C++: how to create thread local/global variable

Question

Hi,

in this code:

int foo() {
   static int x;
}

is the x global to all threads or local in each thread? Or does that depends on a compiler flag and/or the compiler, so I cannot really know what it is from the code?

Several questions (all of them independent from compiler and compiler flags and OS):

1. How can I create a static variable which is global to all threads?
2. How can I create a static variable which is local to each thread?
3. How can I create a global variable which is global to all threads?
4. How can I create a global variable which is local to each thread?

I guess that this is not in C++ itself. (Is it in C++0x?) Some Boost lib which can do this?

Answer 1

Quick partial answers;

(Is it in C++0x?)

Yes. But depends on your compiler's C++0x support too.

Some Boost lib which can do this?

Boost.Threads. See thread local storage therein.

1. How can I create a static variable which is global to all threads?
2. How can I create a static variable which is local to each thread?
3. How can I create a global variable which is global to all threads?
4. How can I create a global variable which is local to each thread?

Note that typically, static refers to duration and global refers to scope.

C++0x thread constructors are variadic: You can pass any number (and type) of arguments. All of these are available to your std::thread object.

#include <thread>

int main()
{
    int foo = 42;
    std::thread t(foo); // copies foo
    std::thread s(&foo); // pass a pointer
    t.join();
}

Answer 2

x is global to all threads. Always, independent of compiler and/or its flags. Independent of whether this is in C++1x or C++03. So if you declare a regular global or static local variable, it will be shared between all threads. In C++1x we will have the thread_local keyword. Until then, you can use thread_specific_ptr from Boost.Thread.

Answer 3

You'd have to be using some kind of cross platform threading library (since you mentioned OS independence), but given pthreads you could do.

template <T>
class myVarStorage
{
    static std::map<int, T> store_;
public:
    myVarStorage();
    T getVar();
    void setVar(T);
}

template <T> void myVarStorage::setVar<T>(T var)
{
     store_[static_cast<int>pthread_self()] = var;
}

template <T> T myVarStorage::getVar()
{
     return store_[static_cast<int>pthread_self()]; //throws exception 
}

I'm sure the code has errors in it and should be treated as pseudo-code, since I'm a pseudo-programmer when it comes to C++. :)