Let's try a more complex example involving an HPX component. An HPX component is a class which exposes HPX actions. HPX components are compiled into dynamically loaded modules called component libraries. Here's the source code:

hello_world_component.cpp

#include "hello_world_component.hpp"
#include <hpx/include/iostreams.hpp>

namespace examples { namespace server
{

void hello_world::invoke()
{
    hpx::cout << "Hello HPX World!\n" << hpx::flush;
}

}}

HPX_REGISTER_COMPONENT_MODULE();

typedef hpx::components::managed_component<
    examples::server::hello_world
> hello_world_type;

HPX_REGISTER_MINIMAL_COMPONENT_FACTORY(hello_world_type, hello_world);

HPX_DEFINE_GET_COMPONENT_TYPE(hello_world_type::wrapped_type);

HPX_REGISTER_ACTION_EX(
    examples::server::hello_world::invoke_action, hello_world_invoke_action);

hello_world_component.hpp

#if !defined(HELLO_WORLD_COMPONENT_HPP)
#define HELLO_WORLD_COMPONENT_HPP

#include <hpx/hpx_fwd.hpp>
#include <hpx/include/actions.hpp>
#include <hpx/include/lcos.hpp>
#include <hpx/include/components.hpp>
#include <hpx/include/serialization.hpp>

namespace examples { namespace server
{

struct HPX_COMPONENT_EXPORT hello_world
  : hpx::components::managed_component_base<hello_world>
{
    void invoke();

    typedef hpx::actions::action0<
        hello_world, 0, &hello_world::invoke
    > invoke_action;
};

}

namespace stubs
{

struct hello_world : hpx::components::stub_base<server::hello_world>
{
    static void invoke(hpx::naming::id_type const& gid)
    {
        hpx::lcos::async<server::hello_world::invoke_action>(gid).get();
    }
};

}

struct hello_world
  : hpx::components::client_base<hello_world, stubs::hello_world>
{
    typedef hpx::components::client_base<hello_world, stubs::hello_world>
        base_type;

    void invoke()
    {
        this->base_type::invoke(this->gid_);
    }
};

}

HPX_REGISTER_ACTION_DECLARATION_EX(
    examples::server::hello_world::invoke_action, hello_world_invoke_action);

#endif // HELLO_WORLD_COMPONENT_HPP

hello_world_client.cpp

#include "hello_world_component.hpp"
#include <hpx/hpx_init.hpp>

int hpx_main(boost::program_options::variables_map&)
{
    {
        examples::hello_world client;

        // Create a single instance of the component on this locality.
        client.create(hpx::find_here());

        // Invoke the components action, which will print "Hello World!".
        client.invoke();
    }

    return hpx::finalize(); // Initiate shutdown of the runtime system.
}

int main(int argc, char* argv[])
{
    // Configure application-specific options.
    boost::program_options::options_description desc_commandline(
        "usage: " HPX_APPLICATION_STRING " [options]");

    return hpx::init(desc_commandline, argc, argv); // Initialize and run HPX.
}

Copy the three source files above into three files (called hello_world_component.cpp, hello_world_component.hpp and hello_world_client.cpp respectively).

Now, in the directory where you put the files, run the following command to build the component library. (where $HPX_LOCATION is the CMAKE_INSTALL_PREFIX you used while building HPX):

$ export PKG_CONFIG_PATH=$PKG_CONFIG_PATH:$HPX_LOCATION/lib/pkgconfig
$ c++ -o libhpx_component_hello_world.so hello_world_component.cpp `pkg-config --cflags --libs hpx_component` -DHPX_COMPONENT_NAME=hello_world
$ mv libhpx_component_hello_world.so $HPX_LOCATION/lib/hpx/libhpx_component_hello_world.so

Now, to build the application that uses this component (hello_world_client.cpp), we do:

$ export PKG_CONFIG_PATH=$PKG_CONFIG_PATH:$HPX_LOCATION/lib/pkgconfig
$ c++ -o hello_world_client hello_world_client.cpp `pkg-config --cflags --libs hpx_application` -lhpx_component_iostreams -lhpx_component_hello_world

	Important
	When using pkg-config with HPX, the pkg-config flags must go after the -o flag.

To test the program, type:

$ ./hello_world_client

which should print Hello HPX World! and exit.