error: conversion from 'Multiplication*' to non-scalar type 'Multiplication' requested

[shadowofwar]

I am trying to finish a program that calculates a simple multiplication using addition and recursivity.

I.e 3 *4 = 3+3+3+3

So I have made a class as follows:

class Multiplication
{
    private:
    int product;
    public:
    Multiplication() //constructor
    {
        product=0;
    }
    int CalculateMultip(int a, int b)
    {
        if(b!=0)
        {
            product=a+CalculateMultip(a,b-1);  //implements recursivity
        }
        return product;
    }
};

Now the main() program can compile if I take away the new Multiplication() part of the following snippet of code :

Multiplication objMultip=new Multiplication();

but when I do, I can only run the program once, because the value of "product" accumulates and if I run the Multiplication subprogram again, it shows the accumulated product, and this is not what I want. Is there any way I can reset it using the "new" function? Because at the moment the error message states:

error: conversion from 'Multiplication*' to non-scalar type 'Multiplication' requested

[shacktar]

Is there any way I can reset it using the "new" function?

"new" will return a pointer to an instance. i.e. it will give you a Multiplication* which you would have to delete when you're done. This is dynamic memory allocation and this is not necessary for your current problem.

I would suggest to write CalculateMultip without the product variable as it is not necessary.

[shadowofwar]

How can I write CalculateMultip without the product
variable? Without it the function wouldn't use recursion, which is what I have been asked to use :S
I've tried changing the line from:
Multiplication objMultip=new Multiplication();
to:
Multiplication* objMultip=new Multiplication();
and even:
Multiplication* objMultip=new Multiplication;

but the same error appears, on line 5 of the main() program below,
where I have a case that says:

case 2: cout << "Write the first number: ";
                    cin>>pA;
                    cout << "Write the second number you would like to multiply: ";
                    cin>>pB;
                    cout << pA<<" * "<<pB<<" = "<<objMultip.CalculateMultip(pA,pB)<<endl;
                    break;

here the error message is:

error: request for member 'CalculateMultip' in 'objMultip', which is of non-class type 'Multiplication*'

[jim80y]

You don't have to use dynamic allocation, you can just create a local instance inside a loop:

while (loop == true)
{
  Multiplication theMultip; // Constructs with default constructor (no params)

  cin >> pA;
  cin >> pB;
  cout << theMultip.CalculateMultip(pA, pB);
}

That way, it creates a Multiplication object every time round the loop.

Alternately, if you don't want to keep creating new objects, you can provide a reset() function in the Multiplication class and call that to clear the results out.

Cheers,
Jim

[shadowofwar]

That works perfectly Jim, thanks to all that replied, with the loop it resets just as I want it to! =)

 case 2:                  
                    loop=true;  
                    while (loop == true)
                    {
                    Multiplication objMultip; // Constructs with default constructor (no params)
                    cout << "Write the first number: ";
                    cin>>pA;
                    cout << "Write the second number you would like to multiply: ";
                    cin>>pB;
                    cout << pA<<" * "<<pB<<" = "<<objMultip.CalculateMultip(pA,pB)<<endl;
                    loop=false;
                    }
                    break;

[shacktar]

How can I write CalculateMultip without the product variable? Without it the function wouldn't use recursion, which is what I have been asked to use :S

This function uses recursion and does not use the product variable.

int CalculateMultip(int a, int b)
{
   if(b == 0)
   {
      return 0;
   }
   else
   {
      return a + CalculateMultip(a,b-1);  //implements recursivity
   }
}

[shadowofwar]

Oh I see what you mean now! Thanks!

[jim80y]

Just one more - now I see how you're using it, you don't need the loop within the case 2: section; you're always setting loop=false after the first iteration, so it can become:

case 2:  {
           Multiplication objMultip;

           // Do prompting/reading

           cout << pA<<" * "<<pB<<" = "<<objMultip.CalculateMultip(pA,pB)<<endl;
         }
         break;

On a related note - this example is fine because the Multiplication class is lightweight and doesn't have much overhead to construct/destruct it.

If it wasn't lightweight (it contained a lot of members that all needed initialising, for example), and you were running around the main loop many many times, it would become a performance issue because it would take time to construct/destruct each instance. In that case you'd be better off just re-using the same instance and resetting it.

Jim