How can I have non-static thread-local variable for each instance

Question

The problem itself:

class B{/*...*/};
class A {
    /* members */
    NON-static thread_local B var; // and a thread_local variable here
            


        

Answer 1

If you're willing to use tbb (which is free even though by Intel), you could use their tbb::enumerable_thread_specific template class (which essentially is something like std::unordered_map but lock free, I understand). Since the A are shared between threads, one such container per instance of A is required, but it appears B is better declared as a nested type. For example

class A
{
  struct B
  {
    B(const A*);
    void call(/* args */);
  };
  tbb::enumerable_thread_specific _B ([&]()->B { return {this}; } );
  void method(/* args */)
  {
    _B.local().call(/* args */);   // lazily creates thread local B if required.
  }
  /* ... */
};