操作符重载 =

注：1、缺省值不要赋值为NULL，空字符串可以写成 “” ；
2、+=操作符特殊，eg：s1+=s2，s1=s1+s2，必须将串接结果赋值给原来的对象；return什么不重要，主要是原有的内存（s1）里最后变成了什么。
#include
#pragma warning(disable:4996);
using namespace std;
class String
{
private:
char str;
public:
String(const char s = NULL) //构造函数
{
if (s != NULL){
str = new char[strlen(s) + 1];
strcpy(str, s);
}
else{
str = NULL;
}

}<br />    String(const String& string)        //复制构造函数<br />    {<br />        if (string.str != NULL){<br />            str = new char[strlen(string.str) + 1];<br />            strcpy(str, string.str);<br />        }<br />        else{<br />            str = NULL;<br />        }
}<br />    ~String()        //析构函数<br />    {<br />        cout << "Destruct!" << endl;<br />    }<br />    void Set(char* s)<br />    {<br />        str = new char[strlen(s) + 1];<br />        strcpy(str, s);<br />    }<br />    void Print()<br />    {<br />        if (str != NULL) {<br />            cout << str << endl;<br />        }<br />        else {<br />            cout << "NULL!" << endl;<br />        }<br />    }<br />    String operator +(const String& string2)<br />    {<br />        String string;<br />        if (this->str != NULL && string2.str != NULL) {<br />            string.str = new char[strlen(this->str) + strlen(string2.str) + 1];<br />            strcpy(string.str, this->str);<br />            strcat(string.str, string2.str);<br />            return string;<br />        }<br />        else if (this->str == NULL) {<br />            return string2;<br />        }<br />        else if (string2.str == NULL) {<br />            return *this;<br />        }<br />        else return NULL;<br />    }<br />    void operator =(const String& string1)<br />    {<br />        this->str = new char[strlen(string1.str) + 1];<br />        strcpy(this->str, string1.str);<br />    }<br />    String operator +=(const String& string)        //操作符重载+=<br />    {<br />        if (this->str == NULL&&string.str!=NULL) {<br />            this->str = new char[strlen(string.str)];<br />            strcpy(this->str, string.str);<br />            return *this;<br />        }<br />        else if (string.str== NULL&&this->str!=NULL) {<br />            return *this;<br />        }<br />        else if (string.str != NULL && this->str != NULL) {<br />            char* temp = new char[strlen(this->str) + 1];<br />            strcpy(temp, this->str);<br />            this->str = new char[strlen(string.str) + strlen(temp) + 1];<br />            strcpy(this->str, temp);<br />            this->str = strcat(this->str, string.str); <br />            return *this;<br />        }<br />        else {<br />            return NULL;<br />        }<br />    }<br />};<br />int main()<br />{<br />    String s1("abcd"), s2("efghij"), s3("lmnop"), s4, s5,s6;<br />    s1 += s2;<br />    s2 += s4;<br />    s3 += s3 += s2;<br />    s4 += s1;<br />    s5 += s6;<br />    s1.Print();<br />    s2.Print();<br />    s3.Print();<br />    s4.Print();<br />    s5.Print();<br />}