Static function in template class

Question

I'm trying to create a sort of factory class in a template. I wanted to do something like a pure virtual function, but it needs to be static as I'm using the function to create types.

What I want to happen is when I declare a class, the template calls the static function. The static function is actually declared in the templatised class.

I've got as far as:

class Base
{

};

template<typename T>
class Type : public Base
{
public:
    static void Create()
    {
        mBase = CreateBase();
    }

private:
    static Base* CreateBase();

    static Base* mBase;
};

class MyType : public Type<MyType>
{
private:        
    static Base* CreateBase()
    {
        return new MyType;
    }
};

template<typename T>
Base* Type<T>::mBase = NULL;

void test()
{
    MyType::Create();
}

I get a link time error:

undefined reference to `Type<MyType>::CreateBase()

See also Curiously recurring template pattern.

Robᵩ
– Robᵩ

2012-05-14 16:42:30 +00:00
Commented May 14, 2012 at 16:42 — Robᵩ
– Robᵩ, Commented May 14, 2012 at 16:42

David Rodríguez - dribeas · Accepted Answer · 2012-05-14 16:41:12Z

2

The CreateBase function is defined in the base type, so just call it:

template<typename T>
class Type : public Base
{
public:
    static void Create()
    {
        mBase = Base::CreateBase();
    }
//...

There is no need to declare another CreateBase in the template.

answered May 14, 2012 at 16:41

David Rodríguez - dribeas

209k23 gold badges304 silver badges496 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Neil Over a year ago

The problem isn't that I can't call it, it's that this code doesn't link.

David Rodríguez - dribeas Over a year ago

@Neil: In Create you are calling Type<>::CreateBase that is not defined, and thus the linker fails. The proposed solution is not calling Type<>::CreateBase (don't even declare it!) and directly call Base::CreateBase, which is defined in MyType and will link appropriately.

Neil · Accepted Answer · 2012-05-15 08:53:52Z

1

Found it.

The problem was I wasn't calling the derived class' function.

Here is the fix:

static void Create()
{
    mBase = T::CreateBase();
}

answered May 15, 2012 at 8:53

Neil

12.1k3 gold badges36 silver badges65 bronze badges

Collectives™ on Stack Overflow

Static function in template class

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related