include/o3tl/lazy_update.hxx

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  19
  20 #ifndef INCLUDED_O3TL_LAZY_UPDATE_HXX
  21 #define INCLUDED_O3TL_LAZY_UPDATE_HXX
  22
  23 #include <utility>
  24
  25 namespace o3tl
  26 {
  27     /** Update output object lazily
  28
  29         This template collects data in input type, and updates the
  30         output type with the given update functor, but only if the
  31         output is requested. Useful if updating is expensive, or input
  32         changes frequently, but output is only comparatively seldom
  33         used.
  34
  35         @example
  36         <pre>
  37 LazyUpdate<InType,OutType,decltype(F)> myValue(F);
  38 *myValue = newInput;
  39 myValue->updateInput( this, that, those );
  40
  41 output( *myValue );
  42         </pre>
  43         or
  44         <pre>
  45 output( myValue.getOutValue() );
  46         </pre>
  47         if the compiler does not recognize the const context.
  48      */
  49     template<typename In, typename Out, typename Func> class LazyUpdate {
  50     public:
  51         LazyUpdate(Func const & func): func_(func), input_(), dirty_(true) {}
  52
  53         In const & getInValue() const { return input_; }
  54
  55         Out const & getOutValue() const { return update(); }
  56
  57         In & operator *() {
  58             dirty_ = true;
  59             return input_;
  60         }
  61
  62         In * operator ->() {
  63             dirty_ = true;
  64             return &input_;
  65         }
  66
  67         Out const & operator *() const { return update();  }
  68
  69         Out const * operator ->() const { return &update(); }
  70
  71     private:
  72         Out const & update() const {
  73             if (dirty_) {
  74                 output_ = func_(input_);
  75                 dirty_ = false;
  76             }
  77             return output_;
  78         }
  79
  80         Func const func_;
  81         In input_;
  82         mutable Out output_;
  83         mutable bool dirty_;
  84     };
  85 }
  86
  87 #endif
  88
  89 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */