[c++11][NOTE] thread

std::thread::hardware_concurrency();
std::lock();
std::recursive_mutex;

Rules to follow:

OPTION 1: PASS IN A REFERENCE
OPTION 2: REQUIRE A NO-THROW COPY CONSTRUCTOR OR MOVE CONSTRUCTOR
OPTION 3: RETURN A POINTER TO THE POPPED ITEM
OPTION 4: PROVIDE BOTH OPTIONS 1 AND EITHER OPTION 2 OR 3.

class X
{
private:
some_big_object some_detail;
mutable std::mutex m;
public:
X(some_big_object const& sd):some_detail(sd){}
friend bool operator<(X const& lhs, X const& rhs)
{
if(&lhs==&rhs)
return false;
std::lock(lhs.m,rhs.m); #1
std::lock_guard<std::mutex> lock_a(lhs.m,std::adopt_lock); #2
std::lock_guard<std::mutex> lock_b(rhs.m,std::adopt_lock); #3
return lhs.some_detail<rhs.some_detail;
}
};

Guidelines for Avoiding Deadlock:

• 
  AVOID NESTED LOCKS
• 
  AVOID CALLING USER-SUPPLIED CODE WHILST HOLDING A LOCK
• 
  ACQUIRE LOCKS IN A FIXED ORDER
• 
  USE A LOCK HIERARCHY
• 
  EXTENDING THESE GUIDELINES BEYOND LOCKS