原子类型

Created: November-22, 2018

std::atomic 模板的每个实例化和完全特化都定义了原子类型。如果一个线程写入原子对象而另一个线程从中读取，则行为是明确定义的（有关数据争用的详细信息，请参阅内存模型）

此外，对原子对象的访问可以建立线程间同步并按照 std::memory_order 的规定来命令非原子存储器访问。

std::atomic 可以用任何 TriviallyCopyable type T. std::atomic 实例化，既不可复制也不可移动。

标准库为以下类型提供 std::atomic 模板的特化：

定义了 bool 及其 typedef 类型的一个完整特化，它被视为非专用 std::atomic<T>，除了它具有标准布局，普通默认构造函数，普通析构函数，并支持聚合初始化语法：

Typedef 名称	完全专业化
`std::atomic_bool`	`std::atomic<bool>`

2）完整类型的完全特化和 typedef，如下所示：

Typedef 名称	完全专业化
`std::atomic_char`	`std::atomic<char>`
`std::atomic_char`	`std::atomic<char>`
`std::atomic_schar`	`std::atomic<signed char>`
`std::atomic_uchar`	`std::atomic<unsigned char>`
`std::atomic_short`	`std::atomic<short>`
`std::atomic_ushort`	`std::atomic<unsigned short>`
`std::atomic_int`	`std::atomic<int>`
`std::atomic_uint`	`std::atomic<unsigned int>`
`std::atomic_long`	`std::atomic<long>`
`std::atomic_ulong`	`std::atomic<unsigned long>`
`std::atomic_llong`	`std::atomic<long long>`
`std::atomic_ullong`	`std::atomic<unsigned long long>`
`std::atomic_char16_t`	`std::atomic<char16_t>`
`std::atomic_char32_t`	`std::atomic<char32_t>`
`std::atomic_wchar_t`	`std::atomic<wchar_t>`
`std::atomic_int8_t`	`std::atomic<std::int8_t>`
`std::atomic_uint8_t`	`std::atomic<std::uint8_t>`
`std::atomic_int16_t`	`std::atomic<std::int16_t>`
`std::atomic_uint16_t`	`std::atomic<std::uint16_t>`
`std::atomic_int32_t`	`std::atomic<std::int32_t>`
`std::atomic_uint32_t`	`std::atomic<std::uint32_t>`
`std::atomic_int64_t`	`std::atomic<std::int64_t>`
`std::atomic_uint64_t`	`std::atomic<std::uint64_t>`
`std::atomic_int_least8_t`	`std::atomic<std::int_least8_t>`
`std::atomic_uint_least8_t`	`std::atomic<std::uint_least8_t>`
`std::atomic_int_least16_t`	`std::atomic<std::int_least16_t>`
`std::atomic_uint_least16_t`	`std::atomic<std::uint_least16_t>`
`std::atomic_int_least32_t`	`std::atomic<std::int_least32_t>`
`std::atomic_uint_least32_t`	`std::atomic<std::uint_least32_t>`
`std::atomic_int_least64_t`	`std::atomic<std::int_least64_t>`
`std::atomic_uint_least64_t`	`std::atomic<std::uint_least64_t>`
`std::atomic_int_fast8_t`	`std::atomic<std::int_fast8_t>`
`std::atomic_uint_fast8_t`	`std::atomic<std::uint_fast8_t>`
`std::atomic_int_fast16_t`	`std::atomic<std::int_fast16_t>`
`std::atomic_uint_fast16_t`	`std::atomic<std::uint_fast16_t>`
`std::atomic_int_fast32_t`	`std::atomic<std::int_fast32_t>`
`std::atomic_uint_fast32_t`	`std::atomic<std::uint_fast32_t>`
`std::atomic_int_fast64_t`	`std::atomic<std::int_fast64_t>`
`std::atomic_uint_fast64_t`	`std::atomic<std::uint_fast64_t>`
`std::atomic_intptr_t`	`std::atomic<std::intptr_t>`
`std::atomic_uintptr_t`	`std::atomic<std::uintptr_t>`
`std::atomic_size_t`	`std::atomic<std::size_t>`
`std::atomic_ptrdiff_t`	`std::atomic<std::ptrdiff_t>`
`std::atomic_intmax_t`	`std::atomic<std::intmax_t>`
`std::atomic_uintmax_t`	`std::atomic<std::uintmax_t>`

使用 std::atomic_int 的简单示例

#include <iostream>       // std::cout
#include <atomic>         // std::atomic, std::memory_order_relaxed
#include <thread>         // std::thread

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 main ()
{
  std::thread first (print_foo);
  std::thread second (set_foo,10);
  first.join();
  //second.join();
  return 0;
}
//output: foo: 10