如何分配线程本地存储？

Question

我的函数中有一个变量是静态的,但我希望它在每个线程的基础上是静态的.

如何为我的C++类分配内存,以便每个线程都有自己的类实例副本？

AnotherClass::threadSpecificAction()
{
  // How to allocate this with thread local storage?
  static MyClass *instance = new MyClass();

  instance->doSomething();
}

这是在Linux上.我没有使用C++ 0x,这是gcc v3.4.6.

Answer 1

#include <boost/thread/tss.hpp>
static boost::thread_specific_ptr< MyClass> instance;
if( ! instance.get() ) {
    // first time called by this thread
    // construct test element to be used in all subsequent calls from this thread
    instance.reset( new MyClass);
}
    instance->doSomething();

Answer 2

值得注意的是,C++ 11引入了thread_local关键字.

以下是存储持续时间说明符的示例:

#include <iostream>
#include <string>
#include <thread>
#include <mutex>

thread_local unsigned int rage = 1; 
std::mutex cout_mutex;

void increase_rage(const std::string& thread_name)
{
    ++rage;
    std::lock_guard<std::mutex> lock(cout_mutex);
    std::cout << "Rage counter for " << thread_name << ": " << rage << '\n';
}

int main()
{
    std::thread a(increase_rage, "a"), b(increase_rage, "b");
    increase_rage("main");

    a.join();
    b.join();

    return 0;
}

可能的输出:

Rage counter for a: 2
Rage counter for main: 2
Rage counter for b: 2

Answer 3

boost::thread_specific_ptr 是便携式解决方案的最佳方式.

在Linux和GCC上,您可以使用__thread修饰符.

所以你的实例变量看起来像:

static __thread MyClass *instance = new MyClass();

Answer 4

如果您正在使用Pthreads,则可以执行以下操作:

//declare static data members
pthread_key_t AnotherClass::key_value;
pthread_once_t AnotherClass::key_init_once = PTHREAD_ONCE_INIT;

//declare static function
void AnotherClass::init_key()
{
    //while you can pass a NULL as the second argument, you 
    //should pass some valid destrutor function that can properly
    //delete a pointer for your MyClass
    pthread_key_create(&key_value, NULL);
}

void AnotherClass::threadSpecificAction()
{
  //Initialize the key value
  pthread_once(&key_init_once, init_key);

  //this is where the thread-specific pointer is obtained
  //if storage has already been allocated, it won't return NULL

  MyClass *instance = NULL;
  if ((instance = (MyClass*)pthread_getspecific(key_value)) == NULL)
  {
    instance = new MyClass;
    pthread_setspecific(key_value, (void*)instance);
  }

  instance->doSomething();
}