c++ - 通过引用传递——为什么要调用这个析构函数？

Question

我找不到(关于析构函数调用主题的许多问题)任何与我的情况完全相同的问题。

为什么传递的参数是引用时调用析构函数？我在我认为执行输出的代码行下方放置了注释(主要在 main 中)。

struct X { // simple test class
   int val;

   void out(const std::string& s, int nv)
   {
      std::cerr << this << "–>" << s << ": " << val << " (" << nv << ")\n";
   }

   // default constructor
   X() { 
      out("X()", 0); 
      val = 0; 
   } 

   X(int v) { 
      val = v; 
      out("X(int)", v); 
   }

   // copy constructor
   X(const X& x) {
      val = x.val; 
      out("X(X&) ", x.val); 
   } 

   // copy assignment
   X& operator=(const X& a)
   {
      out("X::operator=()", a.val); 
      val = a.val; 
      return *this;
   }

   // Destructor
   ~X() { 
      out("~X()", 0); 
   }
};

X glob(2); // a global variable
// Output Line 1: X(int): 2 (2)

X copy(X a) { 
   return a; 
}

主要函数:

    int main()
{
   X loc{ 4 }; // local variable
      // Output Line 2: X(int): 4 (4)
      // ^from X(int v) function
   X loc2{ loc }; // copy construction
      // Output Line 3: X(X&) : 4 (4)
      // ^from X(const X& x) function
   loc = X{ 5 }; // copy assignment 
      // Output Line 4: X(int): 5 (5)
      // ^from X(int v) function
      // Output Line 5: X::operator=(): 4 (5)
      // ^from the '=' operator overload
      // Output Line 6: ~X(): 5 (0) - ???
   loc2 = copy(loc); // call by value and return 
      // Or does Output Line 6 result from here?
   .
   .
   .
}

1) 这个析构函数被调用是因为 loc = X{ 5 };//复制赋值 或之后的行:loc2 = copy(loc);//按值调用并返回?

2) 为什么要调用它？据我所知，析构函数仅在以下情况下调用:

a) names go out of scope
b) program terminates
c) "delete" is used on a pointer to an object

我知道它不是“b”或“c”，所以一定是因为某些事情超出了范围。但我认为超出复制赋值函数范围的引用不会造成这种情况。

Answer 1

您可以看到在复制赋值发生后不久就调用了析构函数。复制赋值完成后，临时(x{5})被销毁。

来自标准的析构函数部分:

15.4 Destructors
...
12. A destructor is invoked implicitly
(12.1) — for a constructed object with static storage duration at program termination,
(12.2) — for a constructed object with thread storage duration at thread exit,
(12.3) — for a constructed object with automatic storage duration when the block in which an object is created exits,
(12.4) — for a constructed temporary object when its lifetime ends.