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Pure virtual templated member function

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Started by Oxyd April 04, 2004 09:23 AM

9 comments, last by Oxyd 20 years ago

Oxyd

1,162

Author

April 04, 2004 09:23 AM

I''m trying to write this:

template  virtual symbol_t getSymbol (const std::string &symbol) = 0;

But compiler complains:

invalid use of `virtual'' in template declaration of `virtual symbol_t Insanity::IModule::getSymbol(const std::string&)''

How can I declare pure virtual templated function inside non-templated class? Oxyd --- - Unreadable code is code written on a piece of paper, but not the one, in which the programmer is using a space in the place you don''t. - Real programmers aren''t afraid of goto

SiCrane

11,840

April 04, 2004 11:53 AM

template member functions can''t be virtual.

Oxyd

1,162

Author

April 04, 2004 12:04 PM

Nice... What should I do then? Declare it pure virtual non-templated and in derived class declare it as virtual templated? Or what?

Oxyd

---
- Unreadable code is code written on a piece of paper, but not the one, in which the programmer is using a space in the place you don''t.
- Real programmers aren''t afraid of goto

SiCrane

11,840

April 04, 2004 12:36 PM

I repeat: template member functions can''t be virtual.
Maybe it makes more sense said this way: virtual member functions can''t be templated.

If you want want runtime polymorphism, design the interface for runtime polymorphism. You can try returning a pointer to an interface, supporting a limited number of concrete types, use discriminated unions like the VARIANT type, etc.

psamty10

148

April 04, 2004 12:41 PM

The correct type of polymorphism using templates is ''template specialization'', not ''virtual functions''.

And also, you cant have templated functions in a non-templated class (hmmm... maybe if they are static, im not sure) - because it really doesnt make sense.

You always pass in an std::string, so what determines the return type. How would the compiler ever know what you are going to return??

VolkerG

151

April 04, 2004 12:48 PM

Sorry psamty10, but why should templated (non virtual) members not be allowed in non template classes? It is allowed and makes sence in some cases.

[edited by - VolkerG on April 4, 2004 1:51:15 PM]

Oxyd

1,162

Author

April 04, 2004 12:54 PM

The function is called like object.getSymbol ("symbol");

What I want is to define different behaviour in each derived class and be templated... (Yes - now I know, it''s impossible to have virtual + template). But well... We''ll see.. Maybe I''ll return void * or something like this...

Oxyd

---
- Unreadable code is code written on a piece of paper, but not the one, in which the programmer is using a space in the place you don''t.
- Real programmers aren''t afraid of goto

psamty10

148

April 04, 2004 02:03 PM

quote:Original post by VolkerG
Sorry psamty10, but why should templated (non virtual) members not be allowed in non template classes? It is allowed and makes sence in some cases.

[edited by - VolkerG on April 4, 2004 1:51:15 PM]

Perhaps, as i said, I wasnt sure about this... what I wanted to know is how hed pass in the template argument with his function...

Oh and Oxyd.... I wouldnt use void*, but then again, you arent afraid of ''goto''

hehe

mfawcett

374

April 04, 2004 02:45 PM

It sounds like maybe you are unsure if you can return the template typename from a function. You can, but the caller has to be specific and tell the compiler what type he is expecting.

MyType temp = getSymbol<MyType>("symbol");

[edited by - mfawcett on April 4, 2004 3:45:56 PM]

--Michael Fawcett

psamty10

148

April 04, 2004 03:13 PM

Exactly, and if youre doing that it makes sense to use separate functions...

Pure virtual templated member function

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