#include <thread>
#include <atomic>
#include <iostream>
#include <vector>

struct AtomicCounter {
    std::atomic<int> value;

    AtomicCounter() : value(0) {}

    void increment(){
        ++value;
    }

    void decrement(){
        --value;
    }

    int get(){
        return value.load();
    }
};

int main(){
    AtomicCounter counter;

    std::vector<std::thread> threads;
    for(int i = 0; i < 10; ++i){
        threads.push_back(std::thread([&counter](){
            for(int i = 0; i < 500; ++i){
                counter.increment();
            }
        }));
    }

    for(auto& thread : threads){
        thread.join();
    }

    std::cout << counter.get() << std::endl;

    return 0;
}

#include <thread>
#include <atomic>
#include <iostream>
#include <vector>

struct AtomicCounter {
    std::atomic<int> value;

    void increment(){
        ++value;
    }

    void decrement(){
        --value;
    }

    int get(){
        return value.load();
    }
};

int main(){
    AtomicCounter counter;

    std::vector<std::thread> threads;
    for(int i = 0; i < 10; ++i){
        threads.push_back(std::thread([&counter](){
            for(int i = 0; i < 500; ++i){
                counter.increment();
            }
        }));
    }

    for(auto& thread : threads){
        thread.join();
    }

    std::cout << counter.get() << std::endl;

    return 0;
}

--- 
+++ 
@@ -5,6 +5,8 @@
 
 struct AtomicCounter {
     std::atomic<int> value;
+
+    AtomicCounter() : value(0) {}
 
     void increment(){
         ++value;
Make sure std::atomic is initialized correctly

G++ seems to have some issues with default initialization of
std::atomic
