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STL - iterators mechanism

Iterators are a generalization of pointers which allow a programmer to work with different data structures in the uniform manner. 

In STL we are using iterators to go through STL container for getting and setting values of its elements. In order to better understand working with iterators, take a look on below example:
Output of this example is:
In point I we are defining two vector containers using C++11 initializer lists.

Point II shows as definition of iterator which we will use to go through our vector structures to examine their elements. We are setting our iterator to first element of our myVector container. Because iterators are generalized pointers we can imagine this operation as setting up pointer to the first element of our structure. This operation can be illustrated like Step 1 on the picture below.
In C++11 we can simplify declaration of STL iterators using auto type as shown in point IIa.

In point III, IV and V we can see process of going through container elements using iterator mechanism. This process has been ilustrated on the picture below. We have while loop where we are checking whether we reached myVector.end() element or not. myVector.end() element is reached when our iterator points to address placed just after last container element (as shown in Step 10 of below picture).



In point IV we have example of getting value of element where our iterator points to. Point V illustrate using operator++() to move iterator to next element of container. Both those operations works similar way as normal pointer dereferencing and pointer arithmetic. Therefore we can see similarity of iterators to normal pointers.

Point VI depicts usage of iterator in for loop, where point VII shows how to assign value to element where iterator points to.

In terms of vector container from above example, using iterators is not the only way to get and set values of container elements. We can use random-access operator[] for it. It is because vector is allocated as continous part of memory. The same situation is for deque container. However other containers like: list, set, map does does not have random access to its elements. Therefore we cannot use operator[] go get its elements. We should use iterators when we are using such containers.

 Code of above examples can be found in GitHub repository of this blog here: STL iterators
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Comments

  1. Anonymous9 December 2013 at 12:50

    interesting explenation and example. Thanx!

    ReplyDelete

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