How can do the argument based constructor for dynamic allocatiom

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#include<iostream>
using namespace std;

class A
{
        public:
                int a;
        A(int a) : a(a) {       }
};

int main()
{
        A a(8);
        A b = a;
        printf("a=%p\n", &a);
        printf("b=%p\n", &b);
        cout<<a.a<<endl;

        A *c = new int();
}

Hi,
how to create an argument based constructor initialization with dynamic allocation.??

What I have tried:

tried to initilize but getting compilation error

Posted 10-Oct-23 7:21am

mathiv327

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3 solutions

Solution 2

For this one I think I can guess what the problem is but you should always include the compiler error messages you get because that makes it much easier for us to help.

Anyway, the problem is your member variable and the argument have the same name. I prefer to prepend all member variables with the "m_" wart so here is what that would look like :

C++

class A
{
public:
   int m_a;

   A( int arg ) : m_a( arg )
   {
   }
};

Posted 10-Oct-23 8:25am

Rick York

Comments

Richard MacCutchan 10-Oct-23 15:25pm

The compiler is clever enough to distinguish between the parameter and the member variable.

Rick York 10-Oct-23 17:19pm

Yes, you are right. It didn't used to be this way so I learned a long time ago not to do it. Regardless, you found the problem so have an up-vote from me.

Richard MacCutchan 11-Oct-23 3:41am

Thanks. And, for the record, I also used to do it this way, based on my experiences in the early days of MFC.

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k5054 · Accepted Answer · 2023-10-10T09:19:00

When creating a dynamic object, you can pass the initialization arguments to the new operator, similar to creating an object on the stack

C++

A *c = new A(17);  // allocates space for a new A object with initializer "17"

cout << c->a << endl;  // prints 17

delete c;  //  returns allocated space to the heap

But these days, you should use smart pointers, instead of new/delete: Smart Pointers in C++ - GeeksforGeeks[^]

Richard MacCutchan · Accepted Answer · 2023-10-10T09:24:00

Solution 4

The problem is at this line:

C++

A *c = new int();

The left hand side is OK, you declare c as a pointer to an A object. But the right hand side creates a pointer to an integer. But c is a pointer to an object of A, not an integer. So change it to:

C++

A *c = new A(0); // c now points to a new A object, initialised with a zero value; but it could be any integer value.

Posted 10-Oct-23 9:24am

Richard MacCutchan

Comments

CPallini 11-Oct-23 2:03am

5.

Richard MacCutchan 11-Oct-23 3:44am

Thanks, Carlo.