customAsPointer.cpp

#include "matchit.h"
#include <iostream>

enum class Kind
{
  kONE,
  kTWO
};

#if __cplusplus > 201703L
#define CPP20_CONSTEXPR constexpr
#else
#define CPP20_CONSTEXPR
#endif

class Num
{
public:
  CPP20_CONSTEXPR virtual Kind kind() const = 0;

protected:
  ~Num() = default;
};

class One : public Num
{
public:
  constexpr static auto k = Kind::kONE;
  CPP20_CONSTEXPR Kind kind() const override { return k; }
};

class Two : public Num
{
public:
  constexpr static auto k = Kind::kTWO;
  CPP20_CONSTEXPR Kind kind() const override { return k; }
};

template <Kind k>
constexpr auto kind = matchit::app(&Num::kind, k);

template <typename T>
auto get_if(Num const *num)
{
  std::cout << "get_if" << std::endl;
  return static_cast<T const *>(num->kind() == T::k ? num : nullptr);
}

constexpr int32_t staticCastAs(Num const &input)
{
  using namespace matchit;
  return match(input)(
      // clang-format off
        pattern | as<One>(_)       = 1,
        pattern | kind<Kind::kTWO> = 2,
        pattern | _                = 3
      // clang-format on
  );
}

#if 0 // fail on gcc, fix me later.
#if __cplusplus > 201703L
static_assert(staticCastAs(One{}) == 1);
#endif
#endif

int32_t main()
{
  std::cout << staticCastAs(One{}) << std::endl;
  return 0;
}