在类的构造函数中异常处理的行为 [英] Behaviour of exception handling in constructor of a class

问题描述

我有这个程序，其中派生类的ctor抛出异常。程序只是一个示例程序，我只是想了解异常处理的概念。

I have this program where the ctor of derived class throws a exception. The program is just an example program where I am just trying to understand the concept of exception handling.

class A{
public:
   A() {}

   ~A(){std::cout << "DTOR called - A!!" << std::endl;}
};

class B : public A
{
public:
   B():A()
   {
      try
      {
         init();
      }
      catch(...)
      {
         std::cout << "Inside catch block in B's Ctor!!" << std::endl;
         throw this;
      }
   }

   void init() { throw 0;  }

   ~B() {std::cout << "DTOR called - B!!" << std::endl; }
};

int main()
{
   try{
      B *b = new B;

      std::cout << "Äfter B's ctor called in try block!!" << std::endl;
      delete b;
      std::cout << "Äfter B's dtor called in try block!!" << std::endl;
   }

   catch(B* b)
   {
      delete b;
      b = NULL;
      std::cout << "Exception Occurred in B!!" << std::endl;
   }

   catch(A* a)
   {
      delete a;
      a = NULL;
      std::cout << "Exception Occurred in A!!" << std::endl;
   }

   catch(...)
   {
      std::cout << "Exception Occured!!" << std::endl;
   }
   return EXIT_SUCCESS;
}

预期的输出是它应该进入B的catch块和第一个B的dtor应该被调用，然后A的dtor。但是上面的程序的输出是：

The expected output is that it should enters catch block of B and first B's dtor should be called and then A's dtor. But the output of above program is:

Inside catch block in B's Ctor!!
DTOR called - A!!
DTOR called - B!!
DTOR called - A!!
Exception Occurred in B!!

我的问题在这里是为什么类A的dtor调用两次，当它只进入catch块B并调用B类的dtor？
另外请告诉我这里是否犯了一些错误。
任何帮助感谢

My question here is why the dtor of class A called twice when it only enters the catch block for class B and calls the dtor of class B only?? Also please do tell if I am making some mistake here. Any help is appreciated

编辑：

class B : public A
{
public:
   B():A()
   {
      try
      {
         szName = new char[100];
         init();
      }
      catch(...)
      {
         std::cout << "Inside catch block in B's Ctor!!" << std::endl;
         throw this;
      }
   }

   void init() { throw 0;  }

   ~B()
   {
      delete szName;
      std::cout << "DTOR called - B!!" << std::endl;
   }

   char *szName;
};

这里我在类B中创建了一个char指针。在Ctor中的try块中分配的内存是异常被抛。现在在这种情况下会有一个内存泄漏，如果我不捕获异常B类

Here I have created a char pointer in class B. Allocated memory in try block in Ctor before exception is being thrown. Now in this case will there be a memory leak if i do not catch exception for class B ??

推荐答案

code>〜A（）被调用两次是因为在 B（）， A ）被完全初始化，所以当发生异常时，将自动调用〜A（），而 B 本身尚未完全初始化，因此〜B（）将不会被调用，并且您不需要也不应该删除指针if构造函数throws，C ++将调用 delete 在这种情况下。请参阅 LIVE DEMO 。

The reason that ~A() is called twice is because inside the body of B(), A() is fully initialized, so when an exception occurs, ~A() will be called automatically, while B() itself is not fully initialized yet, so ~B() won't be called, and you need not and shouldn't delete the pointer if the constructor throws, C++ will call delete for you in that case. See LIVE DEMO.

这篇关于在类的构造函数中异常处理的行为的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！