C++ How to initialize an array of objects in the heap

Question

Given a simple class MyClass with a constructor that accepts two int, how can I initialize an array of MyClass in the heap?

I've tried

MyClass *classes[2] = { new MyClass(1, 2),
                        new MyClass(1, 2) };

But this doesn't seem to work. Thanks

@MooingDuck: No: the pointers are on the stack, and the objects are on the heap. — Kerrek SB
– Kerrek SB, Commented Nov 4, 2011 at 15:55
@KerrekSB: I wrote it wrong. The array of pointers is on the stack, the objects are on the heap. Either way, he says that isn't what he wants. — Mooing Duck
– Mooing Duck, Commented Nov 4, 2011 at 16:07

Mooing Duck · Accepted Answer · 2011-11-04 16:22:36Z

1

Use the std::allocator<MyClass> for this.

std::allocator<MyClass> alloc;
MyClass* ptr = alloc.allocate(2);  //allocate
for(int i=0; i<2; ++i) {
    alloc.construct(ptr+i, MyClass(1, i)); //construct in C++03
    //alloc.construct(ptr+i, 1, i); //construct in C++11
}

//use

for(int i=0; i<2; ++i) {
    alloc.destroy(ptr+i); //destruct
}
alloc.deallocate(ptr); //deallocate

Note that you don't have to construct all that you allocate.

Or, better yet, just use std::vector.

[EDIT] KerrekSB suggested this as simpler:

MyClass** ptr = new MyClass*[3];
for(int i=0; i<4; ++i)
    ptr[i] = new MyClass(1, i);

//use

for(int i=0; i<4; ++i)
   delete ptr[i];
delete[] ptr;

It's slightly slower access, but much easier to use.

edited Nov 4, 2011 at 16:22

answered Nov 3, 2011 at 23:48

Mooing Duck

67.6k19 gold badges107 silver badges169 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Mooing Duck Over a year ago

@KerrekSB: Do you another way to construct an array of objects with varying parameters? I can't think of one. (Not that there isn't, I just can't think of it)

Kerrek SB Over a year ago

Well, the OP's code is actually correct and does basically that, so I'm not sure where the problem is now...

Mooing Duck Over a year ago

@KerrekSB: Since a local, fixed size (array (of pointers) on the stack pointing at (objects on the heap)) wasnt what he wanted, he must want an actual (dynamic array of (objects on the heap)).

Kerrek SB Over a year ago

We clearly need parentheses around those sentences :-) Well, the OP should specify her needs; as it stands, I'm not sure what the problem is.

Mooing Duck Over a year ago

I actually used to parenthesize sentences like that, until people told me to knock it off and rewrite them so they make sense the first time.

|

Collectives™ on Stack Overflow

C++ How to initialize an array of objects in the heap

1 Answer 1

7 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

7 Comments

Your Answer

Sign up or log in

Post as a guest

Related