templates and DLLs

**m37h0d** · 04-16-2009

i am new to creating DLLs and the like, and i have a few questions.

the code i want to package is built heavily upon templates. i am assuming this means that i can only use explicitly declared template parameters into the DLL? For instance, given:

Code:

template<typename T>class foo
{
    T &t;
    public:
    foo(T&_t):
        t(_t)
    {
    }
};

the following can be put into a DLL:

Code:

void makeFoo(int i)
{
        new foo<int>(i);
}

but i am guessing the following cannot, since DLLs are compiled structures, the template type parameters must be explicitly declared at the time it is compiled:

Code:

template<typename T>void makeFoo(T& t)
{
        new foo<T>(t);
}

am i correct in my reasoning?

also, can someone please refer me to a good reference on this topic? thanks!

**matsp** · 04-16-2009

You are correct - templates need to have the implementation available to the compiler that compiles the code where the template is used.

I don't know where there is a document describing this, but there's no count of number of instances where someone has posted "why don't my templates work", when using a .h file and a .cpp file to implement the template.

--
Mats

**Codeplug** · 04-16-2009

MSVC can [dll-]export template instantiations.
msdn.microsoft.com/en-us/library/twa2aw10.aspx

gg

**Elysia** · 04-16-2009

And don't use the "new" keyword to instantiate the templates. That would result in memory leaks...

**valaris** · 04-16-2009

Originally Posted by Elysia

And don't use the "new" keyword to instantiate the templates. That would result in memory leaks...

You can't instantiate a template dynamically? delete wont free its memory?

**matsp** · 04-16-2009

Originally Posted by valaris

You can't instantiate a template dynamically? delete wont free its memory?

Of course you can - but you must assign the value from new into a variable, and then call delete.

The posted code in that respect is incorrect. There is no pointer to the foo<int> object we created, and thus no way to delete it at a later stage.

Code:

void makeFoo(int i)
{
        new foo<int>(i);
}

--
Mats

**Elysia** · 04-16-2009

The problem is more that you create an instance of the object when you don't need it.
You can force the compiler to instantiate templates and then export those types without needing to create objects with them.
http://www.geekinterview.com/question_details/16562

**m37h0d** · 04-16-2009

the idea is that i will use container classes within the DLL, but use access methods with POD arguments for the sake of universality.

i intend to include a done() method in the DLL to invoke the appropriate destructors.

memory management wasn't really germane to my question so i didn't want to bother including all the trifling details required to make it correct.

**brewbuck** · 04-16-2009

Originally Posted by m37h0d

the idea is that i will use container classes within the DLL, but use access methods with POD arguments for the sake of universality.

This is the right method, I think. It's generally considered unsafe to allow non-POD data to cross module boundaries (for multiple reasons).

(The definition of "module" is, of course, specific to a scenario. A single application which just happens to be composed of multiple DLLs might be safe. A DLL intended for third party use is totally different.)

**m37h0d** · 04-16-2009

it is indeed for 3rd party use.

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