C++ Cookbook

12.5. Passing an Argument to a Thread Function

Problem

You have to pass an argument to your thread function, but the thread creation facilities in the Boost Threads library only accept functors that take no arguments.

Solution

Create a functor adapter that takes your parameters and returns a functor that takes no parameters. You can use the functor adapter where you would have otherwise put the thread functor. Take a look at Example 12-6 to see how this is done.

Example 12-6. Passing an argument to a thread function

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

// A typedef to make the declarations below easier to read
typedef void (*WorkerFunPtr)(const std::string&);

template<typename FunT,   // The type of the function being called
         typename ParamT> // The type of its parameter
struct Adapter {
   Adapter(FunT f, ParamT& p) : // Construct this adapter and set the
      f_(f), p_(&p) {}          // members to the function and its arg

   void operator()() { // This just calls the function with its arg
      f_(*p_);         
   }
private:
   FunT    f_;
   ParamT* p_;  // Use the parameter's address to avoid extra copying
};

void worker(const std::string& s) {
   std::cout << s << '\n';
}

int main() {

   std::string s1 = "This is the first thread!";
   std::string s2 = "This is the second thread!";

   boost::thread thr1(Adapter<WorkerFunPtr, std::string>(worker, s1));
   boost::thread thr2(Adapter<WorkerFunPtr, std::string>(worker, s2));

   thr1.join();
   thr2.join();
}

Discussion

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