lst-0021-book.cpp
// https://godbolt.org/z/7GEs9a4n6
#include <thread>
#include <iostream>
int count = 0; // is simultaneously modified
void run() {
for(int i=0; i<1'000'000; ++i) {
count += 1; // unprotected
}
}
int main() {
std::cout << "Start: " << count << '\n'; // Output: Start: 0
std::thread th1{ run };
std::thread th2{ run };
std::thread th3{ run };
th1.join(); th2.join(); th3.join();
std::cout << "End: " << count << '\n'; // Output certainly not: 3000000
}