string -> const char *
用str的c_str()方法或者data()方法均可，这个两个方法返回值为cong char *
string str = "hello";
const char *a = str.c_str();
const char *b = str.c_str();
const char * -> string
const char*转换为 string，直接赋值即可
string -> char *
vs不让用strcpy，只能用strcpy_s代替
string str = "hello";
int str_len = str.length();
char *a = new char[str_len + ];
strcpy_s(a, str_len + , str.c_str());
delete[] a;
char * -> string
直接赋值
const char * -> char *
const char *b = "hello";
char *a = new char[strlen(b) + ];
strcpy_s(a, strlen(b) + , b);
cout << a << endl;
delete[] a;
char * -> const char*
直接赋值
char *a = "hello";
const char *b = a;