本文主要是針對(duì)C++中多線程并發(fā)操作參見(cplusplus)進(jìn)行解釋，文章從下面幾個(gè)方面進(jìn)行學(xué)習(xí)，分別介紹多線程中會(huì)使用到的幾個(gè)文件學(xué)習(xí)。 文中代碼 可編譯運(yùn)行版本已上傳在本人github(地址)
多線程
C++ 中關(guān)于并發(fā)多線程的部分，主要包含 <thread>、<mutex>、<atomic>、<condition_varible>、<future>五個(gè)部分。
- <atomic>：該頭文主要聲明了兩個(gè)類, std::atomic 和 std::atomic_flag，另外還聲明了一套 C 風(fēng)格的原子類型和與 C 兼容的原子操作的函數(shù)。
- <thread>：該頭文件主要聲明了 std::thread 類，另外 std::this_thread 命名空間也在該頭文件中。
- <mutex>：該頭文件主要聲明了與互斥量(mutex)相關(guān)的類，包括 std::mutex 系列類，std::lock_guard, std::unique_lock, 以及其他的類型和函數(shù)。
- <condition_variable>：該頭文件主要聲明了與條件變量相關(guān)的類，包括 std::condition_variable 和 std::condition_variable_any。
- <future>：該頭文件主要聲明了 std::promise, std::package_task 兩個(gè) Provider 類，以及 std::future 和 std::shared_future 兩個(gè) Future 類，另外還有一些與之相關(guān)的類型和函數(shù)，std::async() 函數(shù)就聲明在此頭文件中。
1、 thread
本節(jié)講thread頭文件中的內(nèi)容，練習(xí)代碼地址;
<thread> 頭文件中聲明：thread線程和命名空間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是線程調(diào)用get_id和this_thread::get_id的返回值；thread::id默認(rèn)構(gòu)造函數(shù)的結(jié)果是一個(gè)non-joinable的值；通常用來和其他線程 thread::get_id的結(jié)果做比較。
std::thread::native_handle_type本地句柄類型，如果庫實(shí)現(xiàn)支持它，這個(gè)成員類型只存在于類線程中。是thread類成員函數(shù)thread::native_handle的返回值。
定義: 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;
}
- 構(gòu)造函數(shù)(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)默認(rèn)構(gòu)造函數(shù)
構(gòu)造一個(gè)不表示任何執(zhí)行線程的線程對(duì)象。
(2)初始化的構(gòu)造函數(shù) 模版函數(shù)
構(gòu)建一個(gè)線程對(duì)象，該對(duì)象表示一個(gè)新的可接合線程。新的執(zhí)行線程調(diào)用fn傳遞args作為參數(shù)(使用其lvalue或rvalue引用的衰變副本)。此構(gòu)建的完成開始同步調(diào)用fn副本的。
(3) 拷貝構(gòu)造 不允許拷貝構(gòu)造
(4) 移動(dòng)構(gòu)造 構(gòu)造線程獲取x線程，這個(gè)操作不會(huì)影響移動(dòng)線程的執(zhí)行，它只會(huì)傳輸它的處理程序。完成x將不再表示一個(gè)線程。
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;
}
- 析構(gòu)函數(shù)(destructor)
std::thread::~thread破壞了線程對(duì)象。如果在銷毀時(shí)線程是可接合的，則調(diào)用終止()。 - std::thread::operator=：
move (1) thread& operator= (thread&& rhs) noexcept；
copy [deleted] (2) thread& operator= (const thread&) = delete;
thread不允許拷貝；如果對(duì)象當(dāng)前不是joinable的，它將獲得由rhs(如果有的話)表示的執(zhí)行線程。如果是joinable，則調(diào)用終止()。賦值“=”運(yùn)算符通過右值表達(dá)式，復(fù)制后的thread對(duì)象不再是一個(gè)線程。
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
如果線程對(duì)象是joinable，函數(shù)將返回唯一標(biāo)識(shí)線程的值。
如果線程對(duì)象不可joinable，函數(shù)將返回成員類型線程的默認(rèn)構(gòu)造對(duì)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
返回線程對(duì)象是否可joinable。
如果線程對(duì)象表示執(zhí)行的線程，則是可joinable。
在這些情況下，一個(gè)線程對(duì)象是不可連接的:- 如果是默認(rèn)構(gòu)造。
- 如果它已經(jīng)被移動(dòng)(或者構(gòu)造另一個(gè)線程對(duì)象，或者分配給它)。
- 如果它的成員加入或分離被調(diào)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 函數(shù)在線程執(zhí)行完成的時(shí)候返回；此函數(shù)在函數(shù)返回時(shí)與線程中所有操作的完成是同步的；調(diào)用join直到j(luò)oin被構(gòu)造函數(shù)調(diào)用返回間，阻塞調(diào)用的線程；在調(diào)用此函數(shù)之后，線程對(duì)象變得不可連接，可以安全地銷毀。
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分離出調(diào)用線程對(duì)象所代表的線程，允許它們彼此獨(dú)立地執(zhí)行;這兩個(gè)線程在任何方式上都不阻塞或同步；注意，當(dāng)一個(gè)結(jié)束執(zhí)行時(shí)，它的資源被釋放。在調(diào)用此函數(shù)之后，線程對(duì)象變得不可連接，可以安全地銷毀。
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; // 與X交換對(duì)象狀態(tài)。std::thread::native_handle
獲取本地處理函數(shù)；如果庫實(shí)現(xiàn)支持，這個(gè)成員函數(shù)只存在于類線程中；如果存在，它將返回用于訪問與線程關(guān)聯(lián)的特定于實(shí)現(xiàn)的信息的值。std::thread::hardware_concurrency
static unsigned hardware_concurrency() noexcept; //函數(shù)定義
檢測硬件并發(fā)，返回硬件線程上下文的數(shù)量。對(duì)這個(gè)值的解釋是看具體的系統(tǒng)和實(shí)現(xiàn)，可能不是精確的，只是一個(gè)近似值。請(qǐng)注意，這并不需要匹配系統(tǒng)中可用的處理器或內(nèi)核的實(shí)際數(shù)目:一個(gè)系統(tǒng)可以支持每個(gè)處理單元的多個(gè)線程，或者限制對(duì)程序的資源的訪問。如果此值沒有計(jì)算或被定義好，則函數(shù)返回0。std::swap (thread)
std::swap 跟前面提到的成員函數(shù)有所不同，他不是成員函數(shù)。函數(shù)定義：
void swap (thread& x, thread& y) noexcept;
交換線程對(duì)象x和y的狀態(tài)；就像x.swap(y)被調(diào)用。
本文主要講thread，下篇c++11 多線程（2）mutex總結(jié)。
