use of MultiThread with boost in VC++

Question

I am new to boost, C++ and threading, I am trying to use thread using boost library, but I am not able to access function through objects.

Here is what I am doing:

#include <iostream>
#include <boost/thread.hpp>
using namespace std;
class myclass
{
    int a,b;
    public:
    void print_a();
    void print_b();
};

void myclass::print_a()
{
 for(int i=1;i<1000000;++i) cout<<"printing a"<<endl;
}
void myclass::print_b()
{
    for(int i=1;i<1000000;++i) cout<<"printing b"<<endl;
}

void print_x()
{
    for(int i=1;i<1000000;++i) cout<<"printing x"<<endl;
}

void print_y()
{
    for(int i=1;i<1000000;++i) cout<<"printing y"<<endl;
}

int main()
{
    myclass obj;
    boost::thread thread_a(obj.print_a);
    boost::thread thread_b(obj.print_b);
    boost::thread thread_x(print_x);
    boost::thread thread_y(print_y);
    thread_a.join();
    thread_b.join();
    thread_x.join();
    thread_y.join();
    return 0;
}

Error when calling boost::thread thread_a(obj.print_a) is a pointer to a bound function may only be called to use this function and same with thread_b but for thread_x and thread_y it is working properly. What am I missing, please guide me

Thank You Regards

Answer 1

Unless there is a new syntax I don't know about, you cannot pass obj.print_a as a "runnable" you have to do

boost::bind( &myclass::print_a, &obj ); 

or std::bind if your version supports it.

As it happens boost::thread has a multi-parameter version that does the bind for you. Thus when you create the thread

boost::thread thread_a( &myclass::print_a, &obj );

In C++11 you can also use lambdas

[&obj]{ obj.print_a(); } 

thus in one line

boost::thread thread_a( [&obj]{ obj.print_a(); } );

or if that looks too messy assign the lambda (you can use auto of course as its type) and pass that in a separate line.