C++实现简易版字符串类

用C++实现了简易版字符串类，原理还是用C语言风格的字符指针实现，主要为了练习C++的内存管理。

String有功能构造析构复制赋值和常见操作符，特别需要注意的是赋值和+=的时候相当于把原理的String的字符串抛弃掉了，这时候如果不对之前的字符串进行释放处理的话会造成内存泄漏！！

#include<bits/stdc++.h>
using namespace std;

class String {
private:
    char *ptr;    //字符串首指针 
    size_t len;    //字符串长度 
    
public:
    String(const char *str=NULL);    //构造函数 
    String(const String &str);    //复制构造函数 
    ~String();    //析构函数 
    
    friend String operator + (const String& str1,const String& str2);    //重载加号 
    
    String& operator = (const String& str);    //重载赋值运算符 
    String& operator += (const String& str);    //重载+= 
    bool operator == (const String &str);    //重载相等判断符 
    char& operator [] (int n);    //重载下标取值 
    
    bool operator != (const String& str) const;
    bool operator < (const String& str) const;
    bool operator <= (const String& str) const;
    bool operator > (const String& str) const;
    bool operator >= (const String& str) const;
    
    size_t size() const;    //返回长度 
    const char* c_str() const;    //获得String的C风格字符串 
    
    //输入输出运算 
    friend istream& operator >> (istream &is,String &str);
    friend ostream& operator << (ostream &os,String &str);    
};

//字符串拷贝函数：把strSrc串拷贝到strDest串 
char* strcpy(char *strDest,const char *strSrc) {
    if (strDest==NULL || strSrc==NULL) return NULL;
    if (strDest==strSrc) return strDest;
    char *head=strDest;    //先把首指针保存一份用来输出 
    while ((*strDest++ = *strSrc++) != '');
    return head; 
}

String::String(const char *str) {
    if (str==NULL) {    //构造为空串 
        this->len=0;
        this->ptr=new char[1];
        *ptr='';
    } else {
        this->len=strlen(str);
        this->ptr=new char[this->len+1];    //为什么+1，考虑'' 
        strcpy(this->ptr,str);
    }
}
String::String(const String &str) {
    this->len=str.len;
    this->ptr=new char[this->len+1];
    strcpy(this->ptr,str.ptr);
}
String::~String() {
    delete []this->ptr;
    this->len=0;
}

size_t String::size() const {
    return this->len;
}
const char* String::c_str() const {
    return this->ptr;
}

String operator + (const String& str1,const String& str2) {
    String newString;
    newString.len=str1.len+str2.len;
    newString.ptr=new char[newString.len+1];
    strcpy(newString.ptr,str1.ptr);
    strcat(newString.ptr,str2.ptr);
    return newString;
}
String& String::operator = (const String& str) {
    if (this==(&str)) return *this;
    //这一步很关键，先把以前的空间删掉再赋值，否则空间泄露 
    delete []this->ptr;
    
    this->len=str.len;
    this->ptr=new char[this->len+1];
    strcpy(this->ptr,str.ptr);
    return *this;
} 
String& String::operator += (const String& str) {
    //+=和上面的=同理，注意把以前的空间先释放
    char *tmp=this->ptr;
    *this=*this+str;
    delete []tmp;
    return *this;
}
//内联函数加快运行速度
inline bool String::operator == (const String& str) {
    if (this->len!=str.len) return 0;
    for (int i=0;i<this->len;i++)
        if (*(this->ptr+i) != *(str.ptr+i)) return 0;
    return 1;     
}
char& String::operator [] (int n) {
    //if (n>this->len-1) throw;  //越界异常
    return *(this->ptr+n); 
}

bool String::operator != (const String& str) const {
    if (this->len!=str.len) return 1;
    for (int i=0;i<this->len;i++)
        if (*(this->ptr+i) != *(str.ptr+i)) return 1;
    return 0;
}
bool String::operator < (const String& str) const {
    int minlen=min(this->len,str.len);
    bool ret=0; int equal_point=-1;
    for (int i=0;i<minlen;i++)
        if (*(this->ptr+i) != *(str.ptr+i)) {
            if (*(this->ptr+i) < *(str.ptr+i)) ret=1;
            break;
        } else equal_point=i;
    if (equal_point==minlen-1 && this->len-1==equal_point && str.len-1>equal_point) ret=1;    
    return ret;    
}
bool String::operator > (const String& str) const {
    int minlen=min(this->len,str.len);
    bool ret=0; int equal_point=-1;
    for (int i=0;i<minlen;i++)
        if (*(this->ptr+i) != *(str.ptr+i)) {
            if (*(this->ptr+i) > *(str.ptr+i)) ret=1;
            break;
        } else equal_point=i;
    if (equal_point==minlen-1 && this->len-1>equal_point && str.len-1==equal_point) ret=1;
    return ret;    
}

istream& operator >> (istream &is,String &str) {
    char s[100]; scanf("%s",s);
    str.len=strlen(s);
    strcpy(str.ptr,s);
}
ostream& operator << (ostream &os,String &str) {
    for (int i=0;i<str.len;i++)
        os<<*(str.ptr+i);
    return os;
}

int main()
{
    String s1; cin>>s1; cout<<s1<<endl;
    //这里的串会当成C语言字符串，调用构造函数 
    String s2("AqA"); cout<<s2<<endl;    
    //这里的s2就是String类了，调用复制构造函数 
    String s3(s2); cout<<s3<<endl;
    
    String s4(s2+s1+s3); cout<<s4<<endl;
    //这里的"11"会自动转成String ？ 
    String s5=s4+"11"; cout<<s5<<endl;
    
    cout<<s5[3]<<endl;
    cout<<"s2==s3 ? "<<(s2==s3)<<endl;
    cout<<"s1==s3 ? "<<(s1==s3)<<endl;
    
    //这里测试各种比较符号
    String s6("AAbb");
    String s7("AAbb");
    String s8("AAbba");
    String s9("AAbab");
    cout<<"s6==s7 ? "<<(s6==s7)<<endl;
    cout<<"s6>s7 ? "<<(s6>s7)<<endl;
    cout<<"s6<s7 ? "<<(s6<s7)<<endl;
    cout<<"s6>s8 ? "<<(s6>s8)<<endl;
    cout<<"s6<s8 ? "<<(s6<s8)<<endl;
    cout<<"s8>s9 ? "<<(s8>s9)<<endl;
    cout<<"s7>s9 ? "<<(s7>s9)<<endl; 
    return 0;
}