In addition, if a remove the const on operator()() struct mycallable1 { void operator()() { std::cout << "mycallable1" << std::endl; }; }; void my_thread() { std::cout << "my_thread" << std::endl; mycallable1 x; boost::this_thread::at_thread_exit(x); } I get the following error: gcc.compile.c++ ..\..\..\bin.v2\libs\interthreads\test\hello_world.test\gcc-4.3.2\debug\target-os-cygwin\threadapi-pthread\threading-multi\hello_world.o ../../../boost/thread/detail/thread.hpp: In member function `void boost::detail::thread_exit_function<F>::operator()() const [with F = mycallable1]': ..\example\hello_world.cpp:52: instantiated from here ../../../boost/thread/detail/thread.hpp:474: error: passing `const mycallable1' as `this' argument of `void mycallable1::operator()()' discards qualifiers Why the cleanup function must be const? Best, Vicente ______________________ Vicente Juan Botet Escribá