Boost: can someone explain how to create a thread for a class function?

[Swiftslide]

I'm just learning boost and multithreading in general. In my project I have a class called "HarbourMaster" (it is a boat simulation), which needs to create three threads of functions that belong to it. The functions are named "passTime()", "checkLoadingBays()" and "checkUnloadingBays()".

I have tried to create these threads in several ways. The first is following the syntax provided in my reference material:



boost::thread timeThread(&passTime);
boost::thread lBaysThread(&checkLoadingBays);
boost::thread uBaysThread(&checkUnloadingBays);

But the references seemingly don't apply for class functions; I get the following errors, once for each thread:


ISO C++ forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function. Say ‘&HarbourMaster::passTime’

ISO C++ forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function. Say ‘&HarbourMaster::checkLoadingBays’
ISO C++ forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function. Say ‘&HarbourMaster::checkUnloadingBays’


So despite all my past experiences with the C++ compiler I decide to follow its advice:


boost::thread timeThread(&HarbourMaster::passTime);
boost::thread lBaysThread(&HarbourMaster::checkLoadingBays);
boost::thread uBaysThread(&HarbourMaster::checkUnloadingBays);

Of course, I get another error - though only one this time:


In file included from /usr/include/boost/thread/thread.hpp:22,
/usr/include/boost/thread/detail/thread.hpp:56: error: must use ‘.*’ or ‘->*’ to call pointer-to-member function in ‘((boost::detail::thread_data<void (HarbourMaster::*)()>*)this)->boost::detail::thread_data<void (HarbourMaster::*)()>::f (...)’, e.g. ‘(... ->* ((boost::detail::thread_data<void (HarbourMaster::*)()>*)this)->boost::detail::thread_data<void (HarbourMaster::*)()>::f) (...)’

I tried understanding this, but failed miserably. So I started messing around trying random ways of writing the statement, looking at the error, and trying to figure out what random way to try next. I've been trying for forty-five minutes now and decided that it's rather pointless. Could anyone please tell me how I'm supposed to create a thread? I would love you forever, but in a totally non-invasive way.

Thanks

[dwhitney67]

Your thread class needs to be defined as a functor (function object).

For example:


class MyThread
{
public:
MyThread() {}

void operator()(void)
{
// do whatever here
}
};


Then in your main thread:


MyThread mt1;
MyThread mt2;

boost::thread* bt1 = new boost::thread(mt1);
boost::thread* bt2 = new boost::thread(mt2);

boost::thread_group threads;
threads.add_thread(bt1);
threads.add_thread(bt2);

threads.join_all();

Of course, there are other spins on boost threads; thus far I have only experimented with code similar to what I have shown above.

[dribeas]

The boost threading library is designed to be quite flexible when used in combination with boost::bind. I cannot really point what the best solution would be without more knowledge of your domain, but here are a couple of the things that you can do with boost::thread.

You can create a thread that will execute a free function or a static member function of a class by passing the pointer to function directly to boost::thread:



void foo();
struct bar {
static void foo();
void g();
};
int main() {
boost::thread thr1( &foo );
boost::thread thr2( &bar::foo );
// boost::thread thr3( &bar::g ); [1]
}


The reason that you cannot pass a non-static member function is that non-static member functions are applied to an object, and you are not giving such an object to the boost::thread (nor you can directly).

You can pass in a functor to the thread and the boost thread will execute it (as in dwhitney67) answer:



struct functor {
void operator()();
};
int main() {
boost::thread thr( functor() );
}


Or you can use boost::bind to create a functor that adapt interfaces for you if you have some code that you want to call already written:



struct test {
static void f( int );
void g();
};
void f( double );
int main() {
test t;
boost::thread thr1( boost::bind( &test::f, 5 ) ); // run test::f( 5 )
boost::thread thr2( boost::bind( &test::g, &t ) ); // run t.g()
boost::thread thr3( boost::bind( &f, 10.5 ); // run f( 10.5 )
}


The boost::bind library basically creates a function object with one signature and then forwards the call to the binded element by injecting the extra arguments you provided in the bind call.

[alatnet]

I've havent had any luck with boost::thread but i did find resources on this problem.
The problem is that when you are passing a "class" method to boost::thread(), it doesnt like this cus the class method is also passing "this" as an extra argument to the additional arguments.

What i mean is that say you have a class method foo:

void foo() {}
you see that method foo has no arguments. this isnt true for class methods because "this" is a hidden argument, so what you are really getting is:

void foo(class this) {}

To bypass this, create a static class method that accepts a "void *" argument and call that method within the static method:

void foo() {}
static void fooCaller(void *args){
class* c = (class*)args;
c->foo();
}

Then all you have to do is this:

class* c = new class();
boost::thread(class::fooCaller,(void*)c)

Please note that i dont know how to pass args to boost::thread so look up that and basicly do what i have above and it should work as expected.

[dwhitney67]

You do not need to pass the object, whether it be the class or another object, as a void pointer. Passing as itself would suffice; for example:


static void fooCaller(Class* c)
{
c->foo();
}

[alatnet]

You do not need to pass the object, whether it be the class or another object, as a void pointer. Passing as itself would suffice; for example:


static void fooCaller(Class* c)
{
c->foo();
}

I guess... But isn't the function prototype in the form of "void (prototype*)(void *)" that has to be passed as the first argument to boost::thread()?

[dwhitney67]

I guess... But isn't the function prototype in the form of "void (prototype*)(void *)" that has to be passed as the first argument to boost::thread()?

The boost::thread constructor documentation can be referenced here:
http://www.boost.org/doc/libs/1_38_0/doc/html/thread/thread_management.html#thread.thread_management.th read

Note the third constructor, that accepts a templated typename object (ie. class F), and additional args (ie. class A1, etc).

[dribeas]

To bypass this, create a static class method that accepts a "void *" argument and call that method within the static method:

void foo() {}
static void fooCaller(void *args){
class* c = (class*)args;
c->foo();
}

Then all you have to do is this:

class* c = new class();
boost::thread(class::fooCaller,(void*)c)

Please note that i dont know how to pass args to boost::thread so look up that and basicly do what i have above and it should work as expected.

Or much simpler:


type obj;
boost::thread thr( boost::bind( &type::method, &obj ) );

No need to create the dispatcher static method manually.

[alatnet]

The boost::thread constructor documentation can be referenced here:
http://www.boost.org/doc/libs/1_38_0/doc/html/thread/thread_management.html#thread.thread_management.th read

Note the third constructor, that accepts a templated typename object (ie. class F), and additional args (ie. class A1, etc).
Ah. ok.