Using structures and pointer arrays c++

Question

I am trying to write a pointer array to structs in C++. My main goal is to be able to dynamically add pointers to the array. I am having trouble with the synthax

  struct items
  {
    int member1;
    int member2;
  };

  int N=5;
  items **ptr=new item *[N]; // This is a ptr to an array of ptr's. Each ptr 
                             // in this array needs to point to an items struct.

My question is how to write in the struct's objects from this point on. I know I need to create them first but I don't have any idea how to do that.

The ideal solution is probably a std::vector<item> or std::vector<std::unique_ptr<item>>. — Qaz
– Qaz, Commented Oct 15, 2013 at 17:13

πάντα ῥεῖ · Accepted Answer · 2013-10-15 18:42:35Z

3

You're allocating only an array of pointers of item *, you'll need to allocate the memory for each item also, e.g.:

struct item // naming it 'items' might be confusing, and was used inconsistently
            // in your code sample
{
    int member1;
    int member2;
};

int N=5;
item **ptr=new item *[N];

for(int i = 0; i < N; ++i)
{
    ptr[i] = new item();
}

Accessing your structure members looks like this:

ptr[2]->member1 = 42; // Sets member1 of the 3rd item element to 42

Note that you'll need to free the allocated memory somewhere as follows:

for(int i = 0; i < N; ++i)
{
    delete ptr[i];
}
delete [] ptr;

I general for c++ you'd be better off using a c++ standard container like:

#include <vector>

int N = 5;
std::vector<item> myItems;
myItems.resize(N,item());

myItems[2].member1 = 42; // Sets member1 of the 3rd item element to 42

which would do all the memory management for you internally.
If you're using c++11 and do not need dynamically sized arrays you can even avoid heap allocated memory at all using std::array:

#include <array>

std::array<item,5> myItems; // Size is fixed to 5, you can't use a variable here

myItems[2].member1 = 42; // Sets member1 of the 3rd item element to 42

edited Oct 15, 2013 at 18:42

answered Oct 15, 2013 at 17:19

πάντα ῥεῖ

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4 Comments

Reboot_87 Over a year ago

It's working. What is the advantage in using vectors instead of what I did? I am new to C++ and have not studied about vectors yet.

Reboot_87 Over a year ago

And also, you wrote that accessing structure members is done with:ptr[2]->member1 = 42;. What is its equivalent? for ex: *(ptr[2]).id?

πάντα ῥεῖ Over a year ago

Don't know what exactly you mean with 'What is its equivalent', but I'd say *(ptr[2]).member1 (there's no field id introduced anywhere). What concerns the advantages of using standard containers, it's clearly written in the answer.

Reboot_87 Over a year ago

Thank you for the response! As for the field "id" I meant member1. sorry for the confusion.

Sergey · Accepted Answer · 2013-10-15 17:17:44Z

1

You can add objects to your array by ptr[i] = new items(). And you can access data in items** ptr by ptr[i]->member1. But I'll strongly recommend using stl containers and smart pointers.

answered Oct 15, 2013 at 17:17

Sergey

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Comments

Igor Popov · Accepted Answer · 2013-10-15 17:18:25Z

0

Firstly you have to finish the allocation step.

for(int i=0;i<N;i++)
  ptr[i]=new items;

Then you can access the array using e.g.:

ptr[0]->member1=123;

answered Oct 15, 2013 at 17:18

Igor Popov

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