adv and disadv of the two following data structures:

[kfaday]

Hi i've got these two data structures:

obj *pObj1 = new obj[10000];

obj **pobj2 = new (*obj)[10000];
for (int i=0;i<10000;++i);
pobj2[i]=new obj;

which are the advantages and disadvantages from each one of them?

i know it's an exam question, but it seems to be interesting. i can't think of many.. firstly, i think the first one is easier to use, but the second one is more like ordered.

i'll really appreciate your help!!
thanks!

[Kheun]

I have made some corrections to your code so that it can compile and run.

obj  *pObj1 = new obj [10000];

obj  **pobj2 = new obj *[10000];
for (int i=0;i<10000;++i)
pobj2[i]=new obj;

IMO, although the result from both codes are the same, it is still better to choose the simpler one as it is more readable. Hence easier to maintain or debug.

[Graham]

... and uses about 40,000 fewer bytes of memory (assuming 4-byte pointers).

[Andreas Masur]

...and is much more safe in regard to memory leaks etc.

Nevertheless, I do not know what kind of exam it was, however, the best method would be...

#include <vector>

std::vector<obj> Objects(10000);

Granted...it will only work in C++...

[NMTop40]

so will what he is doing. In C you'd have to use malloc.

Whether to have an array/vector of objects or of pointers depends on the nature of "obj". Using a vector of objects should be done when they are cheap to construct, cheap and safe (and legal) to copy and cheap and safe (and legal) to assign. Otherwise use a vector of pointers, possibly reference-counted ones (i.e. boost::shared_ptr).

[Guysl]

This is an interesting question if we change the original line:

obj  **pobj2 = new (obj *)[10000]; //syntax error

into:

obj  *pobj2 = new (obj *)[10000];

The latter creates an array of 10000 obj's, and returns a "pointer to an array of 10000 obj's"...

[NMTop40]

new T[ N ] produces an array of T and therefore returns T*

so new obj * [ N ] should return obj **

[Guysl]

NMTop40,
Its not a common usage, but there is a "creature" as the OP declared:
new (*obj)[10000];

That "creature" is a pointer to a block of elements (NOT the address of the first element, but as one piece).
That decleration returns 'obj*'.

[Guysl]

My return type is not correct, I check what is it.

[Guysl]

OK, sorry for the previous posts. I was sure for some reason that operator new
can construct the following "pointer to block", apparently it can't:

int (*p)[100]; // ==> a valid pointer to a block of 100 integers.
where sizeof(*p) is 100*sizeof(int)