[Insight-users] using typename

[Luis Ibanez]

Hi Bing, 

The support/requirement for the "typename" keyword 
varies largely among compilers. 

To be more precise: 

1) VC++ 6.0   Just don't care, 
   if you miss the use of "typename", the compiler 
   will not complaint. If you add "typename", the 
   compiler will simply ignore the keyword. 

2) VC++ 7.0  (.NET) Will require it. 

3) Gcc up to 3.2 requires the use of "typename" 
   and will complaint when you miss it.  However 
   it tolerates the use of implicit typenames. 

4) Gcc 3.3 if quite stringent. It requires "typename" 
   and does not tolerate implicite typenames anymore. 



So, you better start using "typename" as required 
by the C++ standard. 

In general the rule is: typename is required when 
you are in a templated class and you are trying to 
access types (traits) from a classes that depends 
on the template paramters. 


For example: 


template <class T> 
class A 
{ 
   typedef typename T::Pointer  TPointer; 
}; 



There is a very particular occassion in which is was not 
possible to unify the use of "typename" between multiple 
compilers. This is when traits of one class are used as 
default template paramters. For this very special case we 
were forced to introduce the macro ITK_TYPENAME. 


You will find the use of "typename" all over the toolkit 
code and the applications. That will help you to get familiar 
with the cases in which this keyword is expected. 


Regards, 


   Luis 


--------------------- 
Bing Jian wrote: 

  Hi, 
     It's an another C++ issue about usage of typename. 
  I write and debug my ITK program in visual studio, but 
  finally I have to move it to Sun and Linux, then compile 
  the code using g++. Seems g++ is always complaining 
  about my usage of typename. I have to add typename 
  at some places and remove typename at other locations. 
  Even the program finally compiles, I get lots of warnings 
  like 
      warning: implicit typename is deprecated 
    Do you guys have any experience on that? Thanks! 


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