converting a vector of strings into an array.

Code:

vector<string> left_args;
 
//left_comm_toks is a vector that stores strings.
// GetNumTokens returns the size of the left_com_toks vector
for(int i = 0; i < left_comm_toks.GetNumTokens(); i++)
{
   left_args.push_back(left_comm_toks[i]);
}
 
vector<char *> left_argv;
 
// loop restores the vector with the same elements but diff type
for(int i = 0; i < left_args.size(); i++)
   left_argv.push_back(const_cast<char *>(left_args[i].c_str()));
 
 
close(p[0]);
dup2(p[1], 1);
close(p[1]);
//argv[2] = 0;
execvp(arg0, left_argv);  // get error right here left_arg is not the type that execvp wants
cerr << "Error on exec of " << arg0 << ": " << strerror(errno) << endl;
_exit(errno == ENOENT ? 127 : 126);

The above code of course does not work. What i think i need to do is copy the left_argv vector into a array so that execvp can use it. How can I go about doing this? Is there a way to cast the vector or something so i dont have to recopy into an array?

PS, this is not the whole code I just posted the part that is giving me trouble, the rest of the program works. Here is the WHOLE function just in case you need it.

Code:

void execute_command(string left_command, string right_command)
{
   //char *argv[] = {"ls", "ls", 0};
   //char *left_arg[] = {"ls", 0};
   //char *right_arg[] = {"grep", "main", 0};
   int p[2];
   int pid;
   int len, len2;
   ParsedTokens left_comm_toks;
   ParsedTokens right_comm_toks;
      
   // int pipe(int filedes[2]);
   // filedes[0] is for reading, filedes[1] is for writing
   if(pipe(p) < 0)
   {
      cerr << strerror(errno) << endl;
      exit(EXIT_FAILURE);
   }

   pid = fork();
   if(pid < 0)
   {
      cerr << strerror(errno) << endl;
      exit(EXIT_FAILURE);
   }
   if(pid == 0)
   {
      cout << "exec left arg" << endl;
	  char * tempstring;
	  tempstring = new char[left_command.length() + 1];
	  strcpy(tempstring, left_command.c_str());
	  left_comm_toks.ParseAndAddTokens(tempstring);
	  len = left_comm_toks[0].length();
	  const char * arg0 = left_comm_toks[0].c_str();
	  // converts left_command into left_argv[] which execvp uses
	  /*len = left_command.length();
	  char * const myArr = new char[len + 1];
	  for(int x = 0; x < len; x++)
	     myArr[x] = left_command[x];
	  myArr[len] = '\0';
	  char * const left_argv[] = {myArr,0};*/

      vector<string> left_args;

      for(int i = 0; i < left_comm_toks.GetNumTokens(); i++)
      {
         left_args.push_back(left_comm_toks[i]);
      }

      vector<char *> left_argv;

      for(int i = 0; i < left_args.size(); i ++)
         left_argv.push_back(const_cast<char *>(left_args[i].c_str()));

      close(p[0]);
      dup2(p[1], 1);
      close(p[1]);
      execvp(arg0, left_argv); 
      cerr << "Error on exec of " << arg0 << ": " << strerror(errno) << endl;
      _exit(errno == ENOENT ? 127 : 126);

   }

   // do the same thing except for the right command
   pid = fork();
   if(pid < 0)
   {
      cerr << strerror(errno) << endl;
      exit(EXIT_FAILURE);
   }
   if(pid == 0)
   {
      cout << "executing right command" << endl;
      // converts left_command into left_argv[] which execvp uses
	  len2 = right_command.length();
	  char * const myArr2 = new char[len +1];
	  for(int x = 0; x < len2; x++)
	     myArr2[x] = right_command[x];
	   myArr2[len2] = '\0';

	  char *const right_argv[] = {myArr2, 0};

	  close(p[1]);
	  dup2(p[0], 0);
      close(p[0]);
      execvp(right_argv[0], right_argv);
      cerr << "Error on exec of " << right_argv[0] << ": " << strerror(errno) << endl;
      _exit(errno == ENOENT ? 127 : 126);
   }
   close(p[0]);
   close(p[1]);

   wait(0);
   wait(0);
   
}

This is a nifty trick that the C++ standard guarantees will work:

Code:

execvp(arg0, &left_argv[0]);

thanks a bunch. everything works now!