本文前驱课程
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
迭代器的能力
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。
cpp
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_data0：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
cpp
//------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)
{
assert(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
assert(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 ints
assert(first.empty() == true && first.size() == 0);
Vector second(4, 100); // four ints with value 100
assert(second.empty() == false);
assert(second.size() == 4);
assert(*second.begin() == 100);
Vector fourth(second); // a copy of third
assert(fourth.size() == second.size());
int myints[] = { 16,2,77,29 };
Vector fifth(myints, myints + sizeof(myints) / sizeof(int));
assert(fifth.empty() == false);
assert(fifth[0] == 16);
assert(fifth[3] == 29);
assert(fifth.size() == sizeof(myints) / sizeof(int));
print(fifth, "fifth");//The contents of fifth are:16 2 77 29
fifth.push_back(30);
assert(fifth[4] == 30);
assert(fifth.size() == 5);
print(fifth, "fifth");//The contents of fifth are:16 2 77 29 30
assert(fifth.size() == sizeof(myints) / sizeof(int) + 1);
first = fifth = fifth;
print(first, "first");//The contents of first are:16 2 77 29 30
assert(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);
assert(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;
}
assert(a1.size() == sizeof(myints) / sizeof(int));
{
Vector c;
c = fifth;
c[0] = 1;
assert(c[0] == 1);
}
}期待输出：
祝你好运！