Boost::Asio::Read is not populating buffer - c++

Question

Here is my server class, which renders an async event to send a string to my client, when connected.
The message is definitely dispatched to the client, as the writehandler is invoked successfully without any errors:
class Server {
private:
void writeHandler(ServerConnection connection, const boost::system::error_code &error_code,
std::size_t bytes_transferred) {
if (!(error_code)) {
std::cout << "SENT "<<bytes_transferred <<" BYTES"<< std::endl;
}
}
void renderWriteEvent(ServerConnection connection, const std::string& str) {
std::cout << "RENDERING WRITE EVENT" << std::endl;
connection->write = str;
boost::asio::async_write(connection->socket, boost::asio::buffer(connection->write),
boost::bind(&Server::writeHandler, this, connection,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}
};
Now on the client side, after successfully connecting to the server, I call
void renderRead(){
std::cout<<"Available Bytes: "<<socket.available()<<std::endl;
std::string foo;
boost::system::error_code error_code;
std::size_t x = socket.read_some(boost::asio::buffer(foo), error_code);
std::cout<<error_code.message()<<std::endl;
std::cout<<"Bytes read: "<<x<<std::endl;
std::cout<<"Available Bytes: "<<socket.available()<<std::endl;
std::cout<<foo<<std::endl;
//boost::asio::async_read(socket, boost::asio::buffer(read_string), boost::bind(&Client::readHandler, this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
}
which outputs "Available Bytes: 12"
Then, in calling boost::asio::read, I get 0 bytes read, and no error. I don't understand what's wrong. After the read, the number of bytes available for reading in the socket stream is still printed to be 12

Answer 1

A key point here is that read_some() doesn't allocate any memory, it fills memory that is provided to it. For your code, this means ASIO will only replace the data already existing inside of foo, and it will never exceed these bounds.
But you have std::string foo;, which is a default-constructed string, aka an empty string.
So ASIO is populating the buffer you are passing just fine. However, you are passing it a buffer with no room in it. ASIO fills it as much as possible: 0 bytes.
You can test this for yourself by adding the following to your code:
std::string foo;
std::cout << "Available room in buffer: "<< foo.size() << std::endl;
The fix would be to pass a buffer with memory already allocated. You could initialize the string with a length, but using a raw block of bytes that you interpret later as a string_view is more explicit.
constexpr std::size_t buffer_size = 32;
std::array<char, buffer_size> foo;
std::size_t x = socket.read_some(boost::asio::buffer(foo), error_code);
//...
std::string_view message(foo.data(), x);
std::cout << message << std::endl;