How to insert a type between all elements of a parameter pack?-CodePudding

I have:

struct spacer : foo<bar>{};

struct sequence : baz<qux, spacer, quz, spacer, plugh>{};

I would like to be able to write (something like this, exact syntax doesn't matter):

struct spaced_sequence : SPACED_BAZ<qux, quz, plugh>{};

Can this be done with macros/templates/anything else?

CodePudding user response：

You can do the following:

Create a base case function template that appends a single type T to some specialization of baz

template<typename T, typename ... Args>
auto append(baz<Args...>) -> baz<Args..., T>;

Note that no spacer is added here, since T is the last type we're adding.

Then write a recursive case, also as a function template, that gets called if there are at least 2 more types T1, and T2 that need to be added

template<typename T1, typename T2, typename ...Rest, typename ... Args>
auto append(baz<Args...>) 
  -> decltype(append<T2, Rest...>(std::declval<baz<Args..., T1, spacer>>()));
                                      // insert spacer after T1 ^
                  // ^ pass the remaining types recursively

and finally add a convenience alias

template<typename ...Ts>
using SPACED_BAZ = decltype(append<Ts...>(std::declval<baz<>>()));
                   // types to add ^  and baz is empty to start

Here's a demo.

CodePudding user response：

You can do something like this (assuming know number of arguments):

template <class T1, class T2, class T3>
using SPACED_BAZ = baz<T1, spacer, T2, spacer, T3>;

If you have more type, you can do other alias as required.