Lære Iterator Categories and Traversal

Iterators are a fundamental part of the C++ Standard Template Library (STL), providing a consistent way to access and traverse the elements of a container. The STL defines several iterator categories, each with different capabilities:

Input Iterators: allow reading elements in a single-pass, forward-only manner;
Output Iterators: allow writing elements in a single-pass, forward-only manner;
Forward Iterators: support both reading and writing, and can be used for multiple passes in the forward direction;
Bidirectional Iterators: extend forward iterators by also supporting backward traversal;
Random Access Iterators: provide all the capabilities of bidirectional iterators, plus direct access to any element in constant time using arithmetic operations.

Each STL container provides iterators of a specific category, depending on its internal structure. Understanding these categories helps you choose the right algorithms and traversal techniques for each container.

main.cpp


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#include <iostream>
#include <vector>
#include <list>

int main()
{
    std::vector<int> v = {1, 2, 3, 4, 5};
    std::list<int> l = {10, 20, 30, 40, 50};

    std::cout << "Traversing std::vector (random access iterator): ";
    for (auto it = v.begin(); it != v.end(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;

    std::cout << "Traversing std::list (bidirectional iterator): ";
    for (auto it = l.begin(); it != l.end(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;

    // Demonstrating random access (only valid for vector, not list)
    std::cout << "Accessing third element in vector using iterator arithmetic: ";
    auto v_it = v.begin();
    v_it += 2; // advance by 2 positions
    std::cout << *v_it << std::endl;

    // Uncommenting the following lines would cause a compile error
    // because std::list iterators do not support random access
    // auto l_it = l.begin();
    // l_it += 2;
    // std::cout << *l_it << std::endl;
}

Iterator Operations and Supported Categories

Iterators provide several core operations that enable traversal and manipulation of container elements. The main operations are:

Understanding which operations are supported by each iterator category is essential for writing efficient and correct code. For example, you can use arithmetic operations with std::vector iterators, but not with std::list iterators. This distinction influences which algorithms and traversal patterns you can use with each container.

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