构造方式

std::thread默认提供了四种构造方式:

  1. default constructor:构造一个不代表任何执行线程的线程对象。
  2. initialization constructor:构造一个新的、joinable的thread对象,该方式需要一个fn函数作为参数,线程会执行这个函数。构造的完成与fn开始调用同步。
  3. copy constructor:不可用,线程对象不可被拷贝。
  4. move constructor:运用了C++11引入的新特性std::move,需要一个已有线程x通过std::move(x)作为参数。该操作不会影响被移动线程的执行,它只是传递其句柄。之后x不再表示任何执行线程。
    注意:一个线程只有在被销毁前被join或detach才是joinable的。

简单例程

cplusplus.com给出这样一个例程:

// 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;
}

该例输出:

increase global counter using 10 threads...
increase counter (foo) with 10 threads using reference...
increase counter (bar) with 10 threads using member...
synchronizing all threads...
global_counter: 10000
foo: 10000
bar: 10000

该例展示了初始化构造函数使用的三种情形,分别为对全局变量、局部变量和成员变量进行++操作。
再来看看其他构造函数如何使用:

#include <iostream>
#include <utility>
#include <thread>
#include <chrono>
#include <functional>
#include <atomic>

void f1(int n)
{
    for (int i = 0; i < 5; ++i) {
        std::cout << "Thread " << n << " executing\n";
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}

void f2(int& n)
{
    for (int i = 0; i < 5; ++i) {
        std::cout << "Thread 2 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}

int main()
{
    int n = 0;
    std::thread t1; // t1 is not a thread
    std::thread t2(f1, n + 1); // pass by value
    std::thread t3(f2, std::ref(n)); // pass by reference
    std::thread t4(std::move(t3)); // t4 is now running f2(). t3 is no longer a thread
    t2.join();
    t4.join();
    std::cout << "Final value of n is " << n << '\n';
}

其可能输出为:

Thread 1Thread 2 executing
 executing
Thread 1 executing
Thread 2 executing
Thread 2 executing
Thread 1 executing
Thread 2 executing
Thread 1Thread 2 executing
 executing
Thread 1 executing
Final value of n is 5

如之前介绍所言,t4通过std::move(t3)构造,并且不影响线程的运行,n的值仍然为5,t3也不再是线程。


本文由 jianhuyy 创作,采用 知识共享署名 3.0,可自由转载、引用,但需署名作者且注明文章出处。

还不快抢沙发

添加新评论