Optimize template replacement of a switch

This is what I call the magic switch problem — how to take a (range of) run time values and turn it into a compile time constant.

Abstractly, you want to generate this switch statement:

switch(n) {
  (case I from 0 to n-1: /* use I as a constant */)...
}

You can use parameter packs to generate code that is similar to this in C++.

I’ll start with c++14-replacing boilerplate:

template<unsigned...> struct indexes {typedef indexes type;};
template<unsigned max, unsigned... is> struct make_indexes: make_indexes<max-1, max-1, is...> {};
template<unsigned... is> struct make_indexes<0, is...>:indexes<is...> {};
template<unsigned max> using make_indexes_t = typename make_indexes<max>::type;

Now we can create a compile-time sequence of unsigned integers from 0 to n-1 easily. make_indexes_t<50> expands to indexes<0,1,2,3, … ,48, 49>. The c++14 version does so in O(1) steps, as most (all?) compilers implement std::make_index_sequence with an intrinsic. The above does it in linear (at compile time — nothing is done at run time) recursive depth, and quadratic compile time memory. This sucks, and you can do better with work (logarithmic depth, linear memory), but do you have more than a few 100 types? If not, this is good enough.

Next, we build an array of callbacks. As I hate C legacy function pointer syntax, I’ll throw in some pointless boilerplate to hide it:

template<typename T> using type = T; // pointless boilerplate that hides C style function syntax

template<unsigned... Is>
Base_Type construct_runtime_helper( indexes<Is...>, Base_Type::type_enum e, QVariant const& v ) {
  // array of pointers to functions:  (note static, so created once)
  static type< Base_Type(const QVariant&) >* const constructor_array[] = {
    (&Base_Type::construct<Is>)...
  };
  // find the eth entry, and call it:
  return constructor_array[ unsigned(e) ](v);
}
Base_Type construct_runtime_helper( Base_Type::type_enum e, QVariant const& v ) {
  return construct_runtime_helper( make_indexes_t< Base_Type::num_types >(), e, v );
}

and Bob is your Uncle^{1. An O(1) array lookup (with an O(n) setup, which in theory could be done prior to your executable launching) for dispatch.}

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1 “Bob’s your Uncle” is a British Commonwealth saying that says “and everything is finished and working” roughly.