C++11 atomic
atomic是在多线程编程中经常使用的一个类。使用的时候需要包含
atomic
atomic内部封装一个值，并保证对该值的访问的互斥性。可以用来进行线程同步。使用非常方便。说白了就是atomic内部保存的值是线程安全的，不用担心多线程访问时的同步问题。也就是说保证对内部值的读取和保存的操作的原子性。
atomic是个模板类，可以保存的常用的值的类型（这些类型在atomic头文件中都有自己的定义）如下表：
| 类型 | atomic内部定义 | 描述 |
| bool | atomic_bool | |
| char | atomic_char | atomics for fundamental integral types. These are either typedefs of the corresponding full specialization of the atomic class template or a base class of such specialization. |
| signed char | atomic_schar | |
| unsigned char | atomic_uchar | |
| short | atomic_short | |
| unsigned short | atomic_ushort | |
| int | atomic_int | |
| unsigned int | atomic_uint | |
| long | atomic_long | |
| unsigned long | atomic_ulong | |
| long long | atomic_llong | |
| unsigned long long | atomic_ullong | |
| wchar_t | atomic_wchar_t | |
| char16_t | atomic_char16_t | |
| char32_t | atomic_char32_t | |
| intmax_t | atomic_intmax_t | atomics for width-based integrals (those defined in ). Each of these is either an alias of one of the above atomics for fundamental integral types or of a full specialization of the atomic class template with an extended integral type. Where N is one in 8, 16, 32, 64, or any other type width supported by the library. |
| uintmax_t | atomic_uintmax_t | |
| int_leastN_t | atomic_int_leastN_t | |
| uint_leastN_t | atomic_uint_leastN_t | |
| int_fastN_t | atomic_int_fastN_t | |
| uint_fastN_t | atomic_uint_fastN_t | |
| intptr_t | atomic_intptr_t | |
| uintptr_t | atomic_uintptr_t | |
| size_t | atomic_size_t | |
| ptrdiff_t | atomic_ptrdiff_t |
总的来说可以总结为，bool和int类(char也可以归为int)类型。
构造函数
| 构造函数 | |
| default (1) | atomic() noexcept = default; |
| initialization (2) | constexpr atomic (T val) noexcept; |
| copy [deleted] (3) | atomic (const atomic&) = delete; |
容易看出atomic对象是不可复制的。
=操作符
| =号操作符 | |
| set value (1) | T operator= (T val) noexcept; |
| T operator= (T val) volatile noexcept; | |
| copy [deleted] (2) | atomic& operator= (const atomic&) = delete; |
| atomic& operator= (const atomic&) volatile = delete; | |
由上表得知，atomic对象可以用其内部存储类型赋值，但是atomic本身是不能进赋值的。
atomic经常用的函数有store和load，如下：
store函数
| store函数 |
|---|
| void store (T val, memory_order sync = memory_order_seq_cst) volatile noexcept; |
| void store (T val, memory_order sync = memory_order_seq_cst) noexcept; |
其实就是用val替换内部保存的值。内部替换的时候其实是加了线程同步的，所以不用担心多线程访问的问题。
load函数
| load函数 |
|---|
| T load (memory_order sync = memory_order_seq_cst) const volatile noexcept; |
| T load (memory_order sync = memory_order_seq_cst) const noexcept; |
就是返回内部保存的值，其实就是访问内部值，没什么要特别说明的。