string实现C++

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#include <iostream>class String{public:    String();    String(char c, int n);    String(const char* c_str);    String(const String& other);    String& operator=(const String& other);    String& operator=(const char* c_str);    ~String();    String& operator+=(const String& other);    String& operator+=(const char* c_str);    char& operator[](int i)const;/*1）必须在类的说明中说明友元函数，说明时以关键字friend开头，后跟友元函数的函数原型，友元函数的说明可以出现在类的任何地方，包括在private和public部分；2）注意友元函数不是类的成员函数，所以友元函数的实现和普通函数一样，在实现时不用"::"指示属于哪个类，只有成员函数才使用"::"作用域符号；3）友元函数不能直接访问类的成员，只能访问对象成员，4）友元函数可以访问对象的私有成员，但普通函数不行；5）调用友元函数时，在实际参数中需要指出要访问的对象，6）类与类之间的友元关系不能继承。7）一个类的成员函数也可以作为另一个类的友元，但必须先定义这个类。*/    friend std::ostream& operator<<(std::ostream& out, const String& other);    friend std::istream& operator>>(std::istream& in, const String& other);    //friend bool operator<(const String& left, const String& right);    //friend bool operator>(const String& left, const String& right);    //friend bool operator==(const String& left, const String& right);    //friend bool operator!=(const String& left, const String& right);    void print()const;    int length()const;    char& back();    char& front();    void append(const String& str);    void insert(int pos,const String& str);    void earse(int index);    void earse(char c);    void replace(char oldch,char newch);    char* c_str()const;private:    char* m_data;    int m_size;};String::String(){    m_data = new char[1];    *m_data = '\0';    m_size = 0;}String::String(char c, int n){    m_data = new char[n + 1];    m_size = n;    while (n > 0)    {        m_data[--n] = c;    }    m_data[m_size] = '\0';}String::String(const char* c_str){    if (c_str == NULL)    {        m_data = new char[1];        *m_data = '\0';        m_size = 0;    }    else    {        m_size = strlen(c_str);        m_data = new char[m_size + 1];        strcpy_s(m_data, m_size+1, c_str);    }}String::String(const String& other){    m_size = other.m_size;    m_data = new char[m_size+ 1];    strcpy_s(m_data, m_size + 1, other.m_data);}String& String::operator=(const String& other){    if (this == &other)    {        return *this;    }    if (m_data != NULL)    {        delete[] m_data;    }    m_size = other.m_size;    m_data = new char[m_size+1];    strcpy_s(m_data, m_size + 1, other.m_data);    return *this;}String& String::operator=(const char* c_str){    if (c_str == NULL)    {        return *this;    }    if (m_data != NULL)    {        delete[] m_data;        m_data = NULL;    }    m_size = strlen(c_str);    m_data = new char[m_size + 1];    strcpy_s(m_data, m_size + 1, c_str);    return *this;}char& String::operator[](int i)const{    if (i < m_size)    {        return m_data[i];    }    return m_data[m_size];;}String& String::operator+=(const String& other){    m_size = strlen(other.m_data) + m_size;    char* c_temp = m_data;    m_data = NULL;    m_data = new char[m_size+1];    strcpy_s(m_data, m_size + 1, c_temp);    strcat_s(m_data, m_size + 1, other.m_data);    delete[] c_temp;    return *this;}String& String::operator+=(const char* c_str){    m_size = strlen(c_str) + m_size;    char* c_temp = m_data;    m_data = NULL;    m_data = new char[m_size + 1];    strcpy_s(m_data, m_size + 1, c_temp);    strcat_s(m_data, m_size + 1, c_str);    delete[] c_temp;    return *this;}String::~String(){    if (m_data != NULL)    {        delete[] m_data;        m_data = NULL;        m_size = 0;    }}void String::print()const{    if (m_data != NULL)    {        std::cout << m_data << std::endl;    }}int String::length()const{    return m_size;}std::ostream& operator<<(std::ostream& out, const String& other){    for (int i = 0; i < other.length(); i++)    {        out << other[i];    }    return out;}//std::istream& operator>>(std::istream& in, const String& other)//bool operator<(const String& left, const String& right)//bool operator>(const String& left, const String& right)//bool operator==(const String& left, const String& right)//bool operator!=(const String& left, const String& right)char& String::back(){    if(m_size>0){        return m_data[m_size-1];    }else{        return m_data[0];    }}char& String::front(){    return m_data[0];}void String::append(const String& str){    m_size = str.m_size + m_size;    char* c_temp = m_data;    m_data = NULL;    m_data = new char[m_size + 1];    strcpy_s(m_data, m_size + 1, c_temp);    strcat_s(m_data, m_size + 1, str.m_data);    delete[] c_temp;}void String::insert(int pos,const String& str){    if(pos < 0 || pos > m_size){        return ;    }    m_size += str.m_size;    char* c_temp = m_data;    m_data = NULL;    m_data = new char[m_size + 1];    int i = 0;    for(; i<pos;++i){        m_data[i] = c_temp[i];    }    for(int j = 0;j < str.m_size;++j){        m_data[i++] = str[j];    }    while(c_temp[pos] != '\0'){        m_data[i++] = c_temp[pos++];    }    m_data[i] = '\0';    delete[] c_temp;}void String::earse(int index){    if(index < m_size){        char* p1 = m_data + index;        char* p2 = p1 + 1;        while(*p2 != '\0'){            *p1++ = *p2++;        }        *p1 = '\0';        m_size -= 1;    }}void String::earse(char c){    char* p1 = m_data;    char* p2 = m_data;    while(*p1 != '\0'){        if(*p1 != c){            *p2++ = *p1++;        }else{            ++p1;        }    }    *p2 = '\0';    m_size = int(p2 - m_data);}void String::replace(char oldch,char newch){    for(int i = 0 ; i < m_size ; ++i){        if(m_data[i] == oldch){            m_data[i] = newch;        }    }}char* String::c_str()const{    return m_data;}int main(){    String str1('1', 9);    String str2("12345");    std::cout << str1 << std::endl;    str2.print();    str1 += "23454";    str1.print();    str1[5] = '9';    str1.print();    std::cout << str1[15] <<std::endl;    str1.replace('4','A');    str1.print();    str1.earse('A');    str1.print();    str1.earse(0);    str1.print();    str1.insert(0,"xiaolixi");    str1.print();    return 0;}
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