原子类型非线程安全

#include <iostream>
#include <thread>int main() {int num = 0;int count = 100000;std::thread thread1([&](){for(int i = 0; i < count; i++){num++;}});std::thread thread2([&](){for(int i = 0; i < count; i++){num++;}});std::thread thread3([&](){for(int i = 0; i < count; i++){num++;}});std::thread thread4([&](){for(int i = 0; i < count; i++){num++;}});thread1.join();thread2.join();thread3.join();thread4.join();std::cout << "num: " << num << std::endl;return 0;
}

预期结果
400000

实际结果

线程安全代码

atomic

#include <iostream>
#include <thread>
#include <atomic>int main() {std::atomic<int> num(0);int count = 100000;std::thread thread1([&](){for(int i = 0; i < count; i++){num++;}});std::thread thread2([&](){for(int i = 0; i < count; i++){num++;}});std::thread thread3([&](){for(int i = 0; i < count; i++){num++;}});std::thread thread4([&](){for(int i = 0; i < count; i++){num++;}});thread1.join();thread2.join();thread3.join();thread4.join();std::cout << "num: " << num << std::endl;return 0;
}

std::mutex

#include <iostream>
#include <thread>
#include <mutex>class Num{
public:Num(){num = 0;}void add(){std::lock_guard<std::mutex> lock(mtx);num++;}int getNum(){std::lock_guard<std::mutex> lock(mtx);return num;}
private:int num;std::mutex mtx;
};int main() {Num num;int count = 100000;std::thread thread1([&](){for(int i = 0; i < count; i++){num.add();}});std::thread thread2([&](){for(int i = 0; i < count; i++){num.add();}});std::thread thread3([&](){for(int i = 0; i < count; i++){num.add();}});std::thread thread4([&](){for(int i = 0; i < count; i++){num.add();}});thread1.join();thread2.join();thread3.join();thread4.join();std::cout << "num: " << num.getNum() << std::endl;return 0;
}