Difference between iterator and pointer

[dekeenfrance]

Hi,

I just figured out that some std functions (for example: copy) need the resource and target objects have iterators to work. Otherwise, the compiler rejects. In case of two arrays, declared as:

double myA[100] = {};
array<double, 100> myB = {};

myA[0] is like a pointer, myB.begin() an iterator, if I do not make any mistake. So, what is exactly the difference between the pointer and the iterator here? They all give the access to elements.

If I need the target of copy to be an array like myA which cannot give an iterator, is there a way to make the command "copy" work for it?

Thanks in advance for replying.

[jumper007]

Firstly, an iterator is a pointer to the dynamic array. It allows you to go through all its elements.
Here is one way of doing it:

#include <iostream>
#include <array>

int main()
{
	int i = 0;  // Counter for number of elements in static array "myA"
	double myA[5] = { 1, 2, 3, 4, 5}; // Static array
	std::array<double,5> myB; // Dynamic array
	std::array<double,5>::iterator it; // Iterator to dynamic array

        // Iterate through the dynamic array, but keeping the counter of the elements at same step. 
        // Each element in the myB is getting the value of the element myA[i] going in an ascending order
	for (it = myB.begin(), i = 0; it != myB.end(); ++it, ++i)
	{
		*it = myA[i]; // Assign value
	}
	for (it = myB.begin();  it != myB.end(); ++it) // Iterate once more to print them out
	{
		std::cout << *it << " "; // Print them out
	}
	std::cout << std::endl;
	system("pause");
	return 0;
}

Then you can use all the functions that the <array> library can take.

Hope I could help,
~ Raul ~

[Peter87]

An iterator is just an object that can be used to iterate the elements in a container. There are different categories of iterators. The difference is what operations they support e.g. with a Forward iterator you you can use ++ to go from one element to the next and with a Random access iterator you can go from one element to another element in one step.
http://www.cplusplus.com/reference/iterator/

myA[0] gives you a reference to the first element in myA. It isn't a pointer, nor is it an iterator. A pointer to an array element is an iterator, because it satisfies all requirements to be a random access iterator.

To copy all elements from myA to myB:
std::copy(myA, myA + 100, myB.begin());

In C++11 you can also use std::begin and std::end that will work the same way on raw arrays and all other containers.
std::copy(begin(myA), end(myA), begin(myB));

[jumper007]

@Peter87 Oh, yes. Didn't think that what you said would work. But it seems it does. :)

[dekeenfrance]

Thank you for your quick replies.

Actually I want to copy from myB to myA. I feel this is a bit more complicated to run... because myA doesn't have iterator and the command "copy" needs one...

So maybe in this case I cannot use the std::copy command, I have to write my own function, like jumper007 did... right?

[Cubbi]

It's the same way: copy(myB.begin(), myB.end(), myA); or copy(begin(myB), end(myB), begin(myA));

[dekeenfrance]

actually I tried with copy(myB.begin(), myB.end(), myA); but it could not compile. the compiler indicated errors with xutility related to iterators...

[zb0S216C]

Iterators are usually structures that house a pointer which are given a common interface that containers can use to transverse their elements. Though, this isn't always the case. In some implementations of standard containers, "std::vector::iterator", for example, "iterator" is merely defined as "typedef T* iterator".

In effect, the difference between a pointer and an iterator really depends on the implementation of the iterator.

_Wazzak

[Cubbi]

dekeenfrance wrote:

the compiler indicated errors with xutility related to iterators.

"xutility" indicates that you're using Visual Studio. It does not indicate errors in this case, it raises a warning because using pointers as iterators circumvents the run-time checks Visual Studio adds to the iterators in Debug mode.

Add -D_SCL_SECURE_NO_WARNINGS to "Project -> Properties -> C/C++ -> Command Line" or #define it before the #includes

(PS: hm, my VS2012 compiles it quietly, perhaps they got rid of that)

[dekeenfrance]

Thanks to you all for your explanations. It's all clear now! :-)