The classical way to protect access to a variable is using a mutex.

Mutex

Protect shared data or shared resources with a mutex

In this example, std::cout is a shared resource that is shared by 6 threads (t1-t5 + main).

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
 
std::mutex mu;
 
void ShowMessage(std::string msg)
{
  std::cout << "Thread " << std::this_thread::get_id() << " says " << msg << std::endl;
}
 
int main()
{
  std::thread t1(ShowMessage, "Hello from Jupiter");
  std::thread t2(ShowMessage, "Hello from Saturn");
  std::thread t3(ShowMessage, "Hello from Mars");
  ShowMessage("Hello from Main/Earth");
  std::thread t4(ShowMessage, "Hello from Uranus");
  std::thread t5(ShowMessage, "Hello from Neptune");
  t1.join();
  t2.join();
  t3.join();
  t4.join();
  t5.join();
  return 0;
}

To make the output more deterministic, the solution is to protect the access to std::cout resource using a std::mutex.

Just change the ShowMessage() to acquire a mutex before using std::cout and release it after it is done.

void ShowMessage(std::string msg)
{
  mu.lock();
  std::cout << "Thread " << std::this_thread::get_id() << " says " << msg << std::endl;
  mu.unlock();
}