Creating Object in a thread safety way

Question

Directly from this web site, I came across the following description about creating object thread safety.

Warning: When constructing an object that will b

Answer 1

As the thread scheduler can stop execution of a thread at any time (even half-way through a high level instruction like instances.push_back(this)) and switch to executing a different thread, unexpected behaviour can happen if you don't synchronize parallel access to objects.

Look at the code below:

#include 
#include 
#include 
#include 

struct A {
    std::vector instances;
    A() { instances.push_back(this); }
    void printSize() { std::cout << instances.size() << std::endl; }
};

int main() {
    std::unique_ptr a; // Initialized to nullptr.

    std::thread t1([&a] { a.reset(new A()); }); // Construct new A.
    std::thread t2([&a] { a->printSize(); }); // Use A. This will fail if t1 don't happen to finish before.

    t1.join();
    t2.join();
}

As the access to a in main()-function is not synchronized execution will fail every once in a while.

This happens when execution of thread t1 is halted before finishing construction of the object A and thread t2 is executed instead. This results in thread t2 trying to access a unique_ptr containing a nullptr.