[boost][caffe] boost::thread_specific_ptr

caffe

common.cpp

namespace caffe{



// Make sure each thread can have different values.

static boost::thread_specific_ptr<Caffe> thread_instance_;

Caffe& Caffe::Get() {

if (!thread_instance_.get()) {

thread_instance_.reset(new Caffe());

}

return *(thread_instance_.get());

}

.......

class Caffe{

...

inline
static bool root_solver() { return Get().root_solver_; }

...

}

}

solver.cpp uses "Caffe::root_solver()" a lot of times, and because of "static", so solver.cpp can use
"Caffe::root_solver()" without creating an instance of Caffe first.

boost
http://www.boost.org/doc/libs/1_55_0/doc/html/thread/thread_local_storage.html
Thread local storage allows multi-threaded applications to have a separate instance of a given data item for each thread. Where a single-threaded application would use static or global data, this could lead to contention, deadlock
or data corruption in a multi-threaded application. One example is the C errno variable, used for storing the error code related to functions from the Standard C library. It is common practice (and required by POSIX) for compilers that support multi-threaded
applications to provide a separate instance of errno for each thread, in order to avoid different threads competing to read or update the value.

boost::thread_specific_ptr provides a portable mechanism for thread-local storage that works on all compilers supported by Boost.Thread. Each instance of boost::thread_specific_ptr represents a pointer to an object (such as errno) where each thread must have
a distinct value. The value for the current thread can be obtained using the get() member function, or by using the * and -> pointer deference operators. Initially the pointer has a value of NULL in each thread, but the value for the current thread can be
set using the reset() member function.
http://www.kingofcoders.com/viewNews.php?type=newsCpp&id=187&number=6571825070
thread_specific_ptr代表了某个全局变量的本地存储，各个线程可以各自独立地通过它访问这个全局变量的本地副本，起到了井水不犯河水的效果。

我们来看一个实际的例子，假如我们要为一个多线程的程序添加一个记录日志的功能，记录程序的运行情况。通常的做法是，建立一个全局的日志对象，然后各个线程相互竞争地访问这个全局对象，将日志消息添加到全局的日志对象中，在这个过程中，涉及共享资源的竞争，必然会影响效率。采用线程本地存储，我们为每个线程创建一个各自独立的日志对象，并交由一个全局的thread_specific_ptr进行管理，在各个线程中，可以通过这个thread_specific_ptr独立地访问各自线程的日志对象，最后在线程退出的时候，我们再将各个线程的独自记录的日志合并到一起，就成了完整的日志。在这个过程中，各个线程通过thread_specific_ptr管理的多个日志对象，各自独立，井水不犯河水，整个过程没有共享资源的竞争，自然可以提高效率。