2.3 Vector 动态数组（迭代器）-编程知识

C++数据结构与算法目录

本文前驱课程

1 C++自学精简教程目录(必读)

2 Vector<T> 动态数组（模板语法）

本文目标

1 熟悉迭代器设计模式；

2 实现数组的迭代器；

3 基于迭代器的容器遍历；

迭代器语法介绍

对迭代器的详细介绍参考：迭代器 iterator 范围for循环删除容器的元素 remove erase

迭代器的功能

迭代器实际上是一个内部类。通过下面的迷你代码，我们可以看到迭代器应该具备的能力。

class Vector
{
public:class Iterator{};Iterator begin() {Iterator itr;/* (1)开始迭代器要能指向第一个元素 m_data[0]*/return itr;};Iterator end(){Iterator itr;/* (2)结束迭代器指向空最后一个元素的下一个位置 m_data+m_size */return itr;};int& operator*();// 重载解引用操作符重载：让迭代器可以通过解引用得到其指向变量的引用  *itr = 5;iterator & operator++(); //用于前置形式  ++itr;int* m_data;//存储动态内存int m_size;
};int main()
{Vector arr;auto itr = arr.begin();for (auto itr = arr.begin(); itr != arr.end()/* (3)迭代器要能够比较相等*/; itr++/* (4)迭代器要能够移动到下一个位置 */ ){cout << *itr/* (5)迭代器要能够解引用得到容器的元素*/ << " ";}return 0;
}

迭代器的实现

现在我们考虑如何实现这种能力。

对于动态数组Vector来说：

(1)开始迭代器要能指向第一个元素 m_data[0]：可以给迭代器添加一个构造函数，传递动态数组的首元素地址给迭代器。

(2)结束迭代器指向空最后一个元素的下一个位置：可以给迭代器的构造函数传入动态数组首地址偏移 m_size 个元素后的地址。m_data + m_size

(3)迭代器要能够比较相等：让迭代器重载，等于操作符 == 不等于操作符 !=

(4)迭代器要能够移动到下一个位置：让迭代器重载自增操作符 ++

(5)迭代器要能够解引用得到容器的元素：让迭代器重载解引用操作符 *

至此，迭代器的功能就全部实现完成了。

完整测试用例

//------下面的代码是用来测试你的代码有没有问题的辅助代码，你无需关注------
#include <algorithm>
#include <cstdlib>
#include <iostream> 
#include <vector>
#include <utility>
using namespace std;
struct Record { Record(void* ptr1, size_t count1, const char* location1, int line1, bool is) :ptr(ptr1), count(count1), line(line1), is_array(is) { int i = 0; while ((location[i] = location1[i]) && i < 100) { ++i; } }void* ptr; size_t count; char location[100] = { 0 }; int line; bool is_array = false; bool not_use_right_delete = false; }; bool operator==(const Record& lhs, const Record& rhs) { return lhs.ptr == rhs.ptr; }std::vector<Record> myAllocStatistic; void* newFunctionImpl(std::size_t sz, char const* file, int line, bool is) { void* ptr = std::malloc(sz); myAllocStatistic.push_back({ ptr,sz, file, line , is }); return ptr; }void* operator new(std::size_t sz, char const* file, int line) { return newFunctionImpl(sz, file, line, false); }void* operator new [](std::size_t sz, char const* file, int line)
{ return newFunctionImpl(sz, file, line, true); }void operator delete(void* ptr) noexcept { Record item{ ptr, 0, "", 0, false }; auto itr = std::find(myAllocStatistic.begin(), myAllocStatistic.end(), item); if (itr != myAllocStatistic.end()) { auto ind = std::distance(myAllocStatistic.begin(), itr); myAllocStatistic[ind].ptr = nullptr; if (itr->is_array) { myAllocStatistic[ind].not_use_right_delete = true; } else { myAllocStatistic[ind].count = 0; }std::free(ptr); } }void operator delete[](void* ptr) noexcept {Record item{ ptr, 0, "", 0, true }; auto itr = std::find(myAllocStatistic.begin(), myAllocStatistic.end(), item); if (itr != myAllocStatistic.end()) { auto ind = std::distance(myAllocStatistic.begin(), itr); myAllocStatistic[ind].ptr = nullptr; if (!itr->is_array) { myAllocStatistic[ind].not_use_right_delete = true; } else { myAllocStatistic[ind].count = 0; }std::free(ptr); }}
#define new new(__FILE__, __LINE__)
struct MyStruct { void ReportMemoryLeak() { std::cout << "Memory leak report: " << std::endl; bool leak = false; for (auto& i : myAllocStatistic) { if (i.count != 0) { leak = true; std::cout << "leak count " << i.count << " Byte" << ", file " << i.location << ", line " << i.line; if (i.not_use_right_delete) { cout << ", not use right delete. "; }	cout << std::endl; } }if (!leak) { cout << "No memory leak." << endl; } }~MyStruct() { ReportMemoryLeak(); } }; static MyStruct my; void check_do(bool b, int line = __LINE__) { if (b) { cout << "line:" << line << " Pass" << endl; } else { cout << "line:" << line << " Ohh! not passed!!!!!!!!!!!!!!!!!!!!!!!!!!!" << " " << endl; exit(0); } }
#define check(msg)  check_do(msg, __LINE__);
//------上面的代码是用来测试你的代码有没有问题的辅助代码，你无需关注------#include <iostream>
#include <cassert>class Vector
{
public:Vector(void);//1 默认构造函数Vector(int count, int value);//2 非默认构造函数Vector(const Vector& from);//4 复制构造函数Vector(int* start, int* end);// 3 非默认构造函数Vector& operator = (const Vector& from);~Vector();
public:size_t size(void) const;bool empty(void) const;const int& operator[] (size_t n) const;int& operator[] (size_t n);void push_back(const int& val);
public:class iterator{friend class  Vector;friend bool operator == (const iterator& lhs, const iterator& rhs);//用于实现!=,因为==非常容易实现friend bool operator != (const iterator& lhs, const iterator& rhs);public:iterator& operator++(); //用于前置形式iterator operator++(int); //用于后置形式，这里有个int参数纯粹是为了区分前自增操作符而加的语法规定int& operator*();//解引用操作符重载private:int* m_hold = nullptr;};class const_iterator{friend class  Vector;public:bool operator == (const const_iterator& rhs) { return this->m_hold == rhs.m_hold; }//用于实现!=,因为==非常容易实现bool operator != (const const_iterator& rhs) { return !(*this == rhs); };const_iterator& operator++(); //用于前置形式 ++itr  先改变自己，指向下一个位置，再返回自己const_iterator operator++(int); //用于后置形式 itr++ 先创建一个改变前的副本用于返回，再在返回前改变自己，指向下一个位置const int& operator*() const;private:const int* m_hold = nullptr;};//noexcept 表示这个函数内不会抛出异常，这样有助于编译器优化代码生成const_iterator begin() const noexcept;iterator begin() noexcept;const_iterator end() const noexcept;iterator end() noexcept;
private:void clear(void);
private:size_t m_size;//当前元素数量size_t m_capacity;//容量int* m_data;//数据部分
};
Vector::Vector(void):m_data(nullptr), m_size(0), m_capacity(0)
{std::cout << "Vector()" << std::endl;
}Vector::Vector(const Vector& from)
{if (from.empty()){m_data = nullptr;m_size = 0;m_capacity = 0;return;}m_capacity = m_size = from.m_size;m_data = new int[m_size];for (auto i = 0; i < m_size; ++i){m_data[i] = from.m_data[i];}std::cout << "Vector(const Vector & from)" << std::endl;
}Vector::Vector(int count, int value): m_data(nullptr)
{if (count <= 0){throw std::runtime_error("size of vector to init must bigger than zero!");}m_data = new int[count];for (size_t i = 0; i < count; i++){m_data[i] = value;}m_capacity = m_size = count;std::cout << "Vector(const, value)" << std::endl;
}Vector::Vector(int* start, int* end): m_data(nullptr), m_size(0), m_capacity(0)
{check(start != nullptr && end != nullptr);m_capacity = m_size = ((size_t)end - (size_t)start) / sizeof(int);//这里如果用int来存放可能会盛不下，size_t可以保证盛放的下check(m_size > 0);m_data = new int[m_size];for (size_t i = 0; i < m_size; i++){m_data[i] = *start++;}std::cout << "Vector(start, end)" << std::endl;
}Vector& Vector::operator=(const Vector& from)
{if (this == &from){return *this;}//先释放自己的数据clear();m_size = from.m_size;m_capacity = from.m_capacity;m_data = new int[m_size];for (size_t i = 0; i < m_size; i++){m_data[i] = from.m_data[i];}return *this;std::cout << "Vector & Vector::operator=(const Vector & from)" << std::endl;
}Vector::~Vector()
{if (m_data){delete[] m_data;}std::cout << "~Vector()" << std::endl;
}size_t Vector::size(void) const
{return m_size;
}bool Vector::empty(void) const
{return m_size == 0;
}const int& Vector::operator[](size_t n) const
{return m_data[n];
}int& Vector::operator[](size_t n)
{return  m_data[n];
}void Vector::push_back(const int& val)
{if (m_capacity > m_size)//直接追加到最后一个{m_data[m_size++] = val;}else//只有满了的那一瞬间，才翻倍开辟新空间{auto pNewArray = new int[m_capacity = m_capacity + m_capacity];//拷贝老数据for (size_t i = 0; i < m_size; i++){pNewArray[i] = m_data[i];}//追加最新的末尾元素pNewArray[m_size++] = val;delete[] m_data;m_data = pNewArray;}
}
//下面的代码 函数名是 Vector::begin
//           返回值类型是 Vector::const_iterator
//返回值类型之所以要加类作用域是因为，返回值类型在函数作用域之外。这是由C语言继承而来的
Vector::const_iterator Vector::begin() const noexcept
{if (empty()){return end();}const_iterator itr;//(1) your code 下面的代码仅仅是让编译通过，可能需要你重新实现return itr;
}Vector::iterator Vector::begin() noexcept
{if (empty()){return end();}iterator itr;//(1) your code 下面的代码仅仅是让编译通过，可能需要你重新实现return itr;
}Vector::const_iterator Vector::end() const noexcept
{const_iterator itr;//(2) your code 下面的代码仅仅是让编译通过，可能需要你重新实现// 如果容器为空，不能返回下标返回的元素位置return itr;
}Vector::iterator Vector::end() noexcept
{iterator itr;//(2) your code 下面的代码仅仅是让编译通过，可能需要你重新实现// 如果容器为空，不能返回下标返回的元素位置return itr;
}void Vector::clear(void)
{//(3) your code 下面的代码仅仅是让编译通过，可能需要你重新实现}bool operator==(const Vector::iterator& lhs, const Vector::iterator& rhs)
{//(4) your code 下面的代码仅仅是让编译通过，可能需要你重新实现return false;
}bool operator!=(const Vector::iterator& lhs, const Vector::iterator& rhs)
{//(5) your code 下面的代码仅仅是让编译通过，可能需要你重新实现return false;
}Vector::iterator& Vector::iterator::operator++()
{//(6) your code 下面的代码仅仅是让编译通过，可能需要你重新实现return *this;
}
Vector::const_iterator& Vector::const_iterator::operator++()
{//(7) your code 下面的代码仅仅是让编译通过，可能需要你重新实现return *this;
}Vector::iterator Vector::iterator::operator++(int)
{//(8) your code 下面的代码仅仅是让编译通过，可能需要你重新实现return iterator();
}
Vector::const_iterator Vector::const_iterator::operator++(int)
{return Vector::const_iterator();
}
int& Vector::iterator::operator*()
{//(9) your code 下面的代码是错误的！不可以返回临时变量的引用！仅仅是让编译通过，需要你重新实现int a = 0;return a;
}
const int& Vector::const_iterator::operator*() const
{//(9) your code 下面的代码是错误的！不可以返回临时变量的引用！仅仅是让编译通过，需要你重新实现int a = 0;return a;
}
void print(const Vector& v, const std::string& msg)
{std::cout << "print The contents of " << msg.c_str() << " are:";for (int i = 0; i < v.size(); ++i){std::cout << ' ' << v[i];}std::cout << '\n';
}
void print_itr(Vector& v, const std::string& msg)
{std::cout << "print_itr The contents of " << msg.c_str() << " are:";for (auto itr = v.begin(); itr != v.end(); ++itr){std::cout << ' ' << *itr;}std::cout << '\n';
}
void print_const_itr(const Vector& v, const std::string& msg)
{std::cout << "print_const_itr The contents of " << msg.c_str() << " are:";for (auto itr = v.begin(); itr != v.end(); ++itr){//*itr = 4;std::cout << ' ' << *itr;}std::cout << '\n';
}int main()
{Vector a;Vector first;                   // empty vector of intscheck(first.empty() == true && first.size() == 0);Vector second(4, 100);                       // four ints with value 100check(second.empty() == false);check(second.size() == 4);check(*second.begin() == 100);Vector fourth(second);                       // a copy of thirdcheck(fourth.size() == second.size());int myints[] = { 16,2,77,29 };Vector fifth(myints, myints + sizeof(myints) / sizeof(int));check(fifth.empty() == false);check(fifth[0] == 16);check(fifth[3] == 29);check(fifth.size() == sizeof(myints) / sizeof(int));print(fifth, "fifth");//The contents of fifth are:16 2 77 29 fifth.push_back(30);check(fifth[4] == 30);check(fifth.size() == 5);print(fifth, "fifth");//The contents of fifth are:16 2 77 29 30 check(fifth.size() == sizeof(myints) / sizeof(int) + 1);first = fifth = fifth;print(first, "first");//The contents of first are:16 2 77 29 30 check(first.empty() == false && first.size() == fifth.size());print_itr(fifth, "fifth");//The contents of fifth are:16 2 77 29 30 print_const_itr(fifth, "fifth");//The contents of fifth are:16 2 77 29 30 Vector a1(myints, myints + sizeof(myints) / sizeof(int));{Vector b(a1);b.push_back(2);check(b[4] == 2);}{Vector c;for (auto i : c){std::cout << i << " ";}c = a1;a1 = c;c = a1;for (auto i : c){std::cout << i << " ";}std::cout << std::endl;}check(a1.size() == sizeof(myints) / sizeof(int));{Vector c;c = fifth;c[0] = 1;check(c[0] == 1);}
}

期待输出：

Vector()
Vector()
line:335 Pass
Vector(const, value)
line:337 Pass
line:338 Pass
line:339 Pass
Vector(const Vector & from)
line:341 Pass
line:118 Pass
line:120 Pass
Vector(start, end)
line:345 Pass
line:346 Pass
line:347 Pass
line:348 Pass
print The contents of fifth are: 16 2 77 29
line:351 Pass
line:352 Pass
print The contents of fifth are: 16 2 77 29 30
line:354 Pass
print The contents of first are: 16 2 77 29 30
line:357 Pass
print_itr The contents of fifth are: 16 2 77 29 30
print_const_itr The contents of fifth are: 16 2 77 29 30
line:118 Pass
line:120 Pass
Vector(start, end)
Vector(const Vector & from)
line:364 Pass
~Vector()
Vector()
16 2 77 29
~Vector()
line:382 Pass
Vector()
line:387 Pass
~Vector()
~Vector()
~Vector()
~Vector()
~Vector()
~Vector()
~Vector()
Memory leak report:
No memory leak.

祝你好运！