C++：string容器（下篇）

1.string浅拷贝的问题

// 为了和标准库区分，此处使用String
class String
{
public :/*String():_str(new char[1]){*_str = '\0';}*///String(const char* str = "\0") // 错误示范//String(const char* str = nullptr) // 错误示范String(const char* str = ""){if (nullptr == str){assert(false);return;} _str = new char[strlen(str) + 1];strcpy(_str, str);   }~String(){if (_str){delete[] _str;_str = nullptr;}}
private:char* _str;
};int main()
{String s1("hello world!");String s2(s1);    // 这里会析构两次，导致程序崩溃return 0;
}

说明：

        上述String类没有显式定义其拷贝构造函数与赋值运算符重载，此时编译器会合成默认的，当用s1构造s2时，编译器会调用默认的拷贝构造。最终导致的问题是，s1、s2共用同一块内存空间，在释放时同一块空间被释放多次而引起程序崩溃，这种拷贝方式，称为浅拷贝。

浅拷贝：

        也称位拷贝，编译器只是将对象中的值拷贝过来。如果对象中管理资源，最后就会导致多个对象共享同一份资源，当一个对象销毁时就会将该资源释放掉，而此时另一些对象不知道该资源已经被释放，以为还有效，所以当继续对资源进项操作时，就会发生发生了访问违规。

        可以采用深拷贝解决浅拷贝问题，即：每个对象都有一份独立的资源，不要和其他对象共享。

2.深拷贝

        如果一个类中涉及到资源的管理，其拷贝构造函数、赋值运算符重载以及析构函数必须要显式给出。一般情况都是按照深拷贝方式提供。

2.1 传统写法的String类

class String
{
public :String(const char* str = ""){if (nullptr == str){assert(false);return;} _str = new char[strlen(str) + 1];strcpy(_str, str);} String(const String& s): _str(new char[strlen(s._str) + 1]){strcpy(_str, s._str);} String& operator=(const String& s){if (this != &s){char* pStr = new char[strlen(s._str) + 1];strcpy(pStr, s._str);delete[] _str;_str = pStr;} return* this;} ~String(){if (_str){delete[] _str;_str = nullptr;}}private:char* _str;
};

2.1 现代写法的String类

class String
{
public :String(const char* str = ""){// 构造String类对象时，如果传递nullptr指针，可以认为程序非if (nullptr == str){assert(false);return;} _str = new char[strlen(str) + 1];strcpy(_str, str);} String(const String& s): _str(new char[strlen(s._str) + 1]){strcpy(_str, s._str);} // 现代版本String & operator=(String s){std::swap(_str, s._str);return *this;}传统版本//String& operator=(const String& s)//{//	if (this != &s)//	{//		char* pStr = new char[strlen(s._str) + 1];//		strcpy(pStr, s._str);//		delete[] _str;//		_str = pStr;//	} //	return* this;//} ~String(){if (_str){delete[] _str;_str = nullptr;}}private:char* _str;
};

3.string类的模拟实现

头文件 string.h：

namespace room
{class string{public:// 迭代器typedef char* iterator;typedef const char* const_iterator;iterator begin(){return _str;}iterator end(){return _str + _size;}const_iterator begin() const{return _str;}const_iterator end() const{return _str + _size;}/*string():_str(new char[1]{'\0'}),_size(0),_capacity(0){}*/void swap(string& s);string(size_t n, char ch);string(const char* str = "");// s2(s1)string(const string& s);// s1 = s2// s1 = s1//string& operator=(const string& s);// s1 = s2string& operator=(string s);~string();void clear(){_str[0] = '\0';_size = 0;}const char* c_str() const{return _str;}void reserve(size_t n);void push_back(char ch);void append(const char* str);string& operator+=(char ch);string& operator+=(const char* str);void insert(size_t pos, size_t n, char ch);void insert(size_t pos, const char* ch);void erase(size_t pos = 0, size_t len = npos);size_t find(char ch, size_t pos = 0);size_t find(const char* str, size_t pos = 0);size_t size() const{return _size;}size_t capacity() const{return _size;}char& operator[](size_t pos){assert(pos < _size);return _str[pos];}const char& operator[](size_t pos) const{assert(pos < _size);return _str[pos];}string substr(size_t pos, size_t len = npos);bool operator==(const string& s) const;bool operator!=(const string& s) const;bool operator<(const string& s) const;bool operator<=(const string& s) const;bool operator>(const string& s) const;bool operator>=(const string& s) const;private:// 声明char* _str = nullptr;size_t _size = 0;size_t _capacity = 0;const static size_t npos;};ostream& operator<<(ostream& out, const string& s);istream& operator>>(istream& in, string& s);istream& getline(istream& in, string& s, char delim);
}

源文件string.cpp：

// 链接时会合并
namespace room 
{const size_t string::npos = -1;string::string(size_t n, char ch):_str(new char[n + 1]),_size(n),_capacity(n){for (size_t i = 0; i < n; ++i){_str[i] = ch;}_str[_size] = '\0';}string::string(const char* str):_size(strlen(str)){_capacity = _size;_str = new char[_size + 1];strcpy(_str, str);}传统写法s2(s1)//string::string(const string& s)//{//	_str = new char[s._capacity + 1];//	strcpy(_str, s._str);//	_size = s._size;//	_capacity = s._capacity;//}void string::swap(string& s){std::swap(_str, s._str);std::swap(_size, s._size);std::swap(_capacity, s._capacity);}// 现代写法// s2(s1)string::string(const string& s){string tmp(s._str);swap(tmp);}// s1 = s2// s1 = s1/*string& string::operator=(const string& s){if (this != &s){delete[] _str;_str = new char[s._capacity + 1];strcpy(_str, s._str);_size = s._size;_capacity = s._capacity;}return *this;}*/// s1 = s2string& string::operator=(string s){swap(s);return *this;}string::~string(){delete[] _str;_str = nullptr;_size = _capacity = 0;}void string::reserve(size_t n){if (n > _capacity){char* tmp = new char[n + 1];strcpy(tmp, _str);delete[] _str;_str = tmp;_capacity = n;}}void string::push_back(char ch){if (_size + 1 > _capacity){// 扩容reserve(_capacity == 0 ? 4 : _capacity * 2);}_str[_size] = ch;++_size;_str[_size] = '\0';	// 末尾得加上一个\0}void string::append(const char* str){size_t len = strlen(str);if (_size + len > _capacity){// 扩容size_t newCapacity = 2 * _capacity;if(_size + len > 2 * _capacity){newCapacity = _size + len;}reserve(newCapacity);}strcpy(_str + _size, str);_size += len;}string& string::operator+=(char ch){push_back(ch);return *this;}string& string::operator+=(const char* str){append(str);return *this;}void string::insert(size_t pos, size_t n, char ch){assert(pos <= _size);assert(n > 0);if (_size + n > _capacity){// 扩容size_t newCapacity = 2 * _capacity;if (_size + n > 2 * _capacity){newCapacity = _size + n;}reserve(newCapacity);}// 挪动数据// 这样挪动数据，头插的时候会越界/*size_t end = _size;while (end >= pos){_str[end + n] = _str[end];--end;}*/size_t end = _size + n;while (end > pos + n - 1){_str[end] = _str[end - n];--end;}for (size_t i = 0; i < n; ++i){_str[pos + i] = ch;}_size += n;/*string tmp(n, ch);insert(pos, tmp.c_str());*/}void string::insert(size_t pos, const char* str){assert(pos <= _size);size_t n = strlen(str);if (_size + n > _capacity){// 扩容size_t newCapacity = 2 * _capacity;if (_size + n > 2 * _capacity){newCapacity = _size + n;}reserve(newCapacity);}size_t end = _size + n;while (end > pos + n - 1){_str[end] = _str[end - n];--end;}for (size_t i = 0; i < n; ++i){_str[pos + i] = str[i];}}void string::erase(size_t pos, size_t len){if (len >= _size - pos){// 删完数据_str[pos] = '\0';_size = pos;}else{size_t end = pos + len;while (end <= _size){_str[end - len] = _str[end];++end;}_size -= len;}}size_t string::find(char ch, size_t pos){for (size_t i = pos; i < _size; ++i){if (_str[i] == ch){return i;}}return npos;}size_t string::find(const char* str, size_t pos){const char* p = strstr(_str + pos, str);if (p == nullptr){return npos;}else{return p - _str;}}string string::substr(size_t pos, size_t len){size_t leftlen = _size - pos;// 给的长度大于剩余的长度时，len等于剩余长度if (len > leftlen)len = leftlen;string tmp;tmp.reserve(len);for (size_t i = 0; i < len; ++i){tmp += _str[pos + i];}return tmp;}bool string::operator==(const string& s) const{return strcmp(_str, s._str) == 0;}bool string::operator!=(const string& s) const{return !(*this == s);}bool string::operator<(const string& s) const{return strcmp(_str, s._str) < 0;}bool string::operator<=(const string& s) const{return *this < s || *this == s;}bool string::operator>(const string& s) const{return !(*this <= s);}bool string::operator>=(const string& s) const{return !(*this < s);}ostream& operator<<(ostream& out, const string& s){for (auto ch : s)out << ch;return out;}istream& operator>>(istream& in, string& s){s.clear();// 输入短串，不会浪费空间// 输入长串，避免不断扩容const size_t N = 1024;char buff[N];int i = 0;//cin >> i;	// 这样是不行的char ch = in.get();	// 用get()才能收到空格和换行// 短串就放入buffwhile (ch != ' ' && ch != '\n')	// 遇到\n就结束{buff[i++] = ch;if (i == N - 1){buff[i] = '\0';s += buff;i = 0;}ch = in.get();}if (i > 0){// 长串才扩容，减小扩容带来的性能消耗buff[i] = '\0';s += buff;}return in;}istream& getline(istream& in, string& s, char delim){s.clear();// 输入短串，不会浪费空间// 输入长串，避免不断扩容const size_t N = 1024;char buff[N];int i = 0;//cin >> i;	// 这样是不行的char ch = in.get();	// 用get()才能收到空格和换行// 短串就放入buffwhile (ch != delim)	// 遇到指定字符delim就结束{buff[i++] = ch;if (i == N - 1){buff[i] = '\0';s += buff;i = 0;}ch = in.get();}if (i > 0){// 长串才扩容，减小扩容带来的性能消耗buff[i] = '\0';s += buff;}return in;}
}

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