Question

Consider a template class like this:

template <int opt1 = 1,
          bool opt2 = false,
          bool opt3 = true,
          int opt4 = 50>
class X  { };

I try to change just one parameter, but it seems that C++ can’t do that. Or am I wrong? How to achieve something like this:

X<opt2 = true> x;

Answer 1

The Boost version mentioned by R. Martinho Fernandes works something like this:

1. define a number of option types (they can be empty tag structures, or templated on bool to enable/disable, or whatever)

   template <int I>  struct option_1: public std::integral_constant<int, I>  {};
   template <bool B> struct option_2: public std::integral_constant<bool, B> {};
   template <bool B> struct option_3: public std::integral_constant<bool, B> {};
   template <int I>  struct option_4: public std::integral_constant<int, I>  {};
   

2. define some kind of typelist of your default values (I’ll hide it in a namespace along with the existing template class)

   namespace impl {
     typedef typelist<option_1<1>, option_2<false>,
                      option_3<true>, option_4<50>> X_defaults;
   
     template <int opt1, bool opt2, bool opt3, int opt4>
     class X { /* ... */ };
   }
   

3. write a get mechanism to extract an option from a suitable typelist (including a version which falls back on the provided default)

   Once you have all that, you can write a wrapper class that handles all the options and delegates to your existing class, eg.

   template <typename... Options>
   class X: public impl::X<
       get<option_1, typelist<Options...>, impl::X_defaults>::value,
       get<option_2, typelist<Options...>, impl::X_defaults>::value,
       get<option_3, typelist<Options...>, impl::X_defaults>::value,
       get<option_4, typelist<Options...>, impl::X_defaults>::value>
   {
   };
   

Your final invocation now looks like:

X<option_2<true>> x;

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The typelist and get parts are left as an exercise (along with lots of errors, most likely) for the OP, a kindly passing editor, or me when I have more time.