Modify data inside thread - c++

Question

I am writing unit tests for one of the classes I want to test. Function infFooLoop() what I want to test is executed in endless loop (request to stop it comes externally). My problem is I want to change the private variable state_ via setter in setState when function executes in separate thread asynchronously. Some simplified code example is here:
enum class State : int
{
Success = 1,
Failure = 2
};
class Foo
{
private:
State state_{State::Success};
bool stop_flag_{false};
public:
void setState(State state) { state_ = state; }
void infFooLoop(bool &start)
{
while (start)
{
std::cout << "While loop executes \n";
if (state_ == State::Failure)
{
stop_flag_ = true;
}
else if (stop_flag_)
{
std::cout << "Program stopped\n";
// Report error but for this example break
break;
}
std::this_thread::sleep_for(std::chrono::milliseconds(200));
}
}
};
int main()
{
Foo obj;
bool flag{true};
std::future<void> future = std::async(std::launch::async, [&](){ obj.infFooLoop(flag);});
// Try to change the data:
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
// I want to change data for `Failure` so stop_flag condition will be used
obj.setState(State::Failure);
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
// terminate loop
flag = false;
return 0;
}
state_ will be taken from external component and stop_flag_ is used to handle some corner-case here (in real it wont break the loop, just report error to different component).
I assume infFooLoop executes on a separate thread so I can't just call another function in a different thread to change this data. stop_flag_ is some internal variable used only in this one function so I want to leave it as simple as possible (do not introduce mutexes/atomic in Foo class only for tests). Can you give me some suggestions on what should I do here?