differences between new and malloc in c++

Question

#include <iostream>
#include <cstdlib>
using namespace std;

class Box {
   public:
      Box() {
         cout << "Constructor called!" <<endl;
      }
      void printer(int x)
    {
        cout<<x<<" printer"<<endl;
    }

      ~Box() {
         cout << "Destructor called!" <<endl;
      }

};

int main( ) {
    Box* myBoxArray = new Box[4];

    Box* myBoxArray2 = (Box*)malloc(sizeof(Box[4]));
    myBoxArray2->printer(23);
    *myBoxArray2;
    *(myBoxArray2).printer(23);

   return 0;
}

the problem simply is that when i use 'new' the constructor is printed out but when i simple derefrence the pointer to myBoxArray2 the constructor is not printed and neither is the funtion printer printed. Also why is it that when i use -> the funnction printer runs but not when i use the equivalent *(myBoxArray2).printer(23)

Your use of `malloc()` is invalid here. Class instances must be allocated on the stack, or via `new`. When you use `malloc()` none of the C++ machinery is invoked. — user207421, Apr 07 '17 at 07:56
`myBoxArray2` is just a pointer to some memory space that's the size of - well, the size of something that's irrelevant. Even if you copied `myBoxArray` into that space, it's never going to call the constructor, because - wait for it - it's not being constructed. — moopet, Apr 07 '17 at 09:59

Some programmer dude · Accepted Answer · 2017-04-07T08:02:57.150

malloc allocates memory only, it doesn't invoke constructors which can leave objects in an indeterminate state.

In C++ you should almost never use malloc, calloc or free. And if possible avoid new and new[] as well, use object instances or vectors of instances instead.

As for your second question (which is really unrelated to the first), *(myBoxArray2).printer(23) is wrong since the the . selection operator have higher precedence than the dereference operator *. That means first of all that you use the . member selector on a pointer which is invalid, and that you attempt to dereference what printer returns which is also wrong since it doesn't return anything.

You want (*myBoxArray2).printer(23) (note the location of the asterisk is inside the parentheses), which is exactly the same as myBoxArray2->printer(23).

Also note that myBoxArray2->printer(23) is the same as myBoxArray2[0].printer(23).

The Quantum Physicist · Answer 2 · 2017-04-07T08:01:07.483

1

The difference is that malloc allocates memory without initializing it at all. On the other hand, new calls the appropriate constructor to initialize that memory (if that constructor is made to initialize that memory) and do other stuff to make a class usable.

Also delete calls the destructor for you.

Rule of thumb is: Never use malloc with C++, unless you know what you're doing.

edited Apr 07 '17 at 08:01

answered Apr 07 '17 at 07:55

The Quantum Physicist

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Unless you really know what you're doing (and why you're doing it) - hence the constructor new. – UKMonkey Apr 07 '17 at 07:58
Even if you know what you're doing you should still *probably* not do it. The compiler might make assumptions about how those objects are initialized due to optimizations and violating that is an express ticket to "Undefined Behaviour". – tadman Apr 07 '17 at 08:24

differences between new and malloc in c++

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