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Design

The Boost.Threads Library


Important Update

For an update of the Boost.Threads library, see the article What's New in Boost Threads? by Anthony Williams, maintainer of the Boost.Threads Library. This update appears in the November 2008 issue of Dr. Dobb's Journal.


Just a few years ago it was uncommon for a program to be written with multiple threads of execution. Today Internet server applications run multiple threads of execution to efficiently handle multiple client connections. To maximize throughput, transaction servers execute services on separate threads. GUI applications perform lengthy operations in a separate thread to keep the user interface responsive. The list goes on.

The C++ Standard doesn’t mention threads, leaving programmers to wonder whether it’s even possible to write multithreaded C++ programs. Though it is not possible to write standards-compliant multithreaded programs, programmers none the less write multithreaded programs in C++ using the libraries provided by their OS that expose the system’s support for threads. However, there are at least two major problems with doing this: these libraries are almost universally C libraries and require careful use in C++, and each OS provides its own set of libraries for handling multithreaded support. Therefore, the resulting code is not only non-standard, but also non-portable [1]. Boost.Threads is a library designed to address both problems.

Boost [2] is an organization started by members of the C++ Standards Committee Library Working Group to develop new source libraries for C++. Its current membership includes approximately 2,000 members. Many libraries can be found in the Boost source distribution [3]. To make these libraries thread-safe, Boost.Threads was created.

Many C++ experts provided input to the design of Boost.Threads. The interface was designed from the ground up and is not just a simple wrapper around any C threading API. Many features of C++ (such as the existence of constructors/destructors, function objects, and templates) were fully utilized to make the interface more flexible. The current implementation works for POSIX, Win32, and Macintosh Carbon platforms.

Thread Creation

The boost::thread class represents a thread of execution in the same way the std::fstream class represents a file. The default constructor creates an instance representing the current thread of execution. An overloaded constructor takes a function object called with no arguments and returning nothing. This constructor starts a new thread of execution, which in turn calls the function object.

At first it appears that this design is less useful than the typical C approach to creating a thread where a void pointer can be passed to the routine called by the new thread, which allows data to be passed. However, because Boost.Threads uses a function object instead of just a function pointer, it is possible for the function object to carry data needed by the thread. This approach is actually more flexible and is type safe. When combined with functional libraries, such as Boost.Bind, this design actually allows you to easily pass any amount of data to the newly created thread.

Currently, not a lot can be done with a thread object created in Boost.Threads. In fact only two operations can be performed. Thread objects can easily be compared for equality or inequality using the == and != operators to verify if they refer to the same thread of execution, and you can wait for a thread to complete by calling boost::thread::join. Other threading libraries allow you to perform other operations with a thread (for example, set its priority or even cancel it). However, because these operations don’t easily map into portable interfaces, research is being done to determine how they can be added to Boost.Threads.

Listing One illustrates a very simple use of the boost::thread class. A new thread is created that simply writes “Hello World” out to std::cout, while the main thread waits for it to complete.

Listing One: The boost::thread class

#include <boost/thread/thread.hpp>
#include <iostream>

void hello()
{
  std::cout <<
    "Hello world, I'm a thread!"
    << std::endl;
}

int main(int argc, char* argv[])
{
  boost::thread thrd(&hello);
  thrd.join();
  return 0;
}

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