C++ <vector> begin / end function

C++ container class <vector>

beginandendis the most important iterator function in vector, used toGet the beginning and ending iterators of the container。

beginandendare member functions of container classes:

  • beginreturns a pointer to the first element of the containerIterator
  • endreturns a position past the last element of the containerIterator(Sentinel iterator)

These two functions are the foundation of C++ standard library algorithms and range-based for loops.

Word Definitions： beginRepresents "begin",endRepresents "end".

Basic syntax and parameters

beginandendare member functions of container classes, and calling them requires no arguments.

Syntax format

iterator begin();
const_iterator begin() const;

iterator end();
const_iterator end() const;

Parameter description

  • Parameter: No parameters
    • These functions do not accept any parameters.

Function description

  • Return Value：
    • begin()Returns an iterator pointing to the first element.
    • end()Returns an iterator (sentinel) pointing to the position after the last element.
  • Effect: The returned iterator can be used to traverse the container or to work with algorithms.
  • Note： end()The returned iterator points to a "virtual" position; it cannot be dereferenced, only used for comparison.

Example

Let us thoroughly master through a series of examplesbeginandendUsage.

Example 1: Basic usage - traversing the container

Example

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {10, 20, 30, 40, 50};

    // Traverse using an iterator
    std::cout << "Traverse using an iterator: ";
    for(auto it = numbers.begin(); it != numbers.end(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;

    return 0;
}

Expected output:

使用迭代器遍历: 10 20 30 40 50

Code analysis:

  1. numbers.begin()Returns an iterator pointing to the first element (10).
  2. numbers.end()Returns an iterator pointing to the position after the last element.
  3. Loop conditionit != numbers.end()Ensures traversal of all elements.

Example 2: Range-based for loop

The range-based for loop introduced in C++11 essentially uses at its corebeginandend。

Example

#include <iostream>
#include <vector>
#include <string>

int main() {
    std::vector<std::string> names = {"Alice", "Bob", "Charlie"};

    // Range-based for loop (internally uses begin/end)
    std::cout << "Use range-based for loop: ";
    for(const auto& name : names) {
        std::cout << name << " ";
    }
    std::cout << std::endl;

    return 0;
}

Expected output:

使用范围 for 循环: Alice Bob Charlie

Code analysis:

  • The range-based for loop is a concise way to use iterators.
  • The compiler will automatically callbegin()andend()。

Example 3: Using STL algorithms

Standard library algorithms require iterators to specify ranges.

Example

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> numbers = {5, 2, 8, 1, 9, 3};

    // Sort
    std::sort(numbers.begin(), numbers.end());

    std::cout << "After sorting: ";
    for(int n : numbers) {
        std::cout << n << " ";
    }
    std::cout << std::endl;

    // Find element
    auto it = std::find(numbers.begin(), numbers.end(), 8);
    if(it != numbers.end()) {
        std::cout << "Found element: " << *it << std::endl;
    }

    return 0;
}

Expected output:

排序后: 1 2 3 5 8 9
找到元素: 8

Code analysis:

  • std::sort(begin, end)Use iterators to specify the sorting range.
  • std::find(begin, end, value)Use iterators to specify the search range.

Example 4: Reverse iterators

UsagerbeginandrendCan traverse the container in reverse.

Example

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};

    // Forward traversal
    std::cout << "Forward: ";
    for(auto it = numbers.begin(); it != numbers.end(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;

    // Reverse traversal (using rbegin/rend)
    std::cout << "Reverse: ";
    for(auto it = numbers.rbegin(); it != numbers.rend(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;

    return 0;
}

Expected output:

正向: 1 2 3 4 5
反向: 5 4 3 2 1

Code analysis:

  • rbegin()Returns a reverse iterator pointing to the last element.
  • rend()Returns a reverse iterator pointing to the position before the first element.

Example 5: Iterator arithmetic operations

Iterators support arithmetic operations, allowing quick jumps to a specified position.

Example

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {10, 20, 30, 40, 50, 60, 70};

    // Get the third element (index 2)
    auto it = numbers.begin() + 2;
    std::cout << "begin() + 2 = " << *it << std::endl;

    // Calculate the distance between two iterators
    auto first = numbers.begin();
    auto last = numbers.end();
    auto distance = last - first;
    std::cout << "Container size (distance): " << distance << std::endl;

    // Get the middle element
    auto mid = numbers.begin() + (numbers.size() / 2);
    std::cout << "Middle element: " << *mid << std::endl;

    return 0;
}

Expected output:

begin() + 2 = 30
容器大小（距离）: 7
中间元素: 40

Code analysis:

  • Iterator support+、-and other arithmetic operations.
  • You can useend() - begin()Compute the size of the container.

Example 6: Conversion between iterators and indices

Iterators and indices can be converted to each other.

Example

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {10, 20, 30, 40, 50};

    // Convert index to iterator
    size_t index = 2;
    auto it = numbers.begin() + index;
    std::cout << "Index " << index << " corresponding element: " << *it << std::endl;

    // Convert iterator to index
    auto iter = numbers.begin() + 3;
    size_t idx = iter - numbers.begin();
    std::cout << "Index corresponding to iterator: " << idx << std::endl;

    return 0;
}

Expected output:

索引 2 对应的元素: 30
迭代器对应的索引: 3

Code analysis:

  • begin() + indexConvert an index to an iterator.
  • iter - begin()Convert an iterator to an index.

Example 7: Using iterators to modify elements

Iterators return references, which can be used to modify elements.

Example

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};

    std::cout << "Before modification: ";
    for(int n : numbers) std::cout << n << " ";
    std::cout << std::endl;

    // Multiply all elements by 2 using iterators
    for(auto it = numbers.begin(); it != numbers.end(); ++it) {
        *it *= 2;
    }

    std::cout << "After modification: ";
    for(int n : numbers) std::cout << n << " ";
    std::cout << std::endl;

    return 0;
}

Expected output:

修改前: 1 2 3 4 5
修改后: 2 4 6 8 10

Code analysis:

  • Iterator dereference (*it) returns a reference to the element.
  • You can directly modify this reference to change the element's value.

C++ container class <vector>

other extensions