C++ 默认构造函数，用新对象初始化指针

C++ default constructor, initializing pointer with new object

提问by Alex

I have the following problem: In myClass I want to default initialize a pointer to yourClass, with a new yourClass adress. Unfortunately, if I want to delete the pointer at any point I get a (core dump).

我有以下问题：在 myClass 中，我想默认初始化一个指向 yourClass 的指针，并使用新的 yourClass 地址。不幸的是，如果我想在任何时候删除指针，我会得到一个（核心转储）。

class myClass
{
      protected:
      yourClass * yc;

      public:
      myClass() { yc = new yourClass(); }

      myClass(yourClass * tyc ) { delete yc; yc = tyc; }

      ~myClass() { delete yc; yc = NULL; }

      void setMyClass (yourClass * tyc) { delete yc; yc = tyc; }

      void print () { yc->print(); }
};

int main()
{
  yourClass b (//parameter);
  myClass * a = new myClass();
  a->print();
  a->setMyClass(&b)
  a->print();

  delete a;
  return 0;
}

The print() of a, should result in two different prints, dependent on //parameters.

a, 的 print() 应该导致两个不同的打印，这取决于 // 参数。

I considered yourClass yc; instead of a yourClass* yc, but I want to know if it is possible.

我考虑过你的Class yc；而不是 yourClass* yc，但我想知道是否可能。

EDIT:I reworked the code in the following way and it works. Still looks complicated, smart pointers seem promising and I still did not apply the "Rule of Three". Here the code. Thanks all.

编辑：我按以下方式重新编写了代码并且它有效。看起来仍然很复杂，智能指针似乎很有希望，我仍然没有应用“三分法则”。这里是代码。谢谢大家。

class myClass
{
      protected:
      yourClass * yc;
      bool dynamic;

      public:
        myClass() { dynamic = true; yc = new yourClass (); }
        myClass (yourClass * tyc ) 
        { 
          // dynamic init (like default)
          if (tyc == NULL ) { dynamic = true; yc = new yourClass (); }
          // static use of yc
          else { dynamic = false; yc = tyc; } 
        }
        // because only if dynamic is true, we need to erase
        ~blu () { if (dynamic) { delete yc; dynamic = false; } } 

        void setMyClass(yourClass* tyc) 
        { 
          // leaving unchanged if new-stuff is NULL or like old-stuff
          if ( tyc == yc || tyc == NULL ) return;
          else // treating dynamic and static differently
          { 
            if (dynamic) // if flag is set, must be deleted 
            {
              delete yc; yc = tyc; dynamic = false;
            }
            else // must not be deleted, dynamic is still false
            {
              yc = tyc;
            }
          }
        }
        void print () { yc->print(); }
};

回答by Lorenzo Dematté

That's because you are trying to delete too much:

那是因为你试图删除太多：

• you are deleting a non-allocated object in the second constructor (remove delete yc;)
• you are trying to delete a stack-allocated object, b. delete a;will try to delete a pointer to b, which is an object on the stack; what happens depend on your OS (I expect an exception/core dump/whatever)

• 您正在删除第二个构造函数中的未分配对象（remove delete yc;）
• 您正在尝试删除堆栈分配的对象 b。delete a;将尝试删除指向 b 的指针，它是堆栈上的一个对象；会发生什么取决于你的操作系统（我希望有一个异常/核心转储/任何）

EDIT: another problem I spotted.. a->setMyClass(NULL)

编辑：我发现的另一个问题.. a->setMyClass(NULL)

I would suggest:

我会建议：

• this poston smart pointers
• this blog poston RAII
• any C/C++ primer explaining stack vs. heap allocation (static vs. dynamic?)

• 这篇关于智能指针的帖子
• 这篇关于 RAII 的博文
• 任何解释堆栈与堆分配的 C/C++ 入门（静态与动态？）

回答by sehe

You're violating the rule of three.

你违反了三原则。

What is The Rule of Three?

三分法则是什么？

Also this is recipe for disaster:

这也是灾难的秘诀：

 myClass(yourClass * tyc ) { delete yc; yc = tyc; }

What happens if tyc==yc? Right. Not pretty :)

如果会发生什么tyc==yc？对。不漂亮 ：）

 myClass(yourClass * tyc ) { if (yc!=tyc) { delete yc; yc = tyc; } }