What is difference between instantiating an object using new vs. without

问题

In C++,

Aside from dynamic memory allocation, is there a functional difference between the following two lines of code:

Time t (12, 0, 0); //t is a Time object

Time* t = new Time(12, 0, 0);//t is a pointer to a dynamically allocated Time object

I am assuming of course that a Time(int, int, int) ctor has been defined. I also realize that in the second case t will need to be deleted as it was allocated on the heap. Is there any other difference?

回答1:

The line:

Time t (12, 0, 0);

... allocates a variable of type Time in local scope, generally on the stack, which will be destroyed when its scope ends.

By contrast:

Time* t = new Time(12, 0, 0);

... allocates a block of memory by calling either ::operator new() or Time::operator new(), and subsequently calls Time::Time() with this set to an address within that memory block (and also returned as the result of new), which is then stored in t. As you know, this is generally done on the heap (by default) and requires that you delete it later in the program, while the pointer in t is generally stored on the stack.

回答2:

One more obvious difference is when accessing the variables and methods of t.

Time t (12, 0, 0);
t.GetTime();

Time* t = new Time(12, 0, 0);
t->GetTime();

回答3:

As far as the constructor is concerned, the two forms are functionally identical: they'll just cause the constructor to be called on a newly allocated object instance. You already seem to have a good grasp on the differences in terms of allocation modes and object lifetimes.

回答4:

I think you already understand all the differences. Assuming that you are well aware about the syntax difference of accessing a member of t through a pointer and through a variable (well, pointer is also a variable but I guess you understand what I mean). And assuming also that you know the difference of call by value and call by reference when passing t to a function. And I think you also understand what will happen if you assign t to another variable and make change through that other variable. The result will be different depending on whether t is pointer or not.

回答5:

There is no functional difference to the object between allocating it on the stack and allocating it on the heap. Both will invoke the object's constructor.

Incidentally I recommend you use boost's shared_ptr or scoped_ptr which is also functionally equivalent when allocating on the heap (with the additional usefulness of scoped_ptr constraining you from copying non-copyable pointers):

scoped_ptr<Time> t(new Time(12, 0, 0));

回答6:

No.. There is no other difference..

回答7:

There is no other difference to what you know already.

Assuming your code is using the service of default operator new.

回答8:

Use new： Call operator new function to get dynamic memory, and then to call the constuctor function.
Not use new: Will not call operator new function, just directly to call the constuctor function. The stack will be used directly, no use to malloc.

回答9:

void foo (Time t)
{
  t = Time(12, 0, 0);
}

void bar (Time* t)
{
  t = new Time(12, 0, 0);
}


int main(int argc, char *argv[])
{
  Time t;
  foo(t);//t is not (12,0,0),its value depends on your defined type Time's default constructor. 

  bar(&t);//t is (12,0,0)
  return 0;
}

来源：https://stackoverflow.com/questions/3673998/what-is-difference-between-instantiating-an-object-using-new-vs-without

标签

c++

constructor

new-operator