毁。

因此，引用h在代码块的范围内无效。

在这个声明中

std::unique_ptr<Sniffer> p(new Sniffer()); // 2

对象p被创建并将一直存在，直到代码块结束。 所以参考h

Sniffer& h = *p;

在此范围内有效。

In this statement

Sniffer& h = *std::unique_ptr<Sniffer>(new Sniffer());

a temporary object of type std::unique_ptr<Sniffer> is created that is destroyed at the end of execution of the statement.

Thus reference h is invalid in the scope of the code block.

In this statement

std::unique_ptr<Sniffer> p(new Sniffer()); // 2

object p is created and will be alive until the end of the code block. So reference h

Sniffer& h = *p;

will be valid in this scope.