How do I utilize boost::packaged_task, function parameters, and boost::asio::io_service?

Question

First and foremost, I want to apologize for the lengthy post. I wanted to be as thorough as possible.

I've been stuck on this issue for a few days now, and there is surprisingly little information regarding the proper use of boost::packaged_task on a function that has input parameters.

System Info

• C++03
• Boost 1.54.0
• CMake 2.8.9

The Initial Requirement

1. I have a setup that consists of client(s), a server, and device(s).
2. A client interacts with a device by sending requests to the server.
   • These requests are examined and routed to the appropriate device.
   • Requests are handled asynchronously, and are occasionally queued up via boost::asio::io_service::strand for various reasons.
3. Requests are placed into a queue, local to the device itself.
   • When the request has been acknowledged (not necessarily completed), it is assigned an ID, and returned to the client.

Packaged Task

After looking through boost::futures we decided that boost::packaged_task would do exactly what we need. However, there appears to be a bug in the implementation of packaged task.

It appears as if packaged_task has a few different templates to choose from:

1. packaged_task<R>
2. packaged_task<R()>
3. packaged_task<R(ArgTypes)>
4. Others that I may be missing.

To ensure that I was using the function correctly, I started simple; using the simple example on the boost::futures page as a starting point. From there, I created four simple functions:

• int return, no parameters.
• int return, with parameters.
• std::string return, no parameters.
• std::string return, with parameters.

Test functions

std::string ans("forty two");

int int_no_params()
{
    return 42;
}

int int_with_params(int param)
{
    return param;
}

std::string string_no_params()
{
    return std::string("forty two");
}

std::string string_with_params(std::string & param) // Have tried both with and without '&'
{
    return param;
}

EXAMPLE 1:

int function(void)

    //! Compiles and produces correct result.  
    {
        boost::packaged_task<int()> example(int_no_params);
        boost::future<int> f = example.get_future();
        boost::thread task(boost::move(example));
        int answer = f.get();
        std::cout << "Answer to life and whatnot, in English: " << answer << std::endl;
        task.join();
    }

EXAMPLE 2:

std::string function(void)

    //! Compiles and produces correct result.
    {
        boost::packaged_task<std::string()> example(string_no_params);
        boost::future<std::string> f = example.get_future();
        boost::thread task(boost::move(example));
        std::string answer = f.get();
        std::cout << "string_no_params: " << answer << std::endl;
        task.join();
    }

EXAMPLE 3:

std::string(std::string& param) No threading

//! Doesn't compile.
//! error: variable ‘boost::packaged_task<std::basic_string<char>(std::basic_string<char>&)> example’ has initializer but incomplete type

{
    boost::packaged_task<std::string(std::string&)> example(string_with_params);
    boost::future<std::string> f = example.get_future();
    example(ans);
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
}

EXAMPLE 4:

using boost::threading

//! Doesn't compile.
//! error: variable ‘boost::packaged_task<std::basic_string<char>(std::basic_string<char>&)> example’ has initializer but incomplete type
{
    boost::packaged_task<std::string(std::string&)> example(string_with_params);
    boost::future<std::string> f = example.get_future();
    boost::thread task(boost::move(example), ans);
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
    task.join();
}

EXAMPLE 5:

Using extended initializers in packaged_task declaration

//! Doesn't compile in C++03, C++11 only.
//! error: extended initializer lists only available with -std=c++11 or -std=gnu++11 [-Werror]
{
    boost::packaged_task<std::string(std::string&)> example
    { boost::bind(&string_with_params, ans) };
    boost::future<std::string> f = example.get_future();
    boost::thread task(boost::move(example), ans);
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
    task.join();
}

EXAMPLE 6:

Threaded, using shared_ptr

The following use typedef boost::packaged_task<std::string(std::string&)> task_t;

Because packaged tasks can't be copied, binding shared_ptr<T>::operator() to task was a suggested solution found here.

// error: invalid use of incomplete type ‘class boost::packaged_task<std::basic_string<char>(std::basic_string<char>&)>’
// error: incomplete type ‘task_t {aka boost::packaged_task<std::basic_string<char>(std::basic_string<char>&)>}’ used in nested name specifier
// boost/thread/future.hpp:1320:11: error: declaration of ‘class boost::packaged_task<std::basic_string<char>(std::basic_string<char>&)>’
{
    boost::shared_ptr<task_t> example = boost::make_shared<task_t>(boost::bind(&string_with_params, ans));
    boost::future<std::string> f = example->get_future();
    boost::thread task(boost::bind(&task_t::operator(), example));
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
    task.join();
}

EXAMPLE 7:

Using boost::asio::io_service and boost::bind

// error: invalid use of incomplete type ‘class boost::packaged_task(std::basic_string&)>’ // error: incomplete type ‘task_t {aka boost::packaged_task(std::basic_string&)>}’ used in nested name specifier // boost/thread/future.hpp:1320:11: error: declaration of ‘class boost::packaged_task(std::basic_string&)>’

{
    boost::asio::io_service io_service;
    boost::thread_group threads;
    boost::asio::io_service::work work(io_service);

    for (int i = 0; i < 3; ++i)
    {
        threads.create_thread(boost::bind(&boost::asio::io_service::run,
            &io_service));
    }

    boost::shared_ptr<task_t> example = boost::make_shared<task_t>(boost::bind(&string_with_params, ans));
    boost::future<std::string> f = example->get_future();
    io_service.post(boost::bind(&task_t::operator(), example));
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
    threads.join_all();
}

Is there something I am doing awfully wrong here? I feel like I've exhaustively tested this and haven't made any headway. I have tried every other combination of binds, threads, and tasks to get this working, but it's simply not happening. I appreciate any help you provide.

As a final note:

I have a working solution using futures and promises, and by using a private function to post to my thread, I return a valid future. This issue just seems to be something that isn't necessarily user error.

Thanks for reading.

Answer 1

While I cannot find the limitation explicitly noted in the documentation, the change history notes that the ability to supply argument types to Boost.Thread's packaged_task is for C++11 compliance:

C++11 compliance: Add ArgTypes to packaged_task template. Provided when BOOST_THREAD_PROVIDES_SIGNATURE_PACKAGED_TASK is defined (Default value from Boost 1.55).

The related ticket states that when variadic templates are not available, only the signature R() will be provided.

As C++03 lacks variadic templates, example 3-7 will fail. Furthermore, there is a type mismatch with examples 6 and 7. While task_t specifies its function type as std::string(std::string&), the first and only argument is being bound to the functor during boost::bind(). As the resulting functor expects no more arguments, the function type provided to packaged_task should be std::string().

While packaged_task does not support arguments in C++03, one intermediate solution is to create a functor type that wraps the lower-level boost::promise. Without the support for variadic templates and perfect forwarding, there will be much boilerplate code for operator() overloads. Nevertheless, here is a basic example functor that ignores the exception handling between promise and future:

/// @brief basic_task to support function types with arguments.  This
///        provides a minimal feature workaround to Boost.Thread's
///        packaged_task not supporting argument types for C++03.
template <typename Fn>
class basic_task
{
public:
  // @brief The type the future will return.
  typedef typename boost::function_types::result_type<Fn>::type result_type;

  typedef boost::promise<result_type> promise_type;

  /// @brief Constructor.
  template <typename F> 
  explicit basic_task(const F& f)
    : fn_(f),
      promise_(boost::make_shared<promise_type>())
  {}

  // Overload operator() functions.

  void operator()()
  {
    promise_->set_value(fn_());
  }

  template <typename A1>
  void operator()(const A1& a1)
  {
    promise_->set_value(fn_(a1));
  }

  template <typename A1>
  void operator()(A1& a1)
  {
    promise_->set_value(fn_(a1));
  }

  /// @brief Get a future for this task' promise.
  boost::unique_future<result_type>
  get_future()
  {
    return promise_->get_future();
  }

private:
  boost::function<Fn> fn_;
  boost::shared_ptr<promise_type> promise_;
};

---

The complete series of examples:

#include <iostream>
#include <string>

#define BOOST_THREAD_PROVIDES_SIGNATURE_PACKAGED_TASK

#include <boost/asio.hpp>
#include <boost/function_types/result_type.hpp>
#include <boost/make_shared.hpp>
#include <boost/thread.hpp>

/// @brief basic_task to support function types with arguments.  This
///        provides a minimal feature workaround to Boost.Thread's
///        packaged_task not supporting argument types for C++03.
template <typename Fn>
class basic_task
{
public:
  // @brief The type the future will return.
  typedef typename boost::function_types::result_type<Fn>::type result_type;

  typedef boost::promise<result_type> promise_type;

  /// @brief Constructor.
  template <typename F> 
  explicit basic_task(const F& f)
    : fn_(f),
      promise_(boost::make_shared<promise_type>())
  {}

  // Overload operator() functions.

  void operator()()
  {
    promise_->set_value(fn_());
  }

  template <typename A1>
  void operator()(const A1& a1)
  {
    promise_->set_value(fn_(a1));
  }

  template <typename A1>
  void operator()(A1& a1)
  {
    promise_->set_value(fn_(a1));
  }

  /// @brief Get a future for this task' promise.
  boost::unique_future<result_type>
  get_future()
  {
    return promise_->get_future();
  }

private:
  boost::function<Fn> fn_;
  boost::shared_ptr<promise_type> promise_;
};

std::string ans("forty two");

int int_no_params()
{
  return 42;
}

int int_with_params(int param)
{
  return param;
}

std::string string_no_params()
{
  return std::string("forty two");
}

std::string string_with_params(std::string & param)
{
  return param;
}

int main()
{
  // example 1
  {
    boost::packaged_task<int()> example(&int_no_params);
    boost::unique_future<int> f = example.get_future();
    boost::thread task(boost::move(example));
    int answer = f.get();
    std::cout << "Answer to life and whatnot, in English: "
              << answer << std::endl;
    task.join();
  }

  // example 2
  {
    boost::packaged_task<std::string()> example(&string_no_params);
    boost::unique_future<std::string> f = example.get_future();
    boost::thread task(boost::move(example));
    std::string answer = f.get();
    std::cout << "string_no_params: " << answer << std::endl;
    task.join();
  }

  // example 3
  {
    basic_task<std::string(std::string&)> example(&string_with_params);
    boost::unique_future<std::string> f = example.get_future();
    example(ans);
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
  }

  // example 4
  {
    basic_task<std::string(std::string&)> example(&string_with_params);
    boost::unique_future<std::string> f = example.get_future();
    boost::thread task(boost::move(example), ans);
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
    task.join();
  }

  // example 5
  {
    basic_task<std::string(std::string&)>
        example(boost::bind(&string_with_params, ans));
    boost::unique_future<std::string> f = example.get_future();
    boost::thread task(boost::move(example), ans);
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
    task.join();
  }

  // example 6
  {
    typedef boost::packaged_task<std::string()> task_t;
    boost::shared_ptr<task_t> example =
        boost::make_shared<task_t>(boost::bind(&string_with_params, ans));
    boost::unique_future<std::string> f = example->get_future();
    boost::thread task(boost::bind(&task_t::operator(), example));
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
    task.join();
  }

  // example 7
  {
    boost::asio::io_service io_service;
    boost::thread_group threads;
    boost::asio::io_service::work work(io_service);

    for (int i = 0; i < 3; ++i)
      threads.create_thread(
          boost::bind(&boost::asio::io_service::run, &io_service));

    typedef boost::packaged_task<std::string()> task_t;
    boost::shared_ptr<task_t> example =
        boost::make_shared<task_t>(boost::bind(&string_with_params, ans));
    boost::unique_future<std::string> f = example->get_future();
    io_service.post(boost::bind(&task_t::operator(), example));
    std::string answer = f.get();
    std::cout << "string_with_params: " << answer << std::endl;
    io_service.stop();
    threads.join_all();
  }
}

And the resulting output:

Answer to life and whatnot, in English: 42
string_no_params: forty two
string_with_params: forty two
string_with_params: forty two
string_with_params: forty two
string_with_params: forty two
string_with_params: forty two