Is it possible to exclude template member variables by conditions on template arguments?

Question

I am currently writing a class which should specialize depending on a template argument.

I am now wondering whether it is possible to leave out certain member variables for specific specializations to be more exact depending on whether a numeric template argument is larger than X.

So for example:

template<int N>
class Test
{
   int a;
   int b;
}

template<N > X>
class Test
{
  int a;
}

I was thinking about the use of std::conditional but that seems to result in at least one type being picked. I could of course the D-Pointer method and std::conditional and put the specialization into different objects that are pointed at but I wondered whether there was a better way.

An alternative I see is to use an abstract base class and have two implementations one for N < X and one for N >= X but I'm not sure that this would be better.

This seems a bit like an [XY problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). What are you trying to do with this model? — TartanLlama, Nov 30 '15 at 14:26
I'm trying to select between two or more different methods of implementing a class based on what would be more efficient depending on the requirements to the class. So to say method X would be more efficient to use when N is smaller than 4 and method Y would be more efficient to use otherwise. However the methods have different member variable requirements. — Blackclaws, Nov 30 '15 at 14:33

score 4 · Accepted Answer · edited Jun 10 '18 at 02:32

4

Just use SFINAE.

template<int N, typename = void>
class Test
{
   int a;
   int b;
};

template<int N>
class Test<N, typename std::enable_if<(N > X)>::type>
{
  int a;
};

edited Jun 10 '18 at 02:32

Nic

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answered Nov 30 '15 at 14:24

ForEveR

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Is it possible to exclude template member variables by conditions on template arguments?

1 Answers1