C++ dynamic allocation

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4.20/5 (2 votes)

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Let's say we have a class A

class A 
{
// blabla
public int x;
}

when we do this:

A a=new A();

is the x also allocated dynamically?
An explanation will be great. Thank you

What I have tried:

I think it is done dynamically since we need to go threw the object to access "x".

Posted 24-Aug-18 1:52am

amine zawix

Updated 24-Aug-18 2:18am

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Jochen Arndt · Accepted Answer · 2018-08-24T02:10:00

Only the non-static member variables are stored within the allocated memory block. With your example the memory holds only the x member and has therefore at least the size of an int.

You can verify this by printing out the addresses:

class A
{
public:
    A() { x = 0; }
    int x;
};

int main()
{
    A *a = new A();
    printf("Size of A = %lu\n", sizeof(*a));
    printf("Address of a = %p\n", a);
    printf("Address of a->x = %p\n", &a->x);
    return 0;
}

which might print

Size of A = 4
Address of a = 0x602010
Address of a->x = 0x602010

Note also the correct syntax used in my example (class definition must be terminated with a semicolon and a is a pointer).

Leo Chapiro · Accepted Answer · 2018-08-24T02:19:00

You can create object on the stack or dynamically on the heap.
On the stack:

C++

void myFunc()
{
   A a;
}

Now exists the "a" as long as you execute the myFunc.

Dynamically on heap:

A* heapA = new A();

Now you have a pointer to A and can use it as long as you need it.
After that you need to destroy it in order to get the memory free:

delete heapA;

I hope that answers your question. For more details take a look at Strategies for the Allocation of Memory - ModernesCpp.com[^]

C++ dynamic allocation

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