to_ref.hpp

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#ifndef STAN_MATH_PRIM_FUN_TO_REF_HPP
#define STAN_MATH_PRIM_FUN_TO_REF_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/functor/make_holder_tuple.hpp>
 
namespace stan {
namespace math {
 
template <typename T, require_not_tuple_t<T>* = nullptr>
inline ref_type_t<T&&> to_ref(T&& a) {
  return std::forward<T>(a);
}
 
template <typename T, require_tuple_t<T>* = nullptr>
inline auto to_ref(T&& a) {
  return apply(
      [](auto&&... args) {
        return make_holder_tuple(to_ref(std::forward<decltype(args)>(args))...);
      },
      std::forward<T>(a));
}
 
template <bool Cond, typename T, std::enable_if_t<!Cond>* = nullptr>
inline T to_ref_if(T&& a) {
  return std::forward<T>(a);
}
 
template <bool Cond, typename T, std::enable_if_t<Cond>* = nullptr>
inline ref_type_t<T&&> to_ref_if(T&& a) {
  return std::forward<T>(a);
}
 
}  // namespace math
}  // namespace stan
#endif