C++ - c++实现String类

#include <iostream>
#include <cstring>
using namespace std;
class String {
public:
    String (const char* str = NULL) {
        m_str = new char[strlen(str?str:"")+1];
        strcpy (m_str, str ? str : "");
    }
    ~String (void) {
        if (m_str) {
            delete[] m_str;
            m_str = NULL;
        }
    }
    String (const String& that) :
        m_str (strcpy (
            new char[strlen(that.m_str)+1],
            that.m_str)) {}
    /* 菜鸟
    void operator= (const String& that) {
        m_str = new char[strlen(that.m_str)+1];
        strcpy (m_str, that.m_str);
    }*/
    String& operator= (const String& that) {
        if (&that != this) {
            /* 小鸟
            delete[] m_str;
            m_str = new char[strlen(that.m_str)+1];
            strcpy (m_str, that.m_str);
            */
            /* 大鸟
            char* str = 
                new char[strlen(that.m_str)+1];
            delete[] m_str;
            m_str = strcpy (str, that.m_str);
            */
            // 老鸟
            String temp (that);
            swap (m_str, temp.m_str);
        }
        return *this;
    }
    const char* c_str (void) const {
        return m_str;
    }
    operator const char* (void) const {
        return c_str ();
    }
private:
    char* m_str;
};
int main (void) {
    String s1 ("Hello, World !");
    cout << s1.c_str () << endl;
    String s2 = s1;
    cout << s2.c_str () << endl;
    String s3 ("Hello, Linux !");
    try {
        s1 = s3;
    }
    catch (exception& ex) {
        cout << ex.what () << endl;
    }
    cout << s1.c_str () << endl;
    const char* psz = s1;
    cout << s1 << endl;
    cout << psz << endl;
    return 0;
}