Re: [boost] RFC: Multithreading design constraints

From: jsiek_at_[hidden]
Date: 2000-06-05 18:02:39

Oops, the lock() function doesn't really solve the unnamed
construction problem (a g++ error made me think it did).
However, the for loop provides that same protection for this
as the if statement. So it might as well be written as:

for (guard g(m); g; g.release()) {
  // critical section
}

jsiek_at_[hidden] writes:
 > 
 > My main complaint was the complexity of the atomic traits... but
 > if that is separate from the mutex, and if it can be made a bit
 > more generic, then I'll be OK with it.
 > 
 > As for the syntax of lock/unlock... well while we're being
 > cute, how about the following abuse of a for loop?
 > 
 > for (guard g(lock(m)); g; g.release()) {
 >   // critical section
 > }
 > 
 > This is pretty close to what John proposed... I've just added this
 > lock() function which solves the accidental default construction
 > problem, and used the "for" loop to create a convenient place to put
 > the g.release().
 > 
 > 
 > #include <iostream>
 > #include <boost/utility.hpp>
 > 
 > using namespace std;
 > 
 > struct mutex { };
 > 
 > namespace detail {
 >   struct guard_initializer { };
 > }
 > 
 > detail::guard_initializer lock(mutex) { 
 >   cout << "lock" << endl;
 >   return detail::guard_initializer(); 
 > }
 > 
 > struct guard : boost::noncopyable {
 >   guard(detail::guard_initializer) : _locked(true) { }
 >   operator bool() { return _locked; }
 >   void release() {
 >     cout << "unlock via release" << endl;
 >     _locked = false; 
 >   }
 >   ~guard() {
 >     if (_locked)
 >       cout << "unlock via destructor" << endl;
 >   }
 >   bool _locked;
 > };
 > 
 > mutex m;
 > 
 > int
 > main(int,char*[])
 > {
 >   try {
 >     cout << "before the critical section" << endl;
 >     for (guard g(lock(m)); g; g.release()) {
 >       cout << "in a critical section" << endl;
 >     }
 >     cout << "after first critical section" << endl;
 >     for (guard g(lock(m)); g; g.release()) {
 >       cout << "in another critical section" << endl;
 >       throw "foo";
 >     }
 >     cout << "after second critical section" << endl;
 >   } catch (...) {
 >     cout << "exception caught" << endl;
 >   }
 > 
 >   return 0;
 > }
 > 
 > 
 > This output is:
 > 
 > before the critical section
 > lock
 > in a critical section
 > unlock via release
 > after first critical section
 > lock
 > in another critical section
 > unlock via destructor
 > exception caught
 > 
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 > 
 >