How to reverse an integer parameter pack?

Question

Sadly, I cannot use any of stl/std libraries from C++, because I am programming for a embedded Operating System which only has available gcc 4.4.4 with bare C++, so, no std::tuple, std::forward, std::apply or std::anything_else.

To help understand meta generic generated code, I am presenting a minimal example code compiled with clang because it has a option to show us the generated template-meta-programming/metaprogramming code.

This question is just for curiosity because instead of generating the integer parameter pack in the wrong order, I can just create it on a correct order. This is what I use to generate my integer packager pack on the wrong order:

template<int ...>
struct MetaSequenceOfIntegers { };

template<int AccumulatedSize, typename Tn, int... GeneratedSequence>
struct GeneratorOfIntegerSequence;

template<int AccumulatedSize, typename Grouper, typename Head, typename... Tail, int... GeneratedSequence>
struct GeneratorOfIntegerSequence< AccumulatedSize, Grouper( Head, Tail... ), GeneratedSequence... >
{
    typedef typename GeneratorOfIntegerSequence
            < AccumulatedSize + sizeof(Head), Grouper( Tail... ), AccumulatedSize, GeneratedSequence...
            >::type type;
};

template<int AccumulatedSize, typename Grouper, int... GeneratedSequence>
struct GeneratorOfIntegerSequence<AccumulatedSize, Grouper(), GeneratedSequence...>
{
  typedef MetaSequenceOfIntegers<GeneratedSequence...> type;
};

template<int ...Sequence>
void intergers_sequencer_generator(MetaSequenceOfIntegers<Sequence...>) {
    int array[] = {Sequence...};
}

int main(int argc, char const *argv[]) {
    intergers_sequencer_generator( GeneratorOfIntegerSequence< 0, int(char, int, char) >::type() );
    return 0;
}

I am only using int array[] = {Sequence...} for demonstration. The real code used is like this:

template<typename ReturnType, typename... Tn>
class Closure
{ 
    // ... other code
    template<int ...Sequence>
    ReturnType _run(MetaSequenceOfIntegers<Sequence...>) {
        return _function_entry_pointer( get_nth_function_argument_on_address<Sequence, Tn>()... );
    }
    // ... other code
}

Which generates this bellow, for an input like create_functor( &function1, 'a', 10, 'b' ):

template <int ...Sequence> char _run(MetaSequenceOfIntegers<Sequence...>);
template<> char _run<<5, 1, 0>>(MetaSequenceOfIntegers<5, 1, 0>)     {
    return this->_function_entry_pointer(
            this->get_nth_function_argument_on_address<5, const char *>(), 
            this->get_nth_function_argument_on_address<1, const char *>(), 
            this->get_nth_function_argument_on_address<0, char>()
        );
}
// and much more

We can see the generated code using clang:

$ clang++ -Xclang -ast-print -fsyntax-only generator.cpp > expanded.cpp

template <int ...> struct MetaSequenceOfIntegers {
};
template<> struct MetaSequenceOfIntegers<<5, 1, 0>> {
};
template <int AccumulatedSize, typename Tn, int ...GeneratedSequence> struct GeneratorOfIntegerSequence
template<> struct GeneratorOfIntegerSequence<0, int (char, int, char), <>> {
    typedef typename GeneratorOfIntegerSequence<0 + sizeof(char), int (int, char), 0>::type type;
}
template<> struct GeneratorOfIntegerSequence<1, int (int, char), <0>> {
    typedef typename GeneratorOfIntegerSequence<1 + sizeof(int), int (char), 1, 0>::type type;
}
template<> struct GeneratorOfIntegerSequence<5, int (char), <1, 0>> {
    typedef typename GeneratorOfIntegerSequence<5 + sizeof(char), int (), 5, 1, 0>::type type;
}
template<> struct GeneratorOfIntegerSequence<6, int (), <5, 1, 0>> {
    typedef MetaSequenceOfIntegers<5, 1, 0> type;
};
template <int AccumulatedSize, typename Grouper, typename Head, typename ...Tail, int ...GeneratedSequence> struct GeneratorOfIntegerSequence<AccumulatedSize, type-parameter-0-1 (type-parameter-0-2, type-parameter-0-3...), <GeneratedSequence...>> {
    typedef typename GeneratorOfIntegerSequence<AccumulatedSize + sizeof(Head), Grouper (Tail...), AccumulatedSize, GeneratedSequence...>::type type;
};
template <int AccumulatedSize, typename Grouper, int ...GeneratedSequence> struct GeneratorOfIntegerSequence<AccumulatedSize, type-parameter-0-1 (), <GeneratedSequence...>> {
    typedef MetaSequenceOfIntegers<GeneratedSequence...> type;
};
template <int ...Sequence> void intergers_sequencer_generator(MetaSequenceOfIntegers<Sequence...>) {
    int array[] = {Sequence...};
}
template<> void intergers_sequencer_generator<<5, 1, 0>>(MetaSequenceOfIntegers<5, 1, 0>) {
    int array[] = {5, 1, 0};
}
int main(int argc, const char *argv[]) {
    intergers_sequencer_generator(GeneratorOfIntegerSequence<0, int (char, int, char)>::type());
    return 0;
}

The meta programming list is generated is on reversed order I need it to be. Instead of int array[] = {5, 1, 0}, it should be int array[] = {0, 1, 5}.

I managed to generated the list on the correct order just changing this line on in example code:

< AccumulatedSize + sizeof(Head), Grouper( Tail... ), GeneratedSequence..., AccumulatedSize
// to -->
< AccumulatedSize + sizeof(Head), Grouper( Tail... ), AccumulatedSize, GeneratedSequence...

But let us supposed I could not do that because the list is input from a third part which I do not have control. How can I reverse the parameter pack <5, 1, 0> to <0, 1, 5> without using any std library functions?

In my first attempt, I tried to use the same strategy as the one I did to generate the integer list, but I could not make it to compile:

template<int ...>
struct MetaSequenceOfIntegers { };

template<int AccumulatedSize, typename Tn, int... GeneratedSequence>
struct GeneratorOfIntegerSequence;

template<int AccumulatedSize, typename Grouper, typename Head, typename... Tail, int... GeneratedSequence>
struct GeneratorOfIntegerSequence< AccumulatedSize, Grouper( Head, Tail... ), GeneratedSequence... >
{
    typedef typename GeneratorOfIntegerSequence
            < AccumulatedSize + sizeof(Head), Grouper( Tail... ), AccumulatedSize, GeneratedSequence...
            >::type type;
};

template<int AccumulatedSize, typename Grouper, int... GeneratedSequence>
struct GeneratorOfIntegerSequence<AccumulatedSize, Grouper(), GeneratedSequence...>
{
  typedef MetaSequenceOfIntegers<GeneratedSequence...> type;
};

// The new code starts here
template<int ...>
struct MetaSequenceReversed { };

template<typename Tn, int... GeneratedSequence>
struct ReversorOfIntegerSequence;

template<typename Grouper, int Head, int... Tail, int... GeneratedSequence>
struct ReversorOfIntegerSequence< Grouper( Head, Tail... ), GeneratedSequence... >
{
    typedef typename ReversorOfIntegerSequence
            < Grouper( Tail... ), GeneratedSequence...
            >::type type;
};

template<typename Grouper, int... GeneratedSequence>
struct ReversorOfIntegerSequence<Grouper(), GeneratedSequence...>
{
  typedef MetaSequenceReversed<GeneratedSequence...> type;
};

template<int ...ReversedSequence>
void intergers_sequencer_reversor(MetaSequenceReversed<ReversedSequence...>) {
    int reversed_array[] = {ReversedSequence...};
}

template<int ...Sequence>
void intergers_sequencer_generator(MetaSequenceOfIntegers<Sequence...>) {
    int array[] = {Sequence...};
    intergers_sequencer_reversor( ReversorOfIntegerSequence< int(Sequence...) >::type() );
}

int main(int argc, char const *argv[])
{
    intergers_sequencer_generator( GeneratorOfIntegerSequence< 0, int(char, int, char) >::type() );
    return 0;
}

When I try to build this, I got this error:

generator.cpp:29:35: error: template argument for template type parameter must be a type
struct ReversorOfIntegerSequence< Grouper( Head, Tail... ), GeneratedSequence... >
                                  ^~~~~~~~~~~~~~~~~~~~~~~~
generator.cpp:25:19: note: template parameter is declared here
template<typename Tn, int... GeneratedSequence>
                  ^
generator.cpp:50:62: error: template argument for template type parameter must be a type
    intergers_sequencer_reversor( ReversorOfIntegerSequence< int(Sequence...) >::type() );
                                                             ^~~~~~~~~~~~~~~~
generator.cpp:25:19: note: template parameter is declared here
template<typename Tn, int... GeneratedSequence>
                  ^

---

References:

1. Variadic templates, parameter pack and its discussed ambiguity in a parameter list
2. "unpacking" a tuple to call a matching function pointer
3. Can we see the template instantiated code by C++ compiler
4. Build function parameters with variadic templates
5. How to reverse the order of arguments of a variadic template function?

Answer 1

How can I reverse the parameter pack <5, 1, 0> to <0, 1, 5> without using any std library functions?

Not sure to understand what do you exactly can use but... it seems easy to me.

Given an helper struct as follows

template <typename, typename>
struct RS_helper;

template <int ... As, int B0, int ... Bs>
struct RS_helper<MetaSequenceOfIntegers<As...>,
                 MetaSequenceOfIntegers<B0, Bs...>>
   : RS_helper<MetaSequenceOfIntegers<B0, As...>,
               MetaSequenceOfIntegers<Bs...>>
 { };

template <typename T>
struct RS_helper<T, MetaSequenceOfIntegers<>>
 { using type = T; };

the revert struct can be simply

template <int ... Is>
struct RevertSequence
   : RS_helper<MetaSequenceOfIntegers<>, MetaSequenceOfIntegers<Is...>>
 { };

I suppose that a reverse function can be useful

template <int ... Is>
constexpr typename RevertSequence<Is...>::type
   revertSequenceFunction (MetaSequenceOfIntegers<Is...> const &)
 { return {}; }

I propose a modified version of your original code with addition of reversed sequence (using also std::cout to print the sequences, but you can remove it, obviously).

#include <iostream>

template <int ...>
struct MetaSequenceOfIntegers
 { };

template <int AccumulatedSize, typename Tn, int ... GeneratedSequence>
struct GeneratorOfIntegerSequence;

template <int AccumulatedSize, typename Grouper, typename Head,
          typename ... Tail, int ... GeneratedSequence>
struct GeneratorOfIntegerSequence<AccumulatedSize, Grouper(Head, Tail...),
                                  GeneratedSequence... >
 { typedef typename GeneratorOfIntegerSequence
            <AccumulatedSize+sizeof(Head), Grouper(Tail...),
             AccumulatedSize, GeneratedSequence...>::type type; };

template <int AccumulatedSize, typename Grouper, int ... GeneratedSequence>
struct GeneratorOfIntegerSequence<AccumulatedSize, Grouper(),
                                  GeneratedSequence...>
 { typedef MetaSequenceOfIntegers<GeneratedSequence...> type; };

template <int ... Sequence>
void intergers_sequencer_generator(MetaSequenceOfIntegers<Sequence...>)
 {
   using unused = int[];

   (void)unused { 0, (std::cout << Sequence << ' ', 0)... };

   std::cout << std::endl;
 }


template <typename, typename>
struct RS_helper;

template <int ... As, int B0, int ... Bs>
struct RS_helper<MetaSequenceOfIntegers<As...>,
                 MetaSequenceOfIntegers<B0, Bs...>>
   : RS_helper<MetaSequenceOfIntegers<B0, As...>,
               MetaSequenceOfIntegers<Bs...>>
 { };

template <typename T>
struct RS_helper<T, MetaSequenceOfIntegers<>>
 { using type = T; };

template <int ... Is>
struct RevertSequence
   : RS_helper<MetaSequenceOfIntegers<>, MetaSequenceOfIntegers<Is...>>
 { };

template <int ... Is>
constexpr typename RevertSequence<Is...>::type
   revertSequenceFunction (MetaSequenceOfIntegers<Is...> const &)
 { return {}; }

int main ()
 {
   intergers_sequencer_generator(
      GeneratorOfIntegerSequence<0, int(char, int, char)>::type());

   intergers_sequencer_generator(
      revertSequenceFunction(
         GeneratorOfIntegerSequence<0, int(char, int, char)>::type()));
 }