boost::bind的返回值类型是什么额

C/C++ code

boost::bind的返回值类型是什么额//这个函数内部用到了bindvoid CSyncTask::execute_at( LPTHREAD_START_ROUTINE lpStartAddress,LPVOID lpParameter,time_t t){    assert(lpStartAddress);    execute_at(boost::bind(&ThreadProc,lpStartAddress,lpParameter),t);            //look here}static void ThreadProc(LPTHREAD_START_ROUTINE lpStartAddress, LPVOID lpParameter)    //look here{    assert(lpStartAddress);    lpStartAddress(lpParameter);}boost::bind(&ThreadProc,lpStartAddress,lpParameter)的返回值类型为：function<void(LPTHREAD_START_ROUTINE lpStartAddress, LPVOID lpParameter)>;但是成员函数execute_at的类型确实：void execute_at( TaskFunction fun,time_t t);     //typedef boost::function<void()> TaskFunction;这是怎么回事