实现String

class String
{
public:
    //构造函数
    String(const char* str = NULL) {
        m_str = new char[strlen(str ? str : "") + 1];
        strcpy(m_str, str ? str : "");
    }
    //析构函数
    ~String(void) {
        if (m_str) {
            //cout << "析构函数:" << (void*)m_str << endl;
            delete[] m_str;
            m_str = NULL;
        }
    }
    //拷贝构造(深拷贝)
    String(const String& that) {
        m_str = new char[strlen(that.m_str)+1];
        strcpy(m_str, that.m_str);
    }
    //拷贝赋值(深拷贝)
    String& operator=(const String& that) {
        if (&that != this) {
            delete[] m_str;
            m_str = new char[strlen(that.m_str) + 1];
            strcpy(m_str, that.m_str);    
            //复用拷贝构造版
            /*String tmp(that);
            swap(m_str,tmp.m_str*/ 
        }
        return *this;
    }
public:
    void print(void)const {
        cout << m_str << endl;
    }
private:
    char* m_str;
};