为什么输出是2020？

Question

我有以下代码：

#include <iostream>
using namespace std;

class Foo {
   int data;
public:
   Foo(int d = 0) {
      data = d;
   }

   ~Foo() {
      cout << data;
   }
};

int main() {
   Foo a;
   a = 20;
   return 0;
}

这段代码的输出是 2020。我想发生的事情是，创建了一个临时对象 a。一旦使用赋值运算符为 a 赋值 20，就会调用析构函数并打印 20。然后 main 函数到达 return 并第二次调用析构函数，再次打印 20.

我说得对吗？

Answer 1

你是对的。其实修改你的代码如下可以更清楚的展示代码的逻辑。

#include <iostream>
using namespace std;

class Foo {
   int data;
public:
   Foo(int d = 0) {
      cout << "call constructor!" << endl;
      data = d;
   }

   ~Foo() {
      cout << data << endl;
   }
};

int main() {
   Foo a; // Foo::Foo(int d = 0) is called which yields the first line of output
   a = 20; // is equal to follows
   
   // 1. a temporary object is constructed which yields the second line of output
   Foo tmp(20);
   // 2. since you do not provide operator= member function,
   // the default one is generated the compiler
   // and a member-wise copy is performed
   a.operator=(&tmp);  
   // after this copy assignment, a.data == 20
   // 3. tmp is destroyed which yields the third line of output
   tmp.~Foo();
   // 4. right before the program exits, a is destroyed which yields the last line of output
   a.~Foo();

   return 0;
}

输出为：

call constructor!

call constructor!

20

20

为什么输出是2020？

Why is the output 2020?

c++

constructor

destructor