Declaring const_iterator vs declaring an iterator const

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An iterator acts much like a T* pointer. 

Declare an iterator const

Declaring an iterator const is nothing but declaring a const pointer which is equivalent to T* const - iterator is not allowed to point to different object, but the data iter points to can be modified. 

Lets look at an example below :

std::vector<std::string> vecOfStrings;

// Push some strings to the vector

// Declare an iterator const

const std::vector<string>::iterator iter; // iter acts like T* const , const pointer

iter = vecOfStrings.begin();

*iter = "newValue"; // Ok, changes the value what iter points to.

++iter; // Compilation error , iter is const


Declare an const_iterator

Declaring an const_iterator is nothing but declaring an pointer to const object which is equivalent to const T* - Iterator can point to different object but modifying the data will throw an compilation error.

Let's look at an example below :
std::vector<std::string> vecOfStrings;

// Push some strings to the vector.

std::vector<string>::const_iterator cIter; // iter acts like const T* , pointer to const                                                                    // object
*cIter= "newValue"; // Compilation error, iter is const

++cIter; //Ok, increments to the next value.


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