Multithread and singleton

Question

is c++ have more usage of multiple threads ? can anyone show it on solo learn to check it !?
And I heard singleton is no more maintain one object in case of more threads... can anyone guide how to achieve in case of multiple thread as well ?

my code for singleton as below :

https://code.sololearn.com/cV96i4sHz99T/?ref=app

Answer 1

Protect it from data races.
Just imagine that 2 thread will increase your counter in the same time. One will rewrite result of other and counter will store wrong value.
You can use std::mutex as simple case.

Answer 2

New standard guarantees that static variable is initialized once in multy thread environment.
There was data race at static variable initialization with old compilers.
All other things work in a same order.
Unfortunately, your code provides multiple data races in multy thread case. It means that you can't use your singleton with many threads.

Answer 3

Sergey Ushakov could you please let me know what to be done to use this code for many threads as singleton?