System::String^    to std::string 

                          void MarshalString ( System::String^ s, std::string& os )

 {   

 using namespace System::Runtime::InteropServices;  

 const char* chars = (const char*)(Marshal::StringToHGlobalAnsi(s)).ToPointer(); 

 os = chars; 

 Marshal::FreeHGlobal(IntPtr((void*)chars));

 }

std::string to System::String^

                         string s="aaaaaaaaaaa";
                         String^ str=gcnew String(s.c_str());

                            char *   to  System::String^

                           char *ch="this is char pointer"; 

  String^ str=gcnew String(ch);//  或 ：System::Runtime::InteropServices::Marshal::PtrToStringAnsi((IntPtr)ch);

                          std::string to char * 

                           string str="hello";

                           char * ch;

                            ch=str;