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"Template"ed classes


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#1 Splat   Members   -  Reputation: 122

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Posted 06 November 1999 - 05:28 PM

Ok. I was just fooling around with creating my own lightweight list class that is templated so that it can be a list of any data type. Anyway, I have it seperated into .cpp and .h files, like any good class - implementation and definition.

Well, when I include the .h file and compile and link the .cpp file, I get linker errors about exported symbols for the member functions, including the constructor and destructor, that can't be found.

The only way I found to fix that is to move all the code into the .h file as inline member functions. And every bit of source code I have from other people does this the same way. In fact, no one has template functions in a .cpp file.

So my question is: Can you create a templated class in two files? And if so how?

- Splat


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#2 Jonathan   Members   -  Reputation: 122

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Posted 06 November 1999 - 04:08 PM

Unfortunately, template function definitions have to be in the same file the template is declared in. I think this is because the class doesn't actually exist until you declare an object using the template(eg. List). I remember this confusing the hell out of me when I was trying to do a cs assignment, so badly that I just dropped using templates altogether

Jonathan


#3 Splat   Members   -  Reputation: 122

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Posted 06 November 1999 - 04:11 PM

Oh well, my worst suspicions are now confirmed ;( A have several objects that are perfect for templating that are VERY large, so moving them all into a single file to be included by a large portion of my game engine will be annoying.

- Splat


#4 Xai   Crossbones+   -  Reputation: 1451

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Posted 06 November 1999 - 04:58 PM

Template definitions must be availible to the code that wants to instantiate them for the same reason that a class declaration must completed before instantiation: The compiler needs enough information to generate code. It's really easy to understand if you think of the template similar to an inline function or a macro.

Tempaltes are simply PROGRAMMING aids that help at compile time, not run time. Templates allow you to code things only once and then compile them over a whole host of particular instances, but they DO NOT provide any special RUN TIME behaviors. Each instantiation of a template using different parameters is a whole different class to the compiler, and at run time. The template keyword just allows the PROGRAMMER to avoid duplication, it does NOT save any code space over writing seperate classes for each of your uses.

Also note that the features provided by polymorphism are a superset of the features templates provide, so if templates don't suit your needs, consider writing the class to manipulate a specific defined interface (preferably a class whose members are ALL virtual, pure or otherwise). Ex, an ObjectList, which is list class which works on classes of type Object, which is the base class for your other types. Please realize I am not advocating only one of these solutions over the other...in fact my current project utilizes both for different circumstances.

BTW - the new standard keyword "export" attempts to allow templated classes to be managed more like other classes.


#5 Alastair   Members   -  Reputation: 122

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Posted 06 November 1999 - 05:28 PM

Actually, the problems described by Splat are just a failing of the C++ compiler. A good C++ compiler does not require that template function definitons to be included by every source file that instantiates them. Every C++ compiler I've used on the PC - Microsoft, Borland and Intel has this problem.

Sun C++ for Solaris has no problem. I've heard Cfront is OK too. Nethier are going to help you write PC games though!





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