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Conditional template specialization

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Conditional template specialization

 
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  #1
Apr 25th, 2009
Hey guys,

How would I partially specilize a template in a manner as below? Is that even possible?

I have these two functions:

cpp Syntax (Toggle Plain Text) 
  1. bool node::read(const char *filename) {
  2.     ifstream f(filename);
  3.  
  4.     if (!f.good()) return false;
  5.  
  6.     bool success = read(f);
  7.     f.close();
  8.  
  9.     return success;
  10. }
  11.  
  12. bool node::write(const char *filename) {
  13.     ofstream f(filename);
  14.  
  15.     if (!f.good()) {
  16.         return false;
  17.     }
  18.  
  19.     bool success = write(f);
  20.     f.close();
  21.  
  22.     return success;
  23. }

And I'd like to create a function like this:

cpp Syntax (Toggle Plain Text) 
  1. template <typename T>
  2. bool node::read_or_write(const char *filename) {
  3.     T f(filename);
  4.  
  5.     if (!f.good()) {
  6.         return false;
  7.     }
  8.  
  9.     bool success;
  10.     if(T == ifstream){
  11.         success = write(f);
  12.     } else if (T == ofstream){
  13.         success = read(f);
  14.     }
  15.     f.close();
  16.  
  17.     return success;
  18. }

How would I do that?

Thanks in advance,
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Re: Conditional template specialization

 
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  #2
Apr 25th, 2009
I think you'll need RunTime Type Identification (RTTI) for that purpose ...
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Re: Conditional template specialization

 
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  #3
Apr 25th, 2009
Well, a design wouldn't need it. I don't know if its needed but can't you say something like: T can only be ifstream, ofstream

Code could look a bit like this:

cpp Syntax (Toggle Plain Text) 
  1. template <typename T = {ifstream, ofstream}>
  2. bool node::read_or_write(const char *filename) {
  3.     T f(filename);
  4.  
  5.     if (!f.good()) {
  6.         return false;
  7.     }
  8.  
  9.     bool success;
  10.     if(T == typename ifstream){
  11.         success = write(f);
  12.     } else if (T == typename ofstream){
  13.         success = read(f);
  14.     }
  15.     f.close();
  16.  
  17.     return success;
  18. }

Compiler should throw an error when you try to instantiate any other type of it.. I'm trying to create such a class now, but I'm not very good at it.

This is the best I could come up with, but I didn't compile/test it yet due to diner being ready.

cpp Syntax (Toggle Plain Text) 
  1. template <typename T>
  2. class filestuffs {
  3.     filestuffs(const char *filename) : filename(filename) {
  4.         f(filename);
  5.         success = f.good();
  6.         success = read_or_write();
  7.         f.close();
  8.     }
  9.  
  10.     //can't do this unless we know T
  11.     bool read_or_write(){ return false; }
  12.  
  13.     T f;
  14.     bool success;
  15.     const char *filename;
  16.     const char [] errmsg = "Can't do this on anything but: ifstream, ofstream";
  17. }
  18. template <>
  19. class filestuffs<ifstream> {
  20.     //now we know T! :D
  21.     bool read_or_write(){
  22.         return read(f);
  23.     }
  24. }
  25.  
  26. template<>
  27. class filestuffs<ofstream> {
  28.     bool read_or_write(){
  29.         return write(f);
  30.     }
  31. }
Last edited by Clockowl; Apr 25th, 2009 at 3:20 pm.
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Re: Conditional template specialization

 
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  #4
Apr 25th, 2009
And what's the result ?
Did it work ?
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Re: Conditional template specialization

 
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  #5
Apr 25th, 2009
Oh I'm sure that doesn't work, never seen it, just made it up. But I want to know how I'd do that in C++.
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Re: Conditional template specialization

 
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  #6
Apr 25th, 2009
Did you try searching google (e.g: typeid , c++ rtti )?
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Re: Conditional template specialization

 
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  #7
Apr 25th, 2009
I am not looking for a runtime solution, that's why I hadn't.

Here's a working example:
cpp Syntax (Toggle Plain Text) 
  1.         template <typename T>
  2.         bool read_or_write(const char *filename) {
  3.             T f(filename);
  4.             if (!f.good()) {
  5.                 return false;
  6.             }
  7.  
  8.             bool success;
  9.             if (typeid(T) == typeid(ofstream)) {
  10.                 success = write(f);
  11.             } else if (typeid(T) == typeid(ifstream)) {
  12.                 success = read(f);
  13.             }
  14.             f.close();
  15.  
  16.             return success;
  17.         }

But this way, I also have to define write(ifstream ... ), while I know 100% sure that the compiler will never have to call such a function. It kinda defeats it's use (making the code smaller) for this small function, but for bigger functions this sort of template is very usable I think.

Any compiler-time suggestions?

PS: For anyone reading this and thinking, why the hell are you creating such a function for 20 lines of code? It's because I'm not going to use read_or_write(), but would like to know how I'd accomplish maximum code re-usage in C++.
Last edited by Clockowl; Apr 25th, 2009 at 4:16 pm.
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Re: Conditional template specialization

 
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  #8
Apr 25th, 2009
Why are you using two separate functions read and write then ?
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Re: Conditional template specialization

 
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  #9
Apr 25th, 2009
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Originally Posted by tux4life
Why are you using two separate functions read and write then ?
Because one reads and one writes? Hahahaha
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Re: Conditional template specialization

 
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  #10
Apr 25th, 2009
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Originally Posted by Clockowl View Post
Because one reads and one writes? Hahahaha
Of course , but I asked why you SEPERATED these from your function, why not implement them directly in your function ?
Last edited by tux4life; Apr 25th, 2009 at 4:31 pm.
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