[Insight-users] error

[Zachary Pincus]

[Oops, forgot to CC the insight list on this one. Well, for the record,  
here is an explanation of why you need const variables, or similar, for  
template parameters.]

Hello,

Recall that templates are a compile-time feature. So the compiler needs  
to have access to the exact values of the template parameters.  
Specifically integer template parameters must be something called an  
"integral constant expression," which is a way of saying that it is an  
expression that the compiler has access to the value of at  
compile-time, and can guarantee that the value will not change during  
run-time.

Here is a list of expression types that are "integral constants" :  
http://www-eleves-isia.cma.fr/documentation/BoostDoc/boost_1_29_0/more/ 
int_const_guidelines.htm

The easiest way to make a value an integral constant is either to  
declare it globally as:
const int size = 5;

or if you are within a class:
class foo {
   static const int size = 5;
};

then

template <int s> class bar {};
bar< size > x;
bar< foo::size> x;

will compile fine.

Simple numeric operators work too:
bar< -size >x;
bar< size + foo::size > x;

will also compile.

Zach Pincus

Department of Biochemistry and Program in Biomedical Informatics
Stanford University School of Medicine



On May 17, 2005, at 9:35 AM, Tina Wang wrote:

> hi,
>
> I am defining a vector type as:
>
> typedef itk::Vector< float, 3 > MeasurementVectorType;
>
> Now I want to change 3 with different value by
> repalcing it with a variable such as
> int size = 4;
> typedef itk::Vector< float, size
>> MeasurementVectorType;
>
> but I always get an error message "error C2973:
> 'Vector' : invalid template argument
> 'NVectorDimension'",
> Could anyone tell me  how to define or change the
> MeasurementVectorType with diefferent size.
>
> Thanks,
>
> tina
>
>
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