string, char*, const char*转换

string -> const char*

   string s = "abc";
   const char* c_s = s.c_str();

const char* -> string

   const char* c_s = "abc";
   string s(c_s);

string -> char*

   string s = "abc";
   char* c;
   const int len = s.length();
   c = new char[len+1];
   strcpy(c,s.c_str());

char* -> string

   char* c = "abc";
   string s(c);

const char* -> char*

   const char* cpc = "abc";
   char* pc = new char[100];//足够长
   strcpy(pc,cpc);

char* -> const char*

   char* pc = "abc";
   const char* cpc = pc;