C++: syntax error detected by MSVC only using Boost::asio

Question

I'm making a basic client-server architecture in C++ using Boost::ASIO.

MSVC reports a syntax error: '.' (C2059) at this line:

void
Server::start()
{
  m_stopped = false;
  listen_one();

  m_runner = std::make_unique<std::thread>([this](){
    io_service.run(); // <- syntax error: '.'
  });

  m_runner->detach();
}

GCC and Clang both compile the same code on Linux and macOS just fine.
The code has been changed several times and compiled several imes, and the error persists.
There is this other block of code, which MSVC does not detect as an error:

void
Client::init()
{
  m_socket->connect(boost::asio::ip::tcp::endpoint(
                              boost::asio::ip::address::from_string(m_ip),
                              static_cast<short unsigned int>(m_port)));
  
  Connection::init();

  m_runner = std::make_unique<std::thread>([this](){
    io_service.run(); // <- this line is fine
  });

  m_runner->detach();
}

Basic debugging:

io_service, m_stopped, m_runner are member variables of Server, and listen_one() is a member function.
Server does not inherit anything. (If relevant, Client inherits another class, Connection)

Why does MSVC detect a syntax error, but neither GCC nor Clang do?

Can you try `this->io_service.run()`? MSVC has been challenged with two-phase lookup for a very long time. Perhaps that's it — sehe, Dec 14 '20 at 21:46
@sehe Well, somehow, it solved the problem. Please post your comment as an answer so I can accept it :) — Sampi, Dec 15 '20 at 02:42

score 1 · Accepted Answer · answered Dec 15 '20 at 10:09

Can you try this->io_service.run()?

MSVC has been challenged with two-phase lookup for a very long time.

Perhaps that's it. What this-> makes explicit is that io_service represents a class member. In the lambda shown it shouldn't usually matter (except perhaps if the class is actually declared [as part of] a template?).

Another factor at play could be name shadowing: io_service shadows the name of the type. If that type name is visible without namespace qualification, then it might cause confusion to MSVC.

C++: syntax error detected by MSVC only using Boost::asio

1 Answers1