Re: two types of heap allocated objects--any benefit?

=?iso-8859-1?q?Erik_Wikstr=F6m?= <>
2 May 2007 23:19:11 -0700
On 3 Maj, 07:47, pmouse <> wrote:

On May 2, 9:50 pm, newbie <> wrote:

On May 2, 6:05 pm, "Victor Bazarov" <> wrote:

newbie wrote:

Let's see two different usage of an STL container. I see (2) more
often when reading code over (1), dose that give any benefit or it's
purely a coding preference?

Also, please see the (3), I see (3) often in ppl' code, does that g=


any benefit over (2)

Thanks for answering in advance:)

OK, let's rewrite this a tiny bit...

class MyData { ... }; // common to every case

class Example {
    std::deque<MyData> storage; // an object
    void Enque(MyData const& d)
     { storage.push_back(d); } // storing a copy


class Example {
    std::deque<MyData> *storage; // a pointer
    void Enque(MyData const& d)
     { storage->push_back(d); } // storing a copy


class Example {
    std::deque<MyData*> *storage; // a pointer
    void Enque(MyData* pd)
     { storage->push_back(pd); } // storing a pointer


Every case has its use. (1) is common and easy to understand
and maintain. Extra copies are made of 'MyData', and it's not
use polymorphically. (2) Is not really different from (1),
except that the member 'storage' is dynamically allocated.
Makes no difference, really. Is harder to maintain than (1).
(3) Stores pointers to MyData. That's a totally different
container since it allows polymorphic use of the objects stored
in 'storage'. The fact that 'storage' is a pointer makes no
difference (and no sense either, like in case 2). If you intend
to store and to use 'MyData' polymorphically, it's better to
have a combination of (1) and (3):

class Example {
    std::deque<MyData*> storage; // an object
    void Enque(MyData* pd)
     { storage.push_back(pd); } // storing a pointer


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Thanks a lot. Very helpful.

There is a case where you would want to keep your member data as
pointers, that's when you don't want to call the default constructor
on them, hence you create them manually.

For example, the stl containers takes iterator inputs as constructor
parameter, if you want to call those constructors, keep them as

Another case when you want to store pointers to the data instead of
copies are when you want to have to same object in multiple containers
(or rather pointers to the same data). Something like:

Example ex1;
Example ex2;
for (int i = 0; i < HUGE; ++i) {
  MyData* a = new MyData();

Than any changes made to objects in ex1 will also be made to those in
ex2, which would not be the case if you used methods 1 or 2.

Erik Wikstr=F6m

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