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#ifndef CommonTools_Utils_LazyConstructed_h
#define CommonTools_Utils_LazyConstructed_h
// -*- C++ -*-
//
// Package: CommonTools/Utils
// Class : LazyConstructed
//
/**\class LazyConstructed LazyConstructed.h "CommonTools/Utils/interface/LazyConstructed.h"
Description: Wrapper around a class for lazy construction.
Usage:
// example: lazy SoA table
auto object = makeLazy<edm::soa::EtaPhiTable>(trackCollection);
Notes:
* See similar class CommonTools/Utils/interface/LazyResult.h for implementation details.
*/
//
// 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
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