【C++】string模拟实现

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目录

构造

 析构

遍历

迭代器

插入

删除、改变容量、赋值

交换

 查找

运算符重载

现代写法

拷贝构造 

 赋值

完整代码呈现

头文件

测试代码

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前言

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             今日更新了string模拟实现的内容
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构造

初始化列表部分，要开辟新空间初始化。三次调用strlen效率太低，改进如下图：

我们的构造函数直接写一个全缺省的，这样就不用分开写无参和有参的构造了。上面缺省值不能给nullptr，因为初始化列表那里会报错，str为空，无法strlen。缺省值也不能给'\0'，而要给"\0"，因为'\0'的类型是char，不能给char*。其实给"\0"也是多余的，虽然可以这样写，我们可以直接给空字符串""，系统会在后面自动加上"\0"的。

string(const char* str=""): _size(strlen(str)){_capacity = _size;_str = new char[_capacity + 1];strcpy(_str, str);}

 析构

~string(){delete[] _str;_str = nullptr;_size = _capacity = 0;}

遍历

//遍历size_t size() const{return _size;}size_t capacity() const {return _capacity;}char& operator[](size_t pos) {assert(pos < _size);return _str[pos];   //_str[pos]在堆上，可用引用返回，支持修改}const char& operator[](size_t pos) const{assert(pos < _size);return _str[pos];   }void test_string1(){string s1("hello world");string s2;for (size_t i = 0; i < s1.size(); i++){s1[i]++;}cout << endl;for (size_t i = 0; i < s1.size(); i++){cout << s1[i] << "";}cout << endl;const string s3("xxxx");for (size_t i = 0; i < s3.size(); i++){//s3[i]++      错误，可读，但不能写cout << s3[i] << "";}cout << endl;}

operator[]需要重载两个，因为string对象可能是const修饰的。 

迭代器

typedef char* iterator;iterator begin(){return _str;}iterator end(){return _str + _size;}void test_string2(){string s3("hello world");string::iterator it3 = s3.begin();while (it3 != s3.end()){//*it3 -= 3;cout << *it3 << " ";++it3;}cout << endl;for (auto ch : s3){cout << ch << " ";}cout << endl;}

这里迭代器的模拟实现是容易的，不同的容器有不同的迭代器，只是名字相同。

如果string对象有const修饰时，就需要const迭代器，如下方代码：

typedef const char* const_iterator;const_iterator begin() const{return _str;}const_iterator end() const{return _str + _size;}const string s2("hello world");string::const_iterator it2 = s2.begin(); while (it2 != s2.end()){cout << *it2 << " ";++it2;}cout << endl;

插入

void reserve(size_t n){if (n > _capacity){char* tmp = new char[n+1];strcpy(tmp, _str);delete[] _str;_str = tmp;_capacity = n;}}void push_back(char ch){/*if (_size == _capacity){reserve(_capacity == 0 ? 4 : 2 * _capacity);}_str[_size] = ch;++_size;_str[_size] = '\0';*/insert(_size, ch);}void append(const char* str){/*size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}strcpy(_str + _size, str);_size += len;*/insert(_size, str);}string& operator+=(char ch){push_back(ch);return *this;}string& operator+=(const char* str){ append(str);return *this;}void insert(size_t pos, char ch){assert(pos <= _size);if (_size == _capacity){reserve(_capacity == 0 ? 4 : 2 * _capacity);}//第一种写法/*int end = _size;while (end >= (int) pos)  //需要强转，pos是size_t类型，end是int，end会被转成size_t类型，即范围小会转向范围大的。{  //当pos等于0，即头插时。结束条件为end小于0，如果不强转，当end小于0时，又会被转成无符号的_str[end + 1] = _str[end];  //就会死循环--end;}*///第二种size_t end = _size+1;while (end > pos){_str[end] = _str[end-1];--end;}_str[pos] = ch;++_size;}void insert(size_t pos, const char* str){assert(pos <= _size);size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}size_t end = _size + len;while (end > pos + len - 1){_str[end] = _str[end - len];end--;}strncpy(_str + pos, str, len);_size += len;}void test_string3(){string s3("hello world"); s3.push_back('1'); s3.push_back('2');  cout << s3.c_str()<< endl; s3 += 'x';  s3 += "yyyy"; cout << s3.c_str() << endl; string s1("hello world");s1.insert(11, 'x');cout << s1.c_str() << endl;s1.insert(0, 'x');cout << s1.c_str() << endl;}void test_string6(){string s1("hello world");cout << s1.c_str() << endl;s1.insert(6, "xxxx");cout << s1.c_str() << endl;}

删除、改变容量、赋值

const char* c_str() const{return _str;}   void resize(size_t n, char ch = '\0'){if (n < _size){_str[n] = '\0';_size = n;}else{reserve(n);for (size_t i = _size; i < n; i++){_str[i] = ch;}_str[n] = '\0';_size = n;}}void erase(size_t pos, size_t len = npos){assert(pos < _size);if (len == npos || len >= _size - pos)  //len +pos>= _size  不能这么写，会有溢出风险{_str[pos] = '\0';_size = pos;}else{strcpy(_str + pos, _str + pos + len);_size -= len;}}//s2(s1)string(const string& s){_str = new char[s._capacity + 1];strcpy(_str, s._str);_size = s._size;_capacity = s._capacity;}//s1=s3string& operator=(const string& s){char* tmp = new char[s._capacity+1];strcpy(tmp, s._str);delete[] _str;_str = tmp;_size = s._size;_capacity = s._capacity;return *this;}void test_string4(){string s1("hello world");cout << s1.c_str() << endl;s1.erase(6, 3);cout << s1.c_str() << endl;string s2("hello world");cout << s2.c_str() << endl;s2.resize(5);cout << s2.c_str() << endl;s2.resize(20, 'x');cout << s2.c_str() << endl;}void test_string5(){string s1("hello world");cout << s1.c_str() << endl;string s2(s1);cout << s2.c_str() << endl;string s3("xxxxx");s1 = s3;cout << s1.c_str() << endl;cout << s3.c_str() << endl;}

交换

当我们用算法库里面的模板swap时，效率很低，它会进行3次拷贝+1次析构。我们需要自己手写一个成员函数swap提高效率。 如下图：

为了避免有人使用效率不高的swap，其实还有一个全局的swap，如下图：

 查找

size_t find(char ch, size_t pos = 0) const{assert(pos < _size);for (size_t i = pos; i < _size; i++){if (_str[i] == ch)return i;}return npos;}size_t find(const char* sub, size_t pos = 0) const{assert(pos < _size);const char* p = strstr(_str+pos, sub);if (p){return p - _str;}else{return npos;} }string substr(size_t pos = 0, size_t len = npos){string sub;if (len >= _size - pos){for (size_t i = pos; i < _size; i++){sub += _str[i];}}else{for (size_t i = pos; i < pos+len; i++){sub += _str[i];}}return sub;}void test_string7(){string url1("https://legacy.cplusplus.com/reference/string/string/substr/");string url2("https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=1&rsv_idx=1&tn=baidu&wd=%E5%90%8E%E7%BC%80%20%E8%8B%B1%E6%96%87&fenlei=256&rsv_pq=0xc61b98d100494ef5&rsv_t=b494uh2Bd6SIxMQ1XY4CwB9MZOD%2BaMk5sx8aiT0NAgUczCRAy8jK7PvP5rxt&rqlang=en&rsv_enter=1&rsv_dl=tb&rsv_sug3=23&rsv_sug1=17&rsv_sug7=100&rsv_sug2=0&rsv_btype=i&inputT=7123&rsv_sug4=8008");string protocol, domain, uri; size_t i1 = url1.find(':');  if (i1 != string::npos) {protocol = url1.substr(0, i1 - 0);//左闭右开，减去后就是个数 cout << protocol.c_str() << endl; }size_t i2 = url1.find('/',i1+3); if (i2 != string::npos)  {domain = url1.substr(i1+3, i2-(i1+3));//左闭右开，减去后就是个数 cout << domain.c_str() << endl;  uri = url1.substr(i2 + 1); cout << uri.c_str() << endl; }}

运算符重载

void clear(){_size = 0;_str[_size] = '\0';}bool operator==(const string& s1, const string& s2){int ret = strcmp(s1.c_str(), s2.c_str());return ret == 0;}bool operator<(const string& s1, const string& s2){int ret = strcmp(s1.c_str(), s2.c_str());return ret < 0;}bool operator<=(const string& s1, const string& s2){return s1 < s2 || s1 == s1;}bool operator>(const string& s1, const string& s2){return !(s1 <= s2);}bool operator>=(const string& s1, const string& s2){return !(s1 < s2);}bool operator!=(const string& s1, const string& s2){return !(s1 == s2);}ostream& operator<<(ostream& out, const string& s){for (auto ch : s){out << ch;}return out;}istream& operator>>(istream& in, string& s) {s.clear(); char ch;//in >> ch; 错误，C++规定 cin,scanf默认空格和换行是分隔符，ch = in.get();//不会取到,get才能取到char buff[128];size_t i = 0;while (ch != ' ' && ch != '\n'){buff[i++] = ch;if (i == 127){buff[127] = '\0';s += buff;i = 0;}ch = in.get();}if (i > 0){buff[i] = '\0';s += buff; }return in;}istream& getline(istream& in, string& s){s.clear();char ch;//in >> ch; 错误，C++规定 cin,scanf默认空格和换行是分隔符，ch = in.get();//不会取到,get才能取到while (ch != '\n'){s += ch;ch = in.get();}return in;}void test_string8(){string s1("hello world");string s2("hello world");cout << (s1 == s2) << endl;cout << ("hello world" == s2) << endl;//因为重载成全局的函数，cout << (s2 == "hello world") << endl;//单参数构造支持隐式类型转换。cout << s1 << endl;cout << s2 << endl;cin >> s1 >> s2;cout << s1 << endl;cout << s2 << endl;getline(cin, s1);cout << s1 << endl;}

现代写法

拷贝构造 

//现代写法string(const string& s){string tmp(s._str);swap(tmp);}

 赋值

string& operator=(string tmp){//现代写法swap(tmp);return *this;}

完整代码呈现

头文件

#pragma once#include<assert.h>namespace bit{class string{public:typedef char* iterator;typedef const char* const_iterator;const_iterator begin() const{return _str;}const_iterator end() const{return _str + _size;}iterator begin(){return _str;}iterator end(){return _str + _size;}//string()//:_str(nullptr)//,_size(0)//,_capacity(0)//{} _str(str)   错误，这样的话声明部分就得加上const，加上后又会导致不能修改   //string(const char* str)//:_str(new char[strlen(str)+1])  //必须自己开辟新的空间，//,_size(strlen(str))          //调用三次strlen，效率不高//,_capacity(strlen(str))//{//strcpy(_str, str);//}const char* c_str() const{return _str;}//string(const char* str = "\0")  里面不能写const char* str =nullptrstring(const char* str=""): _size(strlen(str)){_capacity = _size;_str = new char[_capacity + 1];strcpy(_str, str);}//s2(s1)   传统写法//string(const string& s)//{//_str = new char[s._capacity + 1];//strcpy(_str, s._str);//_size = s._size;//_capacity = s._capacity;//}//现代写法string(const string& s){string tmp(s._str);swap(tmp);}string& operator=(string tmp){ //现代写法swap(tmp);return *this;}//s1=s3  传统写法//string& operator=(const string& s)//{//char* tmp = new char[s._capacity+1];//strcpy(tmp, s._str);//delete[] _str;//_str = tmp;//_size = s._size;//_capacity = s._capacity;////return *this;//}~string(){delete[] _str;_str = nullptr;_size = _capacity = 0;}//遍历size_t size() const{return _size;}size_t capacity() const {return _capacity;}char& operator[](size_t pos) {assert(pos < _size);return _str[pos];   //_str[pos]在堆上，可用引用返回，支持修改}const char& operator[](size_t pos) const{assert(pos < _size);return _str[pos];   }void resize(size_t n, char ch = '\0'){if (n < _size){_str[n] = '\0';_size = n;}else{reserve(n);for (size_t i = _size; i < n; i++){_str[i] = ch;}_str[n] = '\0';_size = n;}}void reserve(size_t n){if (n > _capacity){char* tmp = new char[n+1];strcpy(tmp, _str);delete[] _str;_str = tmp;_capacity = n;}}void push_back(char ch){/*if (_size == _capacity){reserve(_capacity == 0 ? 4 : 2 * _capacity);}_str[_size] = ch;++_size;_str[_size] = '\0';*/insert(_size, ch);}void append(const char* str){/*size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}strcpy(_str + _size, str);_size += len;*/insert(_size, str);}string& operator+=(char ch){push_back(ch);return *this;}string& operator+=(const char* str){ append(str);return *this;}void insert(size_t pos, char ch){assert(pos <= _size);if (_size == _capacity){reserve(_capacity == 0 ? 4 : 2 * _capacity);}//第一种写法/*int end = _size;while (end >= (int) pos)  //需要强转，pos是size_t类型，end是int，end会被转成size_t类型，即范围小会转向范围大的。{  //当pos等于0，即头插时。结束条件为end小于0，如果不强转，当end小于0时，又会被转成无符号的_str[end + 1] = _str[end];  //就会死循环--end;}*///第二种size_t end = _size+1;while (end > pos){_str[end] = _str[end-1];--end;}_str[pos] = ch;++_size;}void insert(size_t pos, const char* str){assert(pos <= _size);size_t len = strlen(str);if (_size + len > _capacity){reserve(_size + len);}size_t end = _size + len;while (end > pos + len - 1){_str[end] = _str[end - len];end--;}strncpy(_str + pos, str, len);_size += len;}void erase(size_t pos, size_t len = npos){assert(pos < _size);if (len == npos || len >= _size - pos)  //len +pos>= _size  不能这么写，会有溢出风险{_str[pos] = '\0';_size = pos;}else{strcpy(_str + pos, _str + pos + len);_size -= len;}}void swap(string& s){std::swap(_str, s._str);std::swap(_size, s._size);std::swap(_capacity, s._capacity);}size_t find(char ch, size_t pos = 0) const{assert(pos < _size);for (size_t i = pos; i < _size; i++){if (_str[i] == ch)return i;}return npos;}size_t find(const char* sub, size_t pos = 0) const{assert(pos < _size);const char* p = strstr(_str+pos, sub);if (p){return p - _str;}else{return npos;} }string substr(size_t pos = 0, size_t len = npos){string sub;if (len >= _size - pos){for (size_t i = pos; i < _size; i++){sub += _str[i];}}else{for (size_t i = pos; i < pos+len; i++){sub += _str[i];}}return sub; }void clear(){_size = 0;_str[_size] = '\0';}private:char* _str=nullptr;size_t _size=0;size_t _capacity=0;public:static const int npos;};const int string::npos = -1;void swap(string& x, string& y){x.swap(y);} bool operator==(const string& s1, const string& s2){int ret = strcmp(s1.c_str(), s2.c_str());return ret == 0;}bool operator<(const string& s1, const string& s2){int ret = strcmp(s1.c_str(), s2.c_str());return ret < 0;}bool operator<=(const string& s1, const string& s2){return s1 < s2 || s1 == s1;}bool operator>(const string& s1, const string& s2){return !(s1 <= s2);}bool operator>=(const string& s1, const string& s2){return !(s1 < s2);}bool operator!=(const string& s1, const string& s2){return !(s1 == s2);}ostream& operator<<(ostream& out, const string& s){for (auto ch : s){out << ch;}return out;}istream& operator>>(istream& in, string& s) {s.clear(); char ch;//in >> ch; 错误，C++规定 cin,scanf默认空格和换行是分隔符，ch = in.get();//不会取到,get才能取到char buff[128];size_t i = 0;while (ch != ' ' && ch != '\n'){buff[i++] = ch;if (i == 127){buff[127] = '\0';s += buff;i = 0;}ch = in.get();}if (i > 0){buff[i] = '\0';s += buff; }return in;}istream& getline(istream& in, string& s){s.clear();char ch;//in >> ch; 错误，C++规定 cin,scanf默认空格和换行是分隔符，ch = in.get();//不会取到,get才能取到while (ch != '\n'){s += ch;ch = in.get();}return in;}void test_string1(){string s1("hello world");string s2;for (size_t i = 0; i < s1.size(); i++){s1[i]++;}cout << endl;for (size_t i = 0; i < s1.size(); i++){cout << s1[i] << "";}cout << endl;const string s3("xxxx");for (size_t i = 0; i < s3.size(); i++){//s3[i]++      错误，可读，但不能写cout << s3[i] << "";}cout << endl;}void test_string2(){string s3("hello world");string::iterator it3 = s3.begin();while (it3 != s3.end()){//*it3 -= 3;cout << *it3 << " ";++it3;}cout << endl;for (auto ch : s3){cout << ch << " ";}cout << endl;const string s2("hello world");string::const_iterator it2 = s2.begin(); while (it2 != s2.end()){cout << *it2 << " ";++it2;}cout << endl;}void test_string3(){string s3("hello world"); s3.push_back('1'); s3.push_back('2');  cout << s3.c_str()<< endl; s3 += 'x';  s3 += "yyyy"; cout << s3.c_str() << endl; string s1("hello world");s1.insert(11, 'x');cout << s1.c_str() << endl;s1.insert(0, 'x');cout << s1.c_str() << endl;}void test_string4(){string s1("hello world");cout << s1.c_str() << endl;s1.erase(6, 3);cout << s1.c_str() << endl;string s2("hello world");cout << s2.c_str() << endl;s2.resize(5);cout << s2.c_str() << endl;s2.resize(20, 'x');cout << s2.c_str() << endl;}void test_string5(){string s1("hello world");cout << s1.c_str() << endl;string s2(s1);cout << s2.c_str() << endl;string s3("xxxxx");s1 = s3;cout << s1.c_str() << endl;cout << s3.c_str() << endl;}void test_string6(){string s1("hello world");cout << s1.c_str() << endl;s1.insert(6, "xxxx");cout << s1.c_str() << endl;string s2("xxxxxx");cout << s1.c_str() << endl;cout << s2.c_str() << endl;//swap(s1, s2);  有了全局的swap，就会先调用全局的，s1.swap(s2);     //而不是算法库里面的模板swapcout << s1.c_str() << endl;cout << s2.c_str() << endl;}void test_string7(){string url1("https://legacy.cplusplus.com/reference/string/string/substr/");string url2("https://www.baidu.com/s?ie=utf-8&f=8&rsv_bp=1&rsv_idx=1&tn=baidu&wd=%E5%90%8E%E7%BC%80%20%E8%8B%B1%E6%96%87&fenlei=256&rsv_pq=0xc61b98d100494ef5&rsv_t=b494uh2Bd6SIxMQ1XY4CwB9MZOD%2BaMk5sx8aiT0NAgUczCRAy8jK7PvP5rxt&rqlang=en&rsv_enter=1&rsv_dl=tb&rsv_sug3=23&rsv_sug1=17&rsv_sug7=100&rsv_sug2=0&rsv_btype=i&inputT=7123&rsv_sug4=8008");string protocol, domain, uri; size_t i1 = url1.find(':');  if (i1 != string::npos) {protocol = url1.substr(0, i1 - 0);//左闭右开，减去后就是个数 cout << protocol.c_str() << endl; }size_t i2 = url1.find('/',i1+3); if (i2 != string::npos)  {domain = url1.substr(i1+3, i2-(i1+3));//左闭右开，减去后就是个数 cout << domain.c_str() << endl;  uri = url1.substr(i2 + 1); cout << uri.c_str() << endl; }} void test_string8(){string s1("hello world");string s2("hello world");cout << (s1 == s2) << endl;cout << ("hello world" == s2) << endl;//因为重载成全局的函数，cout << (s2 == "hello world") << endl;//单参数构造支持隐式类型转换。cout << s1 << endl;cout << s2 << endl;cin >> s1 >> s2;cout << s1 << endl;cout << s2 << endl;getline(cin, s1);cout << s1 << endl;}void test_string10(){string s1("hello world");string s2(s1);cout << s1 << endl;cout << s2 << endl;}}

测试代码

#define _CRT_SECURE_NO_WARNINGS#include<iostream>#include<string>using namespace std;#include"string.h"int main(){bit::test_string10();return 0;}

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