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Class Templates

Started by PorkBag Jun 5, 2004 at 3:02 PM 2 replies 750+ views

Original Post

Jun 5, 2004 at 3:02 PM

In an attempt to use class templates I''ve created a simple class, which of course doesn''t work.

template <class Type>
class Box
{
	Type Width;
	Type Length;
public:
	Box ()
	{
		Width = 25;
		Length = 30;
	};
	Type GetArea ();
};

template <class Type>
Type Box<Type>::GetArea ()
{
	return Width*Height;
};

Box<int> Square;

When i try using Square.GetArea() in the code i get the following error: "unresolved external symbol "public: int __thiscall Box::GetArea(void)" (?GetArea@?$Box@H@@QAEHXZ)" THe .h and .cpp files in which this is located are properly linked (other things in them work). Any idea what''s wrong?

dathui

Jun 5, 2004 at 3:14 PM


template <class Type>
class Box
{
	Type Width;
	Type Length;
public:
	Box ()
	{
		Width = 25;
		Length = 30;
	};
	Type GetArea ();
};

template <class Type>
Type Box<Type>::GetArea ()
{
	return Width*Height;
};

Box<int> Square;

tried removing the ; after the ctor? othervise i cant find anything wrong :/

neoztar

Jun 5, 2004 at 3:20 PM

It looks like you placed the implementation of GetArea() in a cpp file, while it needs to be in the header (because it''s a template).

~nz

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SiCrane

Mod

473

SiCrane

Jun 5, 2004 at 6:08 PM

To expand on what he said: unless your compiler supports the export keyword, and I''m willing to bet that yours doesn''t, then you can''t put the definition of a template in a separate source file. The complete definition of the template needs to be available at point of instantiation, which means, in effect, that the definition needs to go into the header. (Or an inline file of some sort, etc.)

For more details see these articles: "Export" Restrictions, Part 1 and "Export" Restrictions, Part 2.

And since petewood no longer posts on the boards, I suppose I should post this link for him. It''s an article entitled "Instantiator: A (Mostly) Portable Framework for Separate Compilation of Templates"

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