Iterators in C++ STL

C STL (Standard Template Library) is one of the standard libraries of the C programming language. It contains a series of standard data structures and algorithms. In STL, iterator (iterator) is a very important tool for traversing and accessing in STL containers.

An iterator is an object similar to a pointer. It can point to an element in a container (such as vector, list, set, map, etc.), and can move and access elements in the container. Iterators play an important role in STL. They are not only tools for traversing containers, but are also widely used in algorithms, such as sorting, search, copy and other algorithms.

Iterators in STL can be divided into five types:

1. Input Iterator (Input Iterator): used to traverse the elements in the container, but can only traverse once. The element's value cannot be modified.
2. Output Iterator (Output Iterator): used to write elements into the container, and can only be traversed once.
3. Forward Iterator (Forward Iterator): It can traverse forward in the container and can traverse multiple times, but it cannot perform random access.
4. Bidirectional Iterator (Bidirectional Iterator): It can traverse forward and reverse, but it still cannot perform random access.
5. Random Access Iterator (Random Access Iterator): It can perform random access and operations, and provides all operations of pointers, such as addition, subtraction, comparison, etc.

In STL, there are two concepts of iterators: begin and end. begin points to the first element of the container, end points to the last element of the container, and an end() function is also provided to return the next position at the end of the container, which is convenient for loop traversal. In addition, STL provides a large number of algorithms, such as sort, find, copy, etc., all implemented based on iterators.

The following is a sample code that uses an iterator to traverse:

#include <iostream>
#include <vector>

using namespace std;

int main() {
    vector<int> vec{ 1, 2, 3, 4, 5 };
    vector<int>::iterator it;
    for (it = vec.begin(); it != vec.end(); it++) {
        cout << *it << " ";
    }
    return 0;
}

In the above example, we first create a vector container and use an iterator to traverse it. vector::iterator is the iterator type of the vector container. The starting position of the vector container is obtained through vec.begin(), and the end position is obtained through vec.end(). Use a for loop to traverse the entire container, and use the *it operator to obtain the value of the corresponding element and output it.

In short, iterator is one of the very important tools in STL. It makes containers and algorithms in STL more flexible and extensible. Whether you are a beginner or an advanced programming enthusiast, you should master the use of iterators to better use the data structures and algorithms in STL.

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