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C++ Hello World: refer reader to Quick Start
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      examples/cpp/helloworld/README.md

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# gRPC C++ Hello World Tutorial
# gRPC C++ Hello World Example
### Install gRPC
Make sure you have installed gRPC on your system. Follow the
[BUILDING.md](../../../BUILDING.md) instructions.
You can find a complete set of instructions for building gRPC and running the
Hello World app in the [C++ Quick Start][].
### Get the tutorial source code
The example code for this and our other examples lives in the `examples`
directory. Clone this repository at the [latest stable release tag](https://github.com/grpc/grpc/releases)
to your local machine by running the following command:
```sh
$ git clone -b RELEASE_TAG_HERE https://github.com/grpc/grpc
```
Change your current directory to examples/cpp/helloworld
```sh
$ cd examples/cpp/helloworld/
```
### Defining a service
The first step in creating our example is to define a *service*: an RPC
service specifies the methods that can be called remotely with their parameters
and return types. As you saw in the
[overview](#protocolbuffers) above, gRPC does this using [protocol
buffers](https://developers.google.com/protocol-buffers/docs/overview). We
use the protocol buffers interface definition language (IDL) to define our
service methods, and define the parameters and return
types as protocol buffer message types. Both the client and the
server use interface code generated from the service definition.
Here's our example service definition, defined using protocol buffers IDL in
[helloworld.proto](../../protos/helloworld.proto). The `Greeting`
service has one method, `hello`, that lets the server receive a single
`HelloRequest`
message from the remote client containing the user's name, then send back
a greeting in a single `HelloReply`. This is the simplest type of RPC you
can specify in gRPC - we'll look at some other types later in this document.
```protobuf
syntax = "proto3";
option java_package = "ex.grpc";
package helloworld;
// The greeting service definition.
service Greeter {
// Sends a greeting
rpc SayHello (HelloRequest) returns (HelloReply) {}
}
// The request message containing the user's name.
message HelloRequest {
string name = 1;
}
// The response message containing the greetings
message HelloReply {
string message = 1;
}
```
<a name="generating"></a>
### Generating gRPC code
Once we've defined our service, we use the protocol buffer compiler
`protoc` to generate the special client and server code we need to create
our application. The generated code contains both stub code for clients to
use and an abstract interface for servers to implement, both with the method
defined in our `Greeting` service.
To generate the client and server side interfaces:
```sh
$ make helloworld.grpc.pb.cc helloworld.pb.cc
```
Which internally invokes the proto-compiler as:
```sh
$ protoc -I ../../protos/ --grpc_out=. --plugin=protoc-gen-grpc=grpc_cpp_plugin ../../protos/helloworld.proto
$ protoc -I ../../protos/ --cpp_out=. ../../protos/helloworld.proto
```
### Writing a client
- Create a channel. A channel is a logical connection to an endpoint. A gRPC
channel can be created with the target address, credentials to use and
arguments as follows
```cpp
auto channel = CreateChannel("localhost:50051", InsecureChannelCredentials());
```
- Create a stub. A stub implements the rpc methods of a service and in the
generated code, a method is provided to create a stub with a channel:
```cpp
auto stub = helloworld::Greeter::NewStub(channel);
```
- Make a unary rpc, with `ClientContext` and request/response proto messages.
```cpp
ClientContext context;
HelloRequest request;
request.set_name("hello");
HelloReply reply;
Status status = stub->SayHello(&context, request, &reply);
```
- Check returned status and response.
```cpp
if (status.ok()) {
// check reply.message()
} else {
// rpc failed.
}
```
For a working example, refer to [greeter_client.cc](greeter_client.cc).
### Writing a server
- Implement the service interface
```cpp
class GreeterServiceImpl final : public Greeter::Service {
Status SayHello(ServerContext* context, const HelloRequest* request,
HelloReply* reply) override {
std::string prefix("Hello ");
reply->set_message(prefix + request->name());
return Status::OK;
}
};
```
- Build a server exporting the service
```cpp
GreeterServiceImpl service;
ServerBuilder builder;
builder.AddListeningPort("0.0.0.0:50051", grpc::InsecureServerCredentials());
builder.RegisterService(&service);
std::unique_ptr<Server> server(builder.BuildAndStart());
```
For a working example, refer to [greeter_server.cc](greeter_server.cc).
### Writing asynchronous client and server
gRPC uses `CompletionQueue` API for asynchronous operations. The basic work flow
is
- bind a `CompletionQueue` to a rpc call
- do something like a read or write, present with a unique `void*` tag
- call `CompletionQueue::Next` to wait for operations to complete. If a tag
appears, it indicates that the corresponding operation is complete.
#### Async client
The channel and stub creation code is the same as the sync client.
- Initiate the rpc and create a handle for the rpc. Bind the rpc to a
`CompletionQueue`.
```cpp
CompletionQueue cq;
auto rpc = stub->AsyncSayHello(&context, request, &cq);
```
- Ask for reply and final status, with a unique tag
```cpp
Status status;
rpc->Finish(&reply, &status, (void*)1);
```
- Wait for the completion queue to return the next tag. The reply and status are
ready once the tag passed into the corresponding `Finish()` call is returned.
```cpp
void* got_tag;
bool ok = false;
cq.Next(&got_tag, &ok);
if (ok && got_tag == (void*)1) {
// check reply and status
}
```
For a working example, refer to [greeter_async_client.cc](greeter_async_client.cc).
#### Async server
The server implementation requests a rpc call with a tag and then wait for the
completion queue to return the tag. The basic flow is
- Build a server exporting the async service
```cpp
helloworld::Greeter::AsyncService service;
ServerBuilder builder;
builder.AddListeningPort("0.0.0.0:50051", InsecureServerCredentials());
builder.RegisterService(&service);
auto cq = builder.AddCompletionQueue();
auto server = builder.BuildAndStart();
```
- Request one rpc
```cpp
ServerContext context;
HelloRequest request;
ServerAsyncResponseWriter<HelloReply> responder;
service.RequestSayHello(&context, &request, &responder, &cq, &cq, (void*)1);
```
- Wait for the completion queue to return the tag. The context, request and
responder are ready once the tag is retrieved.
```cpp
HelloReply reply;
Status status;
void* got_tag;
bool ok = false;
cq.Next(&got_tag, &ok);
if (ok && got_tag == (void*)1) {
// set reply and status
responder.Finish(reply, status, (void*)2);
}
```
- Wait for the completion queue to return the tag. The rpc is finished when the
tag is back.
```cpp
void* got_tag;
bool ok = false;
cq.Next(&got_tag, &ok);
if (ok && got_tag == (void*)2) {
// clean up
}
```
To handle multiple rpcs, the async server creates an object `CallData` to
maintain the state of each rpc and use the address of it as the unique tag. For
simplicity the server only uses one completion queue for all events, and runs a
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".
[C++ Quick Start]: https://grpc.io/docs/quickstart/cpp

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