Switch with derived class mfc C++

Question

0.00/5 (No votes)

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double GetTime(const CMY * tool)
{
	double d_tAtt;
	double d_tDis;

	switch (tool->GetMode())
	{
	case 0:
		AfxMessageBox(_T("non contemplato"));
		break;
	case 1:
	case 2:
		
	case 3:
	
		break;
	case 4:
		
		break;
	

	}
	return 0.0;
}

in every case I must use a derived class of CMY, but I don't know I can write.

in this way?

CMYdERIV tool1 = CMY tool?

What I have tried:

I searched on internet, I hope I explain well

Posted 8-Nov-23 3:43am

Member 14594285

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3 solutions

Solution 1

You should be able to write:

C++

CMYdERIV* tool1 = (CMYdERIV*)tool

... assuming your class definitions allow it.

Posted 8-Nov-23 4:16am

Richard MacCutchan

Comments

Member 14594285 8-Nov-23 10:34am

thanks, but inside a case Can I insert a definition?

Richard MacCutchan 8-Nov-23 10:37am

Yes, no or maybe. Please update your question, and explain exactly what you are trying to achieve.

Member 14594285 8-Nov-23 10:39am

Can I write:
case 1:
CMYdERIV* tool1 = (CMYdERIV*)tool;
break;

Richard MacCutchan 8-Nov-23 11:06am

The compiler will not like that. You need to declare tool1 before the case statement. But if you really want to isolate it from the other cases then You can do it the following way:

	switch (tool->GetMode())
	{
	case 0:
		AfxMessageBox(_T("non contemplato"));
		break;
	case 1:
    {
        CMYdERIV* tool1 = (CMYdERIV*)tool;
        // use tool1 only in this block
    }
        break;

	case 2:

Rick York 8-Nov-23 18:22pm

It appears we wrote essentially the same thing at the same time and I don't know why you didn't get an up-vote so here you go.

Richard MacCutchan 9-Nov-23 3:26am

Thanks, it's good to know we agree.

Member 14594285 9-Nov-23 4:23am

I tried this but I see that value of tool1 aren't valued

Richard MacCutchan 9-Nov-23 4:28am

Sorry, I don't know what that means. If you have compiler problems then show us the exact message, if something else then please provide complete details.

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Rick York · Accepted Answer · 2023-11-08T05:06:00

Solution 2

You can do initialization and declaration in a case statement but they must enclosed in braces like this :

C++

case 3:
    {
       CMYderived * tool1 = (CMYderived) tool;
    }
    break;

Posted 8-Nov-23 5:06am

Rick York

CPallini · Accepted Answer · 2023-11-08T20:47:00

Solution 3

Well, you could use a dynamic_cast, try for instance

C++

#include <iostream>
using namespace std;

class A
{
protected:
   int a;
public:
  A(int a):a{a}{}
  virtual void show(){ cout <<  "A " << a << "\n"; }
};

class AB : public A
{
public:
  AB(int a):A{a}{}
  void show() override { cout <<  "AB " << a << "\n"; }
};

class AC : public A
{
public:
  AC(int a):A{a}{}
  void show() override { cout <<  "AC " << a << "\n"; }
};


int main()
{

  A a{10};
  AB ab{20};
  AC ac(30);

  cout << "enter your choice: ";
  int choice;
  cin >> choice;
  switch(choice)
  {
  case 0:
    break;
  case 1:
    {
      auto refAB = dynamic_cast<AB &>(ab); // OK, casting to the correct derived class
      refAB.show();
    }
    break;
  case 2:
    {
      auto refAB = dynamic_cast<AB &>(a); // ERROR, trying to cast an instance of the base class
      refAB.show();
    }
  break;
  case 3:
    {
      auto refAB = dynamic_cast<AB &>(ac); // ERROR, trying to cast an instance of a different derived class
      refAB.show();
    }
  break;
  //...
  default:
  //...
    break;
  }
}

Anyway, such behaviors are usually implemented using polymorphism.

Posted 8-Nov-23 20:47pm

CPallini

Comments

Richard MacCutchan 9-Nov-23 3:29am

+5 for a very comprehensive solution.

CPallini 9-Nov-23 3:45am

Thank you.
I posted it because the C-style cast, which always succeeds, doesn't look very appropriate there.

Richard MacCutchan 9-Nov-23 4:30am

That's the problem with C++, you can still use C-code even when it is not the best choice.

CPallini 9-Nov-23 4:37am

That's ONE of the PROBLEMS with C++ (FFY).
:-D :-D :-D

Richard MacCutchan 9-Nov-23 4:46am

You mean there are lots more? :(

Member 14594285 9-Nov-23 4:17am

thanks but I don't understand what refAB means, I have base class CTool, and derived class CToolDerived..can I see an example?

CPallini 9-Nov-23 4:35am

In your case

auto pTD = dynamic_cast<CToolDerived* >(pT);

assuming pT is a pointer to a CToolDerived object, passed as a pointer to CTool.

Member 14594285 9-Nov-23 4:20am

auto refAB = dynamic_cast<ab &="">(ab); // OK, casting to the correct derived class
refAB.show();, in this code where is "A"?

Maxim Kartavenkov 9-Nov-23 4:39am

5.
One note: the casting to the reference raises runtime exception if cast not available, if exceptions not handled in application then better to use casting into pointer type:

auto pAB = dynamic_cast<AB *>(ab);
if (pAB) { // if casting succeeded then pointer is not null
   pAB->show();
}

CPallini 9-Nov-23 5:29am

Thank you.
Exceptions are, sometimes, desirable. See, for instance, the STL at method.