Do I need to have a static lock for a static member variable of a class in C ++

Question

I have a static map as a member variable of my class. Do we need a static lock when we need to access this card?

Answer 1

If your instance is std::mapdeclared static, then your lock should also be static.

Consider two threads working with individual objects using a map when the lock is a non-stationary member, but a map.

• Object 1 blocks the local lock and starts managing the shared card.
• Object 2 locks its local lock (this is a separate lock, remember) and begins to manage the shared card.
• Boom//

static, , , .

, , static, , .

Answer 2

mutex , . , , .

static:

• class static (, , ):

  class X {
    static map<A,B> mMap;
  };
  

  . , , . , .

• local static:

   class X {
     void foo() {
       static map<A,B> theMap{ /* ... */ };
     }
   };
  

  , , foo . . . foo, static global. foo. :

   class X {
     static mutex mapMutex;
  
     static map<A,B>& getMap() {
       static map<A,B> theMap{ /* ... */ };
       return theMap;
     }
  
     void useMap() {
       lock myLock(mutex);
       getMap()[a] = b;
     }
   };
  

• ( " " )

   static map<A,B> gMap;
  
   class X { /* ... */ };
  

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Answer 3

, , smth static (, <). , static .

, , , , - :

class A {
    private:
        static std::map....

    public:
        void doSomethingWithMap() {
            //lock mutex

            //some action with map

            //unlock mutex
        }
}

If this happens, you might want your mutex to be a member of this class. This is the point - it depends on the volume that you need. If you need to lock globally on all objects of this class, you should consider using a static member of the class, if you want to have a lock for each object - you should set only a member of the class.

Answer 4

No, usually you do not need a static lock.