what is the difference between const iterator and iterator

Question

What is difference between these two regarding implementation inside STL  what is the difference regarding performance  I guess when we are traversing the vector in  read only wise   we prefer const iterator  right   Thank you

User · Answer

if you have a list a and then following statements

list<int>::iterator it; // declare an iterator
    list<int>::const_iterator cit; // declare an const iterator 
    it=a.begin();
    cit=a.begin();

you can change the contents of the element in the list using “it” but not “cit”, that is you can use “cit” for reading the contents not for updating the elements.

*it=*it+1;//returns no error
    *cit=*cit+1;//this will return error

User · Answer

Performance wise there is no difference  The only purpose of having const iterator over iterator is to manage the accessesibility of the container on which the respective iterator runs  You can understand it more clearly with an example   std  vector lt int gt  integers  3  4  56  6  778      If we were to read  amp  write the members of a container we will use iterator    for  std  vector lt int gt   iterator it   integers begin     it    integers end       it            it   4   std  cout  lt  lt   it  lt  lt  std  endl      If we were to only read the members of the container integers you might wanna use const iterator which doesn t allow to write or modify members of container   for  std  vector lt int gt   const iterator it   integers begin     it    integers end       it            cout  lt  lt   it  lt  lt  endl      NOTE  if you try to modify the content using  it in second case you will get an error because its read-only

User · Answer

There is no performance difference   A const iterator is an iterator that points to const value  like a const T  pointer   dereferencing it returns a reference to a constant value  const T amp   and prevents modification of the referenced value  it enforces const-correctness   When you have a const reference to the container  you can only get a const iterator   Edited  I mentionned    The const iterator returns constant pointers    which is not accurate  thanks to Brandon for pointing it out   Edit  For COW objects  getting a non-const iterator  or dereferencing it  will probably trigger the copy   Some obsolete and now disallowed implementations of std  string use COW

[c++] what is the difference between const_iterator and iterator?

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