Linking error with static template class members

cunnus88 · 04-01-2009, 04:46 PM

The more I think about it, the more I have a feeling I have to change my design but here's the problem, anyway.

I have a templated class. This class has a static class member variable, which is a templated stl_container. Then there is a class derived from this one that tries to call this member variable in a source file, but results in a linker error. Here's some simplified code:

Code:

// in a header file
template <typename T>
class A {
static stl_container<T*>* container;
};

class B : public A<MyClass> {
void do_something();
};

//in a source file
//memory allocation for "container" is in different source file

void B::do_something() {
do_something_to(container);
}

This gives me a linker error saying it can't find "container" being called from "do_something". Which, if I think about it, makes sense because there is no other object file for it to link from.

So I can think of one solution which would be to dump the definition of "do_something" into the class body so it doesn't have to resolve the linking error. But I've got other non-class functions that have to access "container" so that's a no go.

So how do you declare, allocate and use a templated static class variable? Is it possible?

I need this thing to be static because "class A" is a actually a node in a graph structure that links to other nodes in the graph and info on the overall graph (kept in the container) can be updated by any node.

cunnus88 · 04-01-2009, 09:47 PM

After some googling and tinkering, I've found a solution, but I can't say I understand why it works.

Code:

// in a header file
template <typename T>
class A {
static stl_container<T*>* container;
};

//you need a second declaration outside the body of the class
//in the header
template <typename T>
typename A<T>::stl_container<T*>* container;

Why you need the second declaration is beyond me. But it works.

brewbuck · 04-01-2009, 10:12 PM

Why is that so surprising? You have to do that even without templates.

Code:

class A
{
public:
    static int x;
};

int A::x;

int main()
{
    A::x = 0;
    return 0;
}

How could it be otherwise? How does the compiler magically decide which .cpp module the definition of the static member should come from? The "static int x" is only a declaration -- you still need to define the variable in some specific .cpp module.

cunnus88 · 04-02-2009, 11:59 AM

My previous understanding was that static member variables have to be defined outside the class body, not just declared. Isn't a statement like

Code:

int A::x;

allocating memory as well, and therefore, a definition?

But when you have a template declaration

Code:

template <typename T>
typename A<T>::stl_container<T*>* container;

it's not allocating any memory. It's just repeating a declaration made earlier, n'est ce pas? Seems a little byzantine.

brewbuck · 04-02-2009, 12:02 PM

Why should the template syntax deviate from the non-template syntax?

cunnus88 · 04-02-2009, 12:05 PM

True. Consistency is important.

Elysia · 04-02-2009, 12:31 PM

It's the same as with non-templates.
It may be a declaration or implementation, but one instantiated, when the class is instantiated, it becomes a definition. That is generally a rule of templates - they are a declaration sorts of, but once instantiated, a definition for those particular types is created.

04-01-2009, 04:46 PM	#1
cunnus88 Registered User Join Date: Oct 2005 Posts: 271	Linking error with static template class members The more I think about it, the more I have a feeling I have to change my design but here's the problem, anyway. I have a templated class. This class has a static class member variable, which is a templated stl_container. Then there is a class derived from this one that tries to call this member variable in a source file, but results in a linker error. Here's some simplified code: Code: // in a header file template <typename T> class A { static stl_container<T> container; }; class B : public A<MyClass> { void do_something(); }; //in a source file //memory allocation for "container" is in different source file void B::do_something() { do_something_to(container); } This gives me a linker error saying it can't find "container" being called from "do_something". Which, if I think about it, makes sense because there is no other object file for it to link from. So I can think of one solution which would be to dump the definition of "do_something" into the class body so it doesn't have to resolve the linking error. But I've got other non-class functions that have to access "container" so that's a no go. So how do you declare, allocate and use a templated static class variable? Is it possible? I need this thing to be static because "class A" is a actually a node in a graph structure that links to other nodes in the graph and info on the overall graph (kept in the container) can be updated by any node.
cunnus88 is offline

04-01-2009, 09:47 PM	#2
cunnus88 Registered User Join Date: Oct 2005 Posts: 271	After some googling and tinkering, I've found a solution, but I can't say I understand why it works. Code: // in a header file template <typename T> class A { static stl_container<T> container; }; //you need a second declaration outside the body of the class //in the header template <typename T> typename A<T>::stl_container<T> container; Why you need the second declaration is beyond me. But it works.
cunnus88 is offline

04-01-2009, 10:12 PM	#3
brewbuck Senior software engineer Join Date: Mar 2007 Location: Portland, OR Posts: 5,759	Why is that so surprising? You have to do that even without templates. Code: class A { public: static int x; }; int A::x; int main() { A::x = 0; return 0; } How could it be otherwise? How does the compiler magically decide which .cpp module the definition of the static member should come from? The "static int x" is only a declaration -- you still need to define the variable in some specific .cpp module. __________________ "Congratulations on your purchase. To begin using your quantum computer, set the power switch to both off and on simultaneously." -- raftpeople@slashdot
brewbuck is offline

04-02-2009, 11:59 AM	#4
cunnus88 Registered User Join Date: Oct 2005 Posts: 271	My previous understanding was that static member variables have to be defined outside the class body, not just declared. Isn't a statement like Code: int A::x; allocating memory as well, and therefore, a definition? But when you have a template declaration Code: template <typename T> typename A<T>::stl_container<T> container; it's not allocating any memory. It's just repeating a declaration made earlier, n'est ce pas? Seems a little byzantine.
cunnus88 is offline

04-02-2009, 12:02 PM	#5
brewbuck Senior software engineer Join Date: Mar 2007 Location: Portland, OR Posts: 5,759	Why should the template syntax deviate from the non-template syntax? __________________ "Congratulations on your purchase. To begin using your quantum computer, set the power switch to both off and on simultaneously." -- raftpeople@slashdot
brewbuck is offline

04-02-2009, 12:05 PM	#6
cunnus88 Registered User Join Date: Oct 2005 Posts: 271	True. Consistency is important.
cunnus88 is offline

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