Unable to compile c++ program on Ubuntu

Question

I am unable to compile the following program. I am using g++ version 5 on ubuntu.

#include<iostream>
#include<iterator>
int main()
{
    iterator it;
    return 0;
}

It gives me the following errors,

a.cc: In function ‘int main()’:
a.cc:5:5: error: ‘iterator’ was not declared in this scope
     iterator it;
     ^
a.cc:5:5: note: suggested alternatives:
In file included from /usr/include/c++/5/bits/stl_algobase.h:65:0,
                 from /usr/include/c++/5/bits/char_traits.h:39,
                 from /usr/include/c++/5/ios:40,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from a.cc:1:
/usr/include/c++/5/bits/stl_iterator_base_types.h:118:12: note:   ‘std::iterator’
     struct iterator
            ^
/usr/include/c++/5/bits/stl_iterator_base_types.h:118:12: note:   ‘std::iterator’

The error is pretty self-explanatory. There is no type named `::iterator`. — StoryTeller - Unslander Monica, Jan 03 '18 at 07:44
`iterator` is located in the `std` namespace, so you have to type `std::iterator` or use `using namespace std;`. — lorisleitner, Jan 03 '18 at 07:45
Thx. I used std namespace. Now I get the following error "a.cc:6:14: error: missing template arguments before ‘it’" which is expected. Is there any C++ standard in which the template arguments are not mandatory? — rahul, Jan 03 '18 at 07:51
@rahul no, there is no such standard. What do you want to do? — Captain Giraffe, Jan 03 '18 at 07:52
No. And there is no C++ standard where you should use `std::iterator` to declare variables. What are you *actually* trying to accomplish? Oh, and don't listen to the advice of [`using namespace std;`](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) — StoryTeller - Unslander Monica, Jan 03 '18 at 07:52
In some cases template arguments can be deduced (and don't have to be specified explicitly), this is not such a case — UnholySheep, Jan 03 '18 at 07:52
Actually, I need to compile a binpacking library provided by "https://github.com/betterwaysystems/packer". While trying to compile it, I am getting such errors. — rahul, Jan 03 '18 at 08:02
Well, as much as we'll all like to help, the link you provided is broken. And you should point out what file and line in that repo is causing a problem. — StoryTeller - Unslander Monica, Jan 03 '18 at 08:10
Please ignore the last quote. https://github.com/betterwaysystems/packer — rahul, Jan 03 '18 at 08:13

score -2 · Answer 1 · edited Jan 03 '18 at 18:33

An iterator is an object that points to elements inside a specific container. There is no global iterator type (which is why you got the compiler error). Instead the standard containers, e.g. vector<int> in my example below, define each their own iterator types. The iterator is usually initialized by calling a member function of the container instance over whose elements one wants to iterate. You can think of an iterator as a sophisticated pointer.

You can use built-in functions of a vector to obtain special values of an iterator, such as begin() and end(), which allow you to iterate over all elements it holds.

#include <iostream>
#include <vector>
using namespace std;
int main()
{
    // Declaring a vector
    vector<int> v = { 1, 2, 3 };

    // Declaring an iterator
    vector<int>::iterator i;

    // Accessing the elements through iterators
    // using functions from vector<int>
    for (i = v.begin(); i != v.end(); ++i)
    {
        cout << *i << " ";
    }
}

*"You cannot declare an iterator directly."* - sure you can, though how useful that is is a different question — UnholySheep, Jan 03 '18 at 08:23
Well if you can. Kindly show me how? And why is there an error when the questioner tried to do the same? — Trishant Pahwa, Jan 03 '18 at 08:49

Unable to compile c++ program on Ubuntu

1 Answers1