原子库为细粒度的原子操作提供组件，允许无锁并发编程。涉及同一对象的每个原子操作，相对于任何其他原子操作是不可分的。原子对象不具有数据竞争(data race)。原子类型对象的主要特点就是从不同线程访问不会导致数据竞争。因此从不同线程访问某个原子对象是良性(well-defined)行为，而通常对于非原子类型而言，并发访问某个对象(如果不做任何同步操作)会导致未定义(undifined)行为发生。
atomic是C++标准程序库中的一个头文件，定义了C++11标准中的一些表示线程、并发控制时原子操作的类与方法等。此头文件主要声明了两大类原子对象：std::atomic和std::atomic_flag，另外还声明了一套C风格的原子类型和与C兼容的原子操作的函数。在多线程并发执行时，原子操作是线程不会被打断的执行片段。一些程序设计更为注重性能和效率，需要开发lock-free的算法和数据结构，这就需要更为底层的原子操作与原子类型。原子类型对象的主要特点就是从不同线程并发访问是良性(well-defined)行为，不会导致竞争危害。与之相反，不做适当控制就并发访问非原子对象则会导致未定义(undifined)行为。
atomic_flag类：是一种简单的原子布尔类型，只支持两种操作：test_and_set(flag=true)和clear(flag=false)。跟std::atomic的其它所有特化类不同，它是锁无关的。结合std::atomic_flag::test_and_set()和std::atomic_flag::clear()，std::atomic_flag对象可以当作一个简单的自旋锁(spin lock)使用。atomic_flag只有默认构造函数，禁用拷贝构造函数，移动构造函数实际上也禁用。如果在初始化时没有明确使用宏ATOMIC_FLAG_INIT初始化，那么新创建的std::atomic_flag对象的状态是未指定的(unspecified)，既没有被set也没有被clear；如果使用该宏初始化，该std::atomic_flag对象在创建时处于clear状态。
(1)、test_and_set：返回该std::atomic_flag对象当前状态，检查flag是否被设置，若被设置直接返回true，若没有设置则设置flag为true后再返回false。该函数是原子的。
(2)、clear：清除std::atomic_flag对象的标志位，即设置atomic_flag的值为false。
std::atomic类模板：std::atomic比std::atomic_flag功能更加完善。C++11标准库std::atomic提供了针对bool类型、整形(integral)和指针类型的特化实现。每个std::atomic模板的实例化和完全特化定义一个原子类型。若一个线程写入原子对象，同时另一个线程从它读取，则行为良好定义。而且，对原子对象的访问可以按std::memory_order所指定建立线程间同步，并排序非原子的内存访问。std::atomic可以以任何可平凡复制(Trivially Copyable)的类型T实例化。std::atomic既不可复制亦不可移动。
除了std::atomic和std::atomic_flag外，<atomic>还包括了基于std::atomic_flag类的C风格API和基于std::atomic类模板的C风格API。
与原子对象初始化相关的两个宏：
(1)、ATOMIC_VAR_INIT(val)：初始化std::atomic对象。This macro expands to a token sequence suitable to initialize an atomic object (of static storage duration) with a value of val. This macro exists for compatibility with C implementations, in which it is used as a constructor-like function for(default-constructed) atomic objects; In C++, this initialization may be performed directly by the initialization constructor.
(2)、ATOMIC_FLAG_INIT：初始化std::atomic_flag对象。This macro is defined in such a way that it can be used to initialize an object of type atomic_flag to the clear state.
std::atomic：Objects of atomic types contain a value of a particular type (T). The main characteristic of atomic objects is that access to this contained value from different threads cannot cause data races (i.e., doing that is well-defined behavior, with accesses properly sequenced). Generally, for all other objects, the possibility of causing a data race for accessing the same object concurrently qualifies the operation as undefined behavior. Additionally, atomic objects have the ability to synchronize access to other non-atomic objects in their threads by specifying different memory orders.
std::atomic_flag:Atomic flags are boolean atomic objects that support two operations:test-and-set and clear.
下面是从其他文章中copy的<atomic>测试代码，详细内容介绍可以参考对应的reference：
#include "atomic.hpp"#include <iostream>#include <atomic>#include <vector>#include <thread>#include <sstream>namespace atomic {/////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/atomic/atomic/atomic/std::atomic<bool> ready(false);// atomic_flag::atomic_flag: Constructs an atomic_flag object// The atomic_flag is in an unspecified state on construction (either set or clear),// unless it is explicitly initialized to ATOMIC_FLAG_INIT.std::atomic_flag winner = ATOMIC_FLAG_INIT;void count1m(int id){ while (!ready) { std::this_thread::yield(); } // wait for the ready signal for (volatile int i = 0; i < 1000000; ++i) {} // go!, count to 1 million if (!winner.test_and_set()) { std::cout << "thread #" << id << " won!\n"; }};int test_atomic_atomic(){ // atomic::atomic: Constructs an atomic object std::vector<std::thread> threads; std::cout << "spawning 10 threads that count to 1 million...\n"; for (int i = 1; i <= 10; ++i) threads.push_back(std::thread(count1m, i)); ready = true; for (auto& th : threads) th.join(); return 0;}/////////////////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/atomic/atomic/compare_exchange_weak/// a simple global linked list:struct Node { int value; Node* next; };std::atomic<Node*> list_head(nullptr);void append(int val){ // append an element to the list Node* oldHead = list_head; Node* newNode = new Node{ val, oldHead }; // what follows is equivalent to: list_head = newNode, but in a thread-safe way: while (!list_head.compare_exchange_weak(oldHead, newNode)) newNode->next = oldHead;}int test_atomic_compare_exchange_weak(){ // atomic::compare_exchange_weak: Compares the contents of the atomic object's contained value with expected: // -if true, it replaces the contained value with val(like store). // - if false, it replaces expected with the contained value // spawn 10 threads to fill the linked list: std::vector<std::thread> threads; for (int i = 0; i<10; ++i) threads.push_back(std::thread(append, i)); for (auto& th : threads) th.join(); // print contents: for (Node* it = list_head; it != nullptr; it = it->next) std::cout << ' ' << it->value; std::cout << '\n'; // cleanup: Node* it; while (it = list_head) { list_head = it->next; delete it; } return 0;}///////////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/atomic/atomic/exchange/std::atomic<bool> winner_(false);void count1m_(int id){ while (!ready) {} // wait for the ready signal for (int i = 0; i<1000000; ++i) {} // go!, count to 1 million if (!winner_.exchange(true)) { std::cout << "thread #" << id << " won!\n"; }};int test_atomic_exchange(){ // atomic::exchange: Replaces the contained value by val and returns the value it had immediately before std::vector<std::thread> threads; std::cout << "spawning 10 threads that count to 1 million...\n"; for (int i = 1; i <= 10; ++i) threads.push_back(std::thread(count1m_, i)); ready = true; for (auto& th : threads) th.join(); return 0;}/////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/atomic/atomic/load/std::atomic<int> foo(0);void set_foo(int x){ foo.store(x, std::memory_order_relaxed); // set value atomically}void print_foo(){ int x; do { x = foo.load(std::memory_order_relaxed); // get value atomically } while (x == 0); std::cout << "foo: " << x << '\n';}int test_atomic_load(){ // atomic::load: Returns the contained value. // The operation is atomic and follows the memory ordering specified by sync. std::thread first(print_foo); std::thread second(set_foo, 10); first.join(); second.join(); return 0;}////////////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/atomic/atomic/operator=/std::atomic<int> foo_ = 0;void set_foo_(int x){ foo_ = x;}void print_foo_(){ while (foo_ == 0) { // wait while foo_=0 std::this_thread::yield(); } std::cout << "foo_: " << foo_ << '\n';}int test_atomic_operator(){ // atomic::operator=: Replaces the stored value by val. // This operation is atomic and uses sequential consistency (memory_order_seq_cst). // To modify the value with a different memory ordering std::thread first(print_foo_); std::thread second(set_foo_, 10); first.join(); second.join(); return 0;}///////////////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/atomic/atomic/store/int test_atomic_store(){ // atomic::store: Replaces the contained value with val. // The operation is atomic and follows the memory ordering specified by sync. std::thread first(print_foo); std::thread second(set_foo, 10); first.join(); second.join(); return 0;}/////////////////////////////////////////////////////////////////////// reference: http://www.cplusplus.com/reference/atomic/atomic_flag/clear/std::atomic_flag lock_stream = ATOMIC_FLAG_INIT;std::stringstream stream;void append_number(int x){ while (lock_stream.test_and_set()) {} stream << "thread #" << x << '\n'; lock_stream.clear();}int test_atomic_flag_atomic_clear(){ // atomic_flag::clear: Clears the atomic_flag (i.e., sets it to false) //Clearing the atomic_flag makes the next call to member atomic_flag::test_and_set on this object return false. // The operation is atomic and follows the memory ordering specified by sync. // atomic_flag::test_and_set: Sets the atomic_flag and returns whether it was already set immediately before the call // The entire operation is atomic (an atomic read-modify-write operation): the value is not affected by other threads // between the instant its value is read (to be returned) and the moment it is modified by this function. std::vector<std::thread> threads; for (int i = 1; i <= 10; ++i) threads.push_back(std::thread(append_number, i)); for (auto& th : threads) th.join(); std::cout << stream.str(); return 0;}} // namespace atomicGitHub： https://github.com/fengbingchun/Messy_Test
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