为什么是2020年的产量？

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。我想会发生什么，创建了一个临时对象a。一旦使用赋值运算符将值赋值为 20，就会调用析构函数并打印 20。然后 main 函数到达 return 并再次调用析构函数，再次打印 20。

我对吗？

Answer 1

你是对的。实际上将你的代码修改如下可以更清晰地展示代码的逻辑。

#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;
}

输出是：

调用构造函数！

调用构造函数！

20

20