CopyConstructor

ÿþ/****************************************************************/

void OnStart() {ClassExample(); StructExample();}

/***************************************************************

   Have you ever wondered how to safely return automatic objects

    from functions without memory leaks?

   Here is one option that will work in MQL.

   The technique is called 'copy constructor'. Make sure that

    all fields are copied, including complex data types,

    recursively.

   Please refer to the example code below, you will figure out

    how it works.

*/

void ClassExample()

  {

   Print(__FUNCTION__);

   A a1;

   A a2=Fa(); //object receive from function

   a1.Work();

   a1.Report(); //1 1 1 1

   a2.Report(); //3 3 3 3

   /*CONTINUE USE OBJECT FROM FUNCTION*/

   a2.Work();

   a2.Report(); //4 4 4 4

   return;

  }

/***************************************************************

   PARTS OF CLASS A*/

struct C {int mx; C():mx(0) {}};

class D {public:int mx; D():mx(0) {} D(D &d):mx(d.mx) {}};

/****************************************************************/

class A

  {

protected:

   int               mx;   //simple

   int               ma[]; //array

   C                 mc;   //struct

   D                 d;    //class

public:

   /*COPY CONSTRUCTOR*/

                     A(A &a):mx(a.mx),d(a.d) {ArrayCopy(ma,a.ma); mc=a.mc;}

   /*EXPLICIT DEFAULT CONSTRUCTOR*/

                     A():mx(0) {}

   /*METHODS*/

   void              Work() {mx++; ArrayResize(ma,mx); mc.mx=mx; d.mx=mx;}

   void              Report() {Print(mx," ",ArraySize(ma)," ",mc.mx," ",d.mx);}

  };

/****************************************************************/

A Fa()

  {

   A a;

   for(int i=0; i<3; i++)

     {a.Work();} //a lot of work

   return a; //object return

  }

/***************************************************************

   Want to return a structure?

   Structures can be normally updated with function as parameters

    sent by reference.

   If you wish to RETURN a struct with function, it is also

    possible, but unlike classes, structs do not require an

    explicit 'copy constructor'.

*/

void StructExample()

  {

   Print(__FUNCTION__);

   B b1;

   B b2=Fb(); //struct receive from function

   b1.mx++;

   /**/

   Print(b1.mx," ",ArraySize(b1.ma)); //  1 0

   Print(b2.mx," ",ArraySize(b2.ma)); // 10 9

   return;

  }

/****************************************************************/

struct B

  {

   int               mx;

   int               ma[];

                     B():mx(0) {} //constructor, not needed formally

  };

/****************************************************************/

B Fb()

  {

   B b;

   for(int i=0; i<10; i++)

     {b.mx++; ArrayResize(b.ma,i);} //work hard

   return b; //struct return

  }

/**/