MQL4参考普通函数 GetPointer

GetPointer

The function returns the object pointer.

void*  GetPointer(
   any_class anyobject      // object of any class
   );

Parameters

anyobject

[in] Object of any class.

Return Value

The function returns the object pointer.

Note

Only class objects have pointers. Instances of structures and simple-type variables can't have pointers. The class object not created using the new() operator, but, e.g., automatically created in the array of objects, still has a pointer. But this pointer will be of the automatic type POINTER_AUTOMATIC, therefore the delete() operator can't be applied to it. Aside from that, the type pointer doesn't differ from dynamic pointers of the POINTER_AUTOMATIC type.

Since variables of structure types and simple types do not have pointers, it's prohibited to apply the GetPointer() function to them. It's also prohibited to pass the pointer as a function argument. In all these cases the compiler will notify an error.

An attempt to call an incorrect pointer causes the critical termination of a program. That's why the CheckPointer() function should be called prior to using a pointer. A pointer can be incorrect in the following cases:

the pointer is equal to NULL;
the object has been deleted using the delete operator.

This function can be used to check the validity of a pointer. A non-zero value guarantees, that the pointer can be used for accessing.

Example:

//+------------------------------------------------------------------+
//|                                             Check_GetPointer.mq5 |
//|                        Copyright 2009, MetaQuotes Software Corp. |
//|                                              https://www.mql5.com |
//+------------------------------------------------------------------+
#property copyright "2009, MetaQuotes Software Corp."
#property link      "https://www.mql5.com"
#property version   "1.00"

//+------------------------------------------------------------------+
//| Class implementing the list element                              |
//+------------------------------------------------------------------+
class CItem
  {
   int               m_id;
   string            m_comment;
   CItem*            m_next;
public:
                     CItem() { m_id=0; m_comment=NULL; m_next=NULL; }
                    ~CItem() { Print("Destructor of ",m_id,
                                     (CheckPointer(GetPointer(this))==POINTER_DYNAMIC)?
                                     "dynamic":"non-dynamic"); }
   void              Initialize(int id,string comm) { m_id=id; m_comment=comm; }
   void              PrintMe() { Print(__FUNCTION__,":",m_id,m_comment); }
   int               Identifier() { return(m_id); }
   CItem*            Next() {return(m_next); }
   void              Next(CItem *item) { m_next=item; }
  };
//+------------------------------------------------------------------+
//| Simplest class of the list                                       |
//+------------------------------------------------------------------+
class CMyList
  {
   CItem*            m_items;
public:
                     CMyList() { m_items=NULL; }
                    ~CMyList() { Destroy(); }
    bool             InsertToBegin(CItem* item);
    void             Destroy();
  };
//+------------------------------------------------------------------+
//| Inserts list element at the beginning                            |
//+------------------------------------------------------------------+
bool CMyList::InsertToBegin(CItem* item)
  {
   if(CheckPointer(item)==POINTER_INVALID) return(false);
//---
   item.Next(m_items);
   m_items=item;
//---
   return(true);
  }
//+------------------------------------------------------------------+
//| Deletes the list by deleting elements                            |
//+------------------------------------------------------------------+
void CMyList::Destroy()
  {
//--- service pointer to work in a loop
   CItem* item;
//--- go through the loop and try to delete dynamic pointers
   while(CheckPointer(m_items)!=POINTER_INVALID)
     {
      item=m_items;
      m_items=m_items.Next();
      if(CheckPointer(item)==POINTER_DYNAMIC)
        {
         Print("Dynamyc object ",item.Identifier()," to be deleted");
         delete (item);
        }
      else Print("Non-dynamic object ",item.Identifier()," cannot be deleted");
     }
//---
  }
//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
  {
   CMyList list;
   CItem   items[10];
   CItem*  item;
//--- create and add into the list a dynamic object pointer
   item=new CItem;
   if(item!=NULL)
     {
      item.Initialize(100,"dynamic");
      item.PrintMe();
      list.InsertToBegin(item);
     }
//--- add automatic pointers into the list
   for(int i=0; i<10; i++)
     {
      items[i].Initialize(i,"automatic");
      items[i].PrintMe();
      item=GetPointer(items[i]);
      if(CheckPointer(item)!=POINTER_INVALID)
         list.InsertToBegin(item);
     }
//--- add one more dynamic object pointer at the list beginning
   item=new CItem;
   if(item!=NULL)
     {
      item.Initialize(200,"dynamic");
      item.PrintMe();
      list.InsertToBegin(item);
     }
//--- delete all the list elements
   list.Destroy();
//--- all the list elements will be deleted after the script is over
//--- see the Experts tab in the terminal
  }