Merge pull request #21911 from donnadionne/example2

Add XDS example to the suite of examples.

examples/cpp/helloworld/README.md

@@ -255,6 +255,10 @@ main loop in `HandleRpcs` to query the queue.
 
 For a working example, refer to [greeter_async_server.cc](greeter_async_server.cc).
 
+#### Flags for the client
 
+```sh
+./greeter_client --target="a target string used to create a GRPC client channel"
+```
 
-
+The Default value for --target is "localhost:50051".

examples/cpp/helloworld/greeter_client.cc

@@ -73,11 +73,32 @@ class GreeterClient {
 
 int main(int argc, char** argv) {
   // Instantiate the client. It requires a channel, out of which the actual RPCs
-  // are created. This channel models a connection to an endpoint (in this case,
-  // localhost at port 50051). We indicate that the channel isn't authenticated
-  // (use of InsecureChannelCredentials()).
+  // are created. This channel models a connection to an endpoint specified by
+  // the argument "--target=" which is the only expected argument.
+  // We indicate that the channel isn't authenticated (use of
+  // InsecureChannelCredentials()).
+  std::string target_str;
+  std::string arg_str("--target");
+  if (argc > 1) {
+    std::string arg_val = argv[1];
+    size_t start_pos = arg_val.find(arg_str);
+    if (start_pos != std::string::npos) {
+      start_pos += arg_str.size();
+      if (arg_val[start_pos] == '=') {
+        target_str = arg_val.substr(start_pos + 1);
+      } else {
+        std::cout << "The only correct argument syntax is --target=" << std::endl;
+        return 0;
+      }
+    } else {
+      std::cout << "The only acceptable argument is --target=" << std::endl;
+      return 0;
+    }
+  } else {
+    target_str = "localhost:50051";
+  }
   GreeterClient greeter(grpc::CreateChannel(
-      "localhost:50051", grpc::InsecureChannelCredentials()));
+      target_str, grpc::InsecureChannelCredentials()));
   std::string user("world");
   std::string reply = greeter.SayHello(user);
   std::cout << "Greeter received: " << reply << std::endl;