How make static variable for across threads?

I need a static variable that will be initialized only once for all threads. However, when I create a new thread and check this static variable, it always says it's uninitialized (but asking within the same thread multiple times it says it's already been init's).

Are static variables (declared outside a class) thread-local?

C++ doesn't have a concept of threads, and thus not of thread-local. All variables of static storage duration (globals, function-local statics and class statics) are shared between threads.

I'm more interesting in how you determine that a variable is uninitialized.

Quote:

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Originally Posted by CornedBee

C++ doesn't have a concept of threads, and thus not of thread-local. All variables of static storage duration (globals, function-local statics and class statics) are shared between threads.

I'm more interesting in how you determine that a variable is uninitialized.

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I did:

Code:

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static long index;

class Test
{
public:
      Test()
      {
            if ( !index ) index =  TlsAlloc();
      }
};

---

How should I be doing it?

Sounds reasonable. So it's calling TlsAlloc() again in a new thread? That seems highly unlikely to me.

Unless you have specific reasons to make your variable static OUTSIDE of the class, I'd prefer this:

Code:

---

class Test
{
private:
    static long index;
public:
      Test()
      {
            if ( !index ) index =  TlsAlloc();
      }
};

// This should not be in a header-file!
long Test::index = 0;

---

--
Mats

It occurs to me that, if the class in your case is in a header file, as is the non-class static, you've got undefined behaviour anyway.

So yeah, definitely make index a class-static variable.

Quote:

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Originally Posted by CornedBee

It occurs to me that, if the class in your case is in a header file, as is the non-class static, you've got undefined behaviour anyway.

So yeah, definitely make index a class-static variable.

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I tried that but it's a template class and for some reason it threw an error when I did that. I was calling it inside the templated class like:

Code:

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MyClass<T>::index

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But it still wouldn't compile. It fails with "undefined reference to 'MyClass<int>::index'" (The int is because that's the version I am creating)

Quote:

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Originally Posted by 6tr6tr

I tried that but it's a template class and for some reason it threw an error when I did that. I was calling it inside the templated class like:

Code:

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MyClass<T>::index

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But it still wouldn't compile. It fails with "undefined reference to 'MyClass<int>::index'" (The int is because that's the version I am creating)

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Static variables in a templated class can be a bit more tricky.

Since using TLS is a global type of operation, perhaps a singleton is a more suitable solution to the problem?

--
Mats

Quote:

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Originally Posted by matsp

Static variables in a templated class can be a bit more tricky.

Since using TLS is a global type of operation, perhaps a singleton is a more suitable solution to the problem?

--
Mats

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OK, I finally got it to compile but I want to be sure I'm doing this properly. Here's the steps:

1. Declare the class (this is simplified with no error checking)

Code:

---

template <typename T> class Test
{
    static long index;

public:
    void setIndex();
    T* get();
    ...elided...
};

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2. Define the static variable and functions

Code:

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/*
 * This is what makes it compile BUT only
 * if I leave out static. So is it defining the
 * proper variable and will it still be static?
 */
template <typename T> long Test<T>::index;

template <typename T> void Test<T>::setIndex()
{
    if ( !index ) index = TlsAlloc();
}

template <typename T> T* Test<T>::get()
{
    return ( T* ) TlsGetValue( index );
}

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You don't need static in the defining of the static member, correct. You will, as far as I understand, however, get one index per type T - which may or may not be what you want.

--
Mats

Quote:

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Originally Posted by matsp

You don't need static in the defining of the static member, correct. You will, as far as I understand, however, get one index per type T - which may or may not be what you want.

--
Mats

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Thanks! Yeah, that's exactly what i want. I want there to be one and only one for all threads in which a Test<X> runs but for that to be diff from Test<Y> because I don't want to end up storing the object of type "Y" in the same TLS spot as an object of type "X".

Thanks again for all the help! :)