LazyConstructed.h

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#ifndef CommonTools_Utils_LazyConstructed_h
#define CommonTools_Utils_LazyConstructed_h
// -*- C++ -*-
//
// Package:     CommonTools/Utils
// Class  :     LazyConstructed
//
//
// Original Author:  Jonas Rembser
//         Created:  Mon, 14 Aug 2020 16:05:45 GMT
//
//
#include <tuple>
#include <optional>
 
template <class WrappedClass, class... Args>
class LazyConstructed {
public:
  LazyConstructed(Args const&... args) : args_(args...) {}
 
  WrappedClass& value() {
    if (!object_) {
      evaluate();
    }
    return object_.value();
  }
 
private:
  void evaluate() { evaluateImpl(std::make_index_sequence<sizeof...(Args)>{}); }
 
  template <std::size_t... ArgIndices>
  void evaluateImpl(std::index_sequence<ArgIndices...>) {
    object_ = WrappedClass(std::get<ArgIndices>(args_)...);
  }
 
  std::optional<WrappedClass> object_ = std::nullopt;
  std::tuple<Args const&...> args_;
};
 
// helper function to create a LazyConstructed where the Args are deduced from the function argument types
template <class WrappedClass, class... Args>
auto makeLazy(Args&&... args) {
  return LazyConstructed<WrappedClass, Args...>(std::forward<Args>(args)...);
}
 
#endif