Option Three
Ok. sobering time. Cold head. Of course we know very well about the good old Function Object aka “Functor”. So let’s do a proverbial step back and use what we already know.
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/* No stunts. A plain old function object. Simple usable and working */ template<typename T> class holder { // mutable members of const class instances are modifiable mutable T default_value_{}; // in version 0.1 hide ctors, movers and copiers holder(); holder(const holder &); holder(const holder &&); holder & operator = (const holder &); holder && operator = (const holder &&); public: holder(const T & defval_) : default_value_(defval_) { }; const T & operator () () const noexcept { return default_value_; } /* The operator declaration bellow allows for const isntances to be used through the operator bellow const holder<REAL> width{ 10 }; width(1024) ; // OK */ const volatile T & operator () (const T & new_ ) const volatile noexcept { if (new_ != default_value_) default_value_ = new_; return default_value_; } }; namespace { using namespace Gdiplus; // test the const behaviour const holder<REAL> width{ 10 }; holder<SmoothingMode> smoothnes{ SmoothingMode::SmoothingModeAntiAlias }; holder<LineCap> linecap{ LineCap::LineCapRound }; inline void usage() { auto width_ = width( 1024 ); const auto w1_ = width(); auto w2_ = width(); assert(width_ == 1024); assert(w2_ == w1_ == width_); auto lc_ = linecap(); auto sm_ = smoothnes(); } } |
This is one sober solution indeed. And working too. Usage seems simple and with no surprises.
Also the default argument is solved the C++ way. Modern or not. By using () operator overload.
What also is delivered is correct behavior in case of const required. Code above is documented. In case of popular vote I can deliver more explanations.
