LazyResult< Func, Args > Class Template Reference

#include "CommonTools/Utils/interface/LazyResult.h"

Public Types
using	Result = typename std::invoke_result< Func, Args...>::type

Public Member Functions
	LazyResult (Func func, Args const &...args)
Result const &	value ()

Private Member Functions
void	evaluate ()
template<std::size_t... ArgIndices>
void	evaluateImpl (std::index_sequence< ArgIndices...>)

Private Attributes
std::tuple< Args const &...>	args_
bool	evaluated_ = false
Func	func_
Result	result_

Detailed Description

template<class Func, class... Args>
class LazyResult< Func, Args >

Description: Wrapper around a function call for lazy execution.

Usage: example: lazy addition auto result = LazyResult(std::plus<int>, x, x); std::cout << result.value() << std::endl;

Notes: The arguments for the delayed call are stored by reference (watch their lifetime). The overhead in memory compared to just storing the result is small: one reference per argument, one bool flag and a function pointer (on my system: 1 byte for lambda function, 8 bytes for global function and 16 bytes for member function due to possible index to virtual table).

Implementation:

For the Args... we explicitly add const& (also in the the args_ tuple).
Otherwise, the arguments will be stored by value which comes with too much
overhead. This implies that the lifetime of the arguments passed to
LazyResult neet to live longer than the LazyResult instance. Function pointers
are small, so no need for const& to the Func.
An alternative to using a ::value() member function to get the result could
be a cast operator: operator Result const &(). This might be pretty because
the result is automatically evaluated the first time you try to bind it to
a Result const &. I think this would however be too implicit and dangerous.

Definition at line 46 of file LazyResult.h.

Member Typedef Documentation

template<class Func , class... Args>

using LazyResult< Func, Args >::Result = typename std::invoke_result<Func, Args...>::type

Definition at line 48 of file LazyResult.h.

Constructor & Destructor Documentation

template<class Func , class... Args>

LazyResult< Func, Args >::LazyResult ( Func  func, Args const &...  args  )

inline

Definition at line 50 of file LazyResult.h.

: func_(func), args_(args...) {}

Member Function Documentation

template<class Func , class... Args>

void LazyResult< Func, Args >::evaluate ( )

inlineprivate

Definition at line 60 of file LazyResult.h.

References LazyResult< Func, Args >::evaluateImpl().

Referenced by LazyResult< Func, Args >::value().

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

template<class Func , class... Args>

template<std::size_t... ArgIndices>

void LazyResult< Func, Args >::evaluateImpl ( std::index_sequence< ArgIndices...>  )

inlineprivate

Definition at line 63 of file LazyResult.h.

References LazyResult< Func, Args >::args_, LazyResult< Func, Args >::evaluated_, LazyResult< Func, Args >::func_, and LazyResult< Func, Args >::result_.

Referenced by LazyResult< Func, Args >::evaluate().

                                                      {
    result_ = func_(std::get<ArgIndices>(args_)...);
    evaluated_ = true;
  }

template<class Func , class... Args>

Result const& LazyResult< Func, Args >::value ( )

inline

Definition at line 52 of file LazyResult.h.

References LazyResult< Func, Args >::evaluate(), LazyResult< Func, Args >::evaluated_, and LazyResult< Func, Args >::result_.

Referenced by Types.int32::__nonzero__(), Types.uint32::__nonzero__(), Types.int64::__nonzero__(), Types.uint64::__nonzero__(), Types.double::__nonzero__(), Types.bool::__nonzero__(), Types.string::__nonzero__(), average.Average::average(), Types.string::configValue(), Types.FileInPath::configValue(), Mixins.UsingBlock::dumpPython(), Mixins.UsingBlock::insertInto(), Types.int32::insertInto(), Types.uint32::insertInto(), Types.int64::insertInto(), Types.uint64::insertInto(), Types.double::insertInto(), Types.bool::insertInto(), Types.string::insertInto(), Types.FileInPath::insertInto(), Types.vint32::insertInto(), Types.vuint32::insertInto(), Types.vint64::insertInto(), Types.vuint64::insertInto(), Types.vdouble::insertInto(), Types.vbool::insertInto(), and Types.vstring::insertInto().

                        {
    if (!evaluated_) {
      evaluate();
    }
    return result_;
  }

Member Data Documentation

template<class Func , class... Args>

std::tuple<Args const&...> LazyResult< Func, Args >::args_

private

Definition at line 72 of file LazyResult.h.

Referenced by LazyResult< Func, Args >::evaluateImpl(), and batchmanager.BatchManager::ParseOptions().

template<class Func , class... Args>

bool LazyResult< Func, Args >::evaluated_ = false

private

Definition at line 70 of file LazyResult.h.

Referenced by LazyResult< Func, Args >::evaluateImpl(), and LazyResult< Func, Args >::value().

template<class Func , class... Args>

Func LazyResult< Func, Args >::func_

private

Definition at line 71 of file LazyResult.h.

Referenced by LazyResult< Func, Args >::evaluateImpl().

template<class Func , class... Args>

Result LazyResult< Func, Args >::result_

private

Definition at line 73 of file LazyResult.h.

Referenced by LazyResult< Func, Args >::evaluateImpl(), and LazyResult< Func, Args >::value().