自定义String

class String
{
private:
    char* m_str;
public:
    String(const char* str = NULL)
    {
        m_str = NULL;
        if(NULL == str) return;
        int len = strlen(str);
        m_str = new char[len+1];
        strcpy(m_str,str);
    }
    ~String()
    {
        if(m_str) delete m_str;
    }
    String(const String& that)
    {
        m_str = NULL;
        if(that.m_str==NULL)
        {
            return;
        }
        int len = strlen(that.m_str);
        m_str = new char[len+1];
        strcpy(m_str,that.m_str);
    }
    String &operator=(const String& that)
    {
        if(&that != this)
        {
            String temp;
            swap(m_str,temp.m_str);
        }
        return *this;
    }
    const char* c_str(){return m_str;}
};