本文主要是针对C++中多线程并发操作参见(cplusplus)进行解释，文章从下面几个方面进行学习，分别介绍多线程中会使用到的几个文件学习。 文中代码 可编译运行版本已上传在本人github(地址)
多线程
C++ 中关于并发多线程的部分，主要包含 <thread>、<mutex>、<atomic>、<condition_varible>、<future>五个部分。
- <atomic>：该头文主要声明了两个类, 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。
- <thread>：该头文件主要声明了 std::thread 类，另外 std::this_thread 命名空间也在该头文件中。
- <mutex>：该头文件主要声明了与互斥量(mutex)相关的类，包括 std::mutex 系列类，std::lock_guard, std::unique_lock, 以及其他的类型和函数。
- <condition_variable>：该头文件主要声明了与条件变量相关的类，包括 std::condition_variable 和 std::condition_variable_any。
- <future>：该头文件主要声明了 std::promise, std::package_task 两个 Provider 类，以及 std::future 和 std::shared_future 两个 Future 类，另外还有一些与之相关的类型和函数，std::async() 函数就声明在此头文件中。
1、 thread
本节讲thread头文件中的内容，练习代码地址;
<thread> 头文件中声明：thread线程和命名空间this_thread； thread包含如下：
| (一)、Member types | |
|---|---|
| id | Thread id (public member type ) |
| native_handle_type | Native handle type (public member type ) |
std::thread::id是线程调用get_id和this_thread::get_id的返回值；thread::id默认构造函数的结果是一个non-joinable的值；通常用来和其他线程 thread::get_id的结果做比较。
std::thread::native_handle_type本地句柄类型，如果库实现支持它，这个成员类型只存在于类线程中。是thread类成员函数thread::native_handle的返回值。
定义: typedef /* implementation-defined */ native_handle_type;
| (二)、Member functions | |
|---|---|
| (constructor) | Construct thread (public member function ) |
| (destructor) | Thread destructor (public member function ) |
| operator= | Move-assign thread (public member function ) |
| get_id | Get thread id (public member function ) |
| joinable | Check if joinable (public member function ) |
| join | Join thread (public member function ) |
| detach | Detach thread (public member function ) |
| swap | Swap threads (public member function ) |
| native_handle | Get native handle (public member function ) |
| hardware_concurrency [static] | Detect hardware concurrency (public static member function ) |
示例 1:
// thread example
#include <iostream> // std::cout
#include <thread> // std::thread
void foo() {
std::cout << "foo is called" << std::endl;
}
void bar(int x) {
std::cout << "bar is called" << std::endl;
}
int main()
{
std::thread first (foo); // spawn new thread that calls foo()
std::thread second (bar,0); // spawn new thread that calls bar(0)
std::cout << "main, foo and bar now execute concurrently...\n";
// synchronize threads:
first.join(); // pauses until first finishes
second.join(); // pauses until second finishes
std::cout << "foo and bar completed.\n";
return 0;
}
- 构造函数(constructor)
default (1) thread() noexcept;
initialization (2) template <class Fn, class... Args>
explicit thread (Fn&& fn, Args&&... args);
copy [deleted] (3) thread (const thread&) = delete;
move (4) thread (thread&& x) noexcept;
(1)默认构造函数
构造一个不表示任何执行线程的线程对象。
(2)初始化的构造函数 模版函数
构建一个线程对象，该对象表示一个新的可接合线程。新的执行线程调用fn传递args作为参数(使用其lvalue或rvalue引用的衰变副本)。此构建的完成开始同步调用fn副本的。
(3) 拷贝构造 不允许拷贝构造
(4) 移动构造 构造线程获取x线程，这个操作不会影响移动线程的执行，它只会传输它的处理程序。完成x将不再表示一个线程。
示例2：
// constructing threads
#include <iostream> // std::cout
#include <atomic> // std::atomic
#include <thread> // std::thread
#include <vector> // std::vector
std::atomic<int> global_counter (0);
void increase_global (int n) { for (int i=0; i<n; ++i) ++global_counter; }
void increase_reference (std::atomic<int>& variable, int n) { for (int i=0; i<n; ++i) ++variable; }
struct C : std::atomic<int> {
C() : std::atomic<int>(0) {}
void increase_member (int n) { for (int i=0; i<n; ++i) fetch_add(1); }
};
int main ()
{
std::vector<std::thread> threads;
std::cout << "increase global counter with 10 threads...\n";
for (int i=1; i<=10; ++i)
threads.push_back(std::thread(increase_global,1000));
std::cout << "increase counter (foo) with 10 threads using reference...\n";
std::atomic<int> foo(0);
for (int i=1; i<=10; ++i)
{
threads.push_back(std::thread(increase_reference,std::ref(foo),1000));
}
std::cout << "increase counter (bar) with 10 threads using member...\n";
C bar;
for (int i=1; i<=10; ++i)
{
threads.push_back(std::thread(&C::increase_member,std::ref(bar),1000)) ;
}
std::cout << "synchronizing all threads...\n";
for (auto& th : threads) th.join();
std::cout << "global_counter: " << global_counter << '\n';
std::cout << "foo: " << foo << '\n';
std::cout << "bar: " << bar << '\n';
return 0;
}
- 析构函数(destructor)
std::thread::~thread破坏了线程对象。如果在销毁时线程是可接合的，则调用终止()。 - std::thread::operator=：
move (1) thread& operator= (thread&& rhs) noexcept；
copy [deleted] (2) thread& operator= (const thread&) = delete;
thread不允许拷贝；如果对象当前不是joinable的，它将获得由rhs(如果有的话)表示的执行线程。如果是joinable，则调用终止()。赋值“=”运算符通过右值表达式，复制后的thread对象不再是一个线程。
示例3：
// example for thread::operator=
#include <iostream> // std::cout
#include <thread> // std::thread, std::this_thread::sleep_for
#include <chrono> // std::chrono::seconds
void pause_thread(int n)
{
std::this_thread::sleep_for (std::chrono::seconds(n));
std::cout << "pause of " << n << " seconds ended\n";
}
int main()
{
std::thread threads[5]; // default-constructed threads
std::cout << "Spawning 5 threads...\n";
for (int i=0; i<5; ++i)
threads[i] = std::thread(pause_thread,i+1); // move-assign threads
std::cout << "Done spawning threads. Now waiting for them to join:\n";
for (int i=0; i<5; ++i)
threads[i].join();
std::cout << "All threads joined!\n";
return 0;
}
- std::thread::get_id
如果线程对象是joinable，函数将返回唯一标识线程的值。
如果线程对象不可joinable，函数将返回成员类型线程的默认构造对象:id。
示例4：
// thread::get_id / this_thread::get_id
#include <iostream> // std::cout
#include <thread> // std::thread, std::thread::id, std::this_thread::get_id
#include <chrono> // std::chrono::seconds
std::thread::id main_thread_id = std::this_thread::get_id();
void is_main_thread() {
if ( main_thread_id == std::this_thread::get_id() )
std::cout << "This is the main thread.\n";
else
std::cout << "This is not the main thread.\n";
}
int main()
{
is_main_thread();
std::thread th (is_main_thread);
th.join();
}
-
std::thread::joinable
返回线程对象是否可joinable。
如果线程对象表示执行的线程，则是可joinable。
在这些情况下，一个线程对象是不可连接的:- 如果是默认构造。
- 如果它已经被移动(或者构造另一个线程对象，或者分配给它)。
- 如果它的成员加入或分离被调用。
示例5：
// example for thread::joinable
#include <iostream> // std::cout
#include <thread> // std::thread
void mythread()
{
// do stuff...
}
int main()
{
std::thread foo;
std::thread bar(mythread);
std::cout << "Joinable after construction:\n" << std::boolalpha;
std::cout << "foo: " << foo.joinable() << '\n';
std::cout << "bar: " << bar.joinable() << '\n';
if (foo.joinable()) foo.join();
if (bar.joinable()) bar.join();
std::cout << "Joinable after joining:\n" << std::boolalpha;
std::cout << "foo: " << foo.joinable() << '\n';
std::cout << "bar: " << bar.joinable() << '\n';
return 0;
}
- std::thread::join
join 函数在线程执行完成的时候返回；此函数在函数返回时与线程中所有操作的完成是同步的；调用join直到join被构造函数调用返回间，阻塞调用的线程；在调用此函数之后，线程对象变得不可连接，可以安全地销毁。
示例6：
// example for thread::join
#include <iostream> // std::cout
#include <thread> // std::thread, std::this_thread::sleep_for
#include <chrono> // std::chrono::seconds
void pause_thread(int n)
{
std::this_thread::sleep_for (std::chrono::seconds(n));
std::cout << "pause of " << n << " seconds ended\n";
}
int main()
{
std::cout << "Spawning 3 threads...\n";
std::thread t1 (pause_thread,1);
std::thread t2 (pause_thread,2);
std::thread t3 (pause_thread,3);
std::cout << "Done spawning threads. Now waiting for them to join:\n";
t1.join();
t2.join();
t3.join();
std::cout << "All threads joined!\n";
return 0;
}
- std::thread::detach
detach分离出调用线程对象所代表的线程，允许它们彼此独立地执行;这两个线程在任何方式上都不阻塞或同步；注意，当一个结束执行时，它的资源被释放。在调用此函数之后，线程对象变得不可连接，可以安全地销毁。
示例7：
#include <iostream> // std::cout
#include <thread> // std::thread, std::this_thread::sleep_for
#include <chrono> // std::chrono::seconds
void pause_thread(int n)
{
std::this_thread::sleep_for (std::chrono::seconds(n));
std::cout << "pause of " << n << " seconds ended\n";
}
int main()
{
std::cout << "Spawning and detaching 3 threads...\n";
std::thread (pause_thread,1).detach();
std::thread (pause_thread,2).detach();
std::thread (pause_thread,3).detach();
std::cout << "Done spawning threads.\n";
std::cout << "(the main thread will now pause for 5 seconds)\n";
// give the detached threads time to finish (but not guaranteed!):
pause_thread(5);
return 0;
}
std::thread::swap
void swap (thread& x) noexcept; // 与X交换对象状态。std::thread::native_handle
获取本地处理函数；如果库实现支持，这个成员函数只存在于类线程中；如果存在，它将返回用于访问与线程关联的特定于实现的信息的值。std::thread::hardware_concurrency
static unsigned hardware_concurrency() noexcept; //函数定义
检测硬件并发，返回硬件线程上下文的数量。对这个值的解释是看具体的系统和实现，可能不是精确的，只是一个近似值。请注意，这并不需要匹配系统中可用的处理器或内核的实际数目:一个系统可以支持每个处理单元的多个线程，或者限制对程序的资源的访问。如果此值没有计算或被定义好，则函数返回0。std::swap (thread)
std::swap 跟前面提到的成员函数有所不同，他不是成员函数。函数定义：
void swap (thread& x, thread& y) noexcept;
交换线程对象x和y的状态；就像x.swap(y)被调用。
本文主要讲thread，下篇c++11 多线程（2）mutex总结。
