C++ Primer 学习笔记_27_操作符重载与转换（2）--++/--运算符重载、！运算符重载、赋值运算符重载 、String类（[]、 +、 += 运算符重载）、>>和<<运算符重载

#include <iostream>
#include <string>
#include <string.h>
using namespace std;

class String
{
public:
    String(const char *str = "");
    String(const String &other);
    String &operator=(const String &other);
    String &operator=(const char *str);

    bool operator!() const;
    char &operator[](unsigned int index);
    const char &operator[](unsigned int index) const;

    friend String operator+(const String &s1, const String &s2);
    String &operator+=(const String &other);

    friend ostream &operator<<(ostream &os, const String &str);
    friend istream &operator>>(istream &is, String &str);
    ~String(void);

    void Display() const;
    int Length() const;
    bool IsEmpty() const;

private:
    String &Assign(const char *str);
    char *AllocAndCpy(const char *str);
    char *str_;
};

String::String(const char *str)
{
    str_ = AllocAndCpy(str);
}

String::String(const String &other)
{
    str_ = AllocAndCpy(other.str_);
}

String &String::operator=(const String &other)
{
    if (this == &other)
        return *this;

    return Assign(other.str_);
}

String &String::operator=(const char *str)
{
    return Assign(str);
}

String &String::Assign(const char *str)
{
    delete[] str_;
    str_ = AllocAndCpy(str);
    return *this;
}

bool String::operator!() const
{
    return strlen(str_) != 0;
}

char &String::operator[](unsigned int index)
{
    return const_cast<char &>(static_cast<const String &>(*this)[index]);
}

const char &String::operator[](unsigned int index) const
{
    return str_[index];
}

String::~String()
{
    delete[] str_;
}

char *String::AllocAndCpy(const char *str)
{
    int len = strlen(str) + 1;
    char *newstr = new char[len];
    memset(newstr, 0, len);
    strcpy(newstr, str);

    return newstr;
}

void String::Display() const
{
    cout << str_ << endl;
}

int String::Length() const
{
    return strlen(str_);
}

bool String::IsEmpty() const
{
    return Length() == 0;
}

String operator+(const String &s1, const String &s2)
{
    String str = s1;
    str += s2;
    return str;
}

String &String::operator+=(const String &other)
{
    int len = strlen(str_) + strlen(other.str_) + 1;
    char *newstr = new char[len];
    memset(newstr, 0, len);
    strcpy(newstr, str_);
    strcat(newstr, other.str_);

    delete[] str_;

    str_ = newstr;
    return *this;
}

ostream &operator<<(ostream &os, const String &str)
{
    os << str.str_;
    return os;
}

istream &operator>>(istream &is, String &str)
{
    char tmp[1024];
    cin >> tmp;
    str = tmp;
    return is;
}

int main(void)
{
    String s1("abcdefg");

    char ch = s1[2];
    cout << ch << endl;

    s1[2] = 'A';
    s1.Display();

    const String s2("xyzabc");
    ch = s2[2];
    s2.Display();


    String s3 = "xxx";
    String s4 = "yyy";

    String s5 = s3 + s4;
    s5.Display();

    String s6 = "aaa" + s3 + "sdfadfa" + "xxxx";
    s6.Display();

    s3 += s4;
    s3.Display();

    cout << s3 << endl;

    String s7;
    cin >> s7;
    cout << s7 << endl;

    if (!s7.IsEmpty())
        cout<<s7.Length()<<endl;

    return 0;
}