16.7 Client Implementation

Now that we know the protocol, we can show how DistWebArray was implemented. Let's start with the implementation of the subscripting operator.

Document* DistWebArray::operator[](const RWCString& s)
{
  RWPortalStreambuf sbuf(RWSocketPortal(this->addr_));       //1
  RWvostream& ostrm = RWpostream(&sbuf);
  RWvistream& istrm = RWpistream(&sbuf);
  
  ostrm << RWCString("find") << s << flush;                  //2

  Document *web;
  istrm >> web;                                              //3
  
  return web;
}

`//1`	Create the virtual streams `ostrm` and `istrm` for communication with the server. Both virtual streams use as a *streambuf* an *RWPortalStreambuf* connected to the server. Since this is a one-shot protocol, we must reconnect to the server for each request. Why use `this->addr_` instead of just `addr_`? Without `this->` the compiler thinks this is a declaration of a function called `sbuf` that takes an *RWSocketPortal* named `addr_` as a parameter!
`//2`	Make the request, according to the protocol we have defined. If we forget to flush the *streambuf*'s buffer by inserting `flush`, the request will not get sent, the server will not reply, and the extraction operator on line (`//3`) will cause deadlock.
`//3`	Extract the web which the server has sent as its reply.

The implementation of the insertion operation is just as straightforward. Here is the code.

void DistWebArray::insertKeyAndValue(
  const RWCString& s, Document* doc)
{
  RWPortalStreambuf sbuf(RWSocketPortal(this->addr_));      //1
  RWvostream& ostrm = RWpostream(&sbuf); 
  RWvistream& istrm = RWpistream(&sbuf);
  ostrm << RWCString("insert") << s << doc << flush;        //2

  RWCString response;
  istrm >> response;                                        //3
  if (response!="ok") {
    throw RWExternalErr("insertKeyAndValue protocol failure");
  }
}

`//1`	As before, set up a pair of streams for communicating with the server.
`//2`	Request insertion of the web, according to the protocol we defined.
`//3`	The protocol stipulates that the string "ok" is returned if all went well. Check that this string is actually returned. If it isn't, then throw an exception.