LazyResult.h

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#ifndef CommonTools_Utils_LazyResult_h
#define CommonTools_Utils_LazyResult_h
// -*- C++ -*-
//
// Package:     CommonTools/Utils
// Class  :     LazyResult
//
//
// Original Author:  Jonas Rembser
//         Created:  Mon, 14 Jun 2020 00:00:00 GMT
//

#include <tuple>
#include <type_traits>

template <class Func, class... Args>
class LazyResult {
public:
  using Result = typename std::invoke_result<Func, Args...>::type;

  LazyResult(Func func, Args const&... args) : func_(func), args_(args...) {}

  Result const& value() {
    if (!evaluated_) {
      evaluate();
    }
    return result_;
  }

private:
  void evaluate() { evaluateImpl(std::make_index_sequence<sizeof...(Args)>{}); }

  template <std::size_t... ArgIndices>
  void evaluateImpl(std::index_sequence<ArgIndices...>) {
    result_ = func_(std::get<ArgIndices>(args_)...);
    evaluated_ = true;
  }

  // having evaluated_ and the potentially small func_ together might
  // save alignemnt bits (e.g. a lambda function is just 1 byte)
  bool evaluated_ = false;
  Func func_;
  std::tuple<Args const&...> args_;
  Result result_;
};

#endif