<sizeof(T)> as inherited class template parameter in c++ -

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i've been trying make transition simple self-made game engine written in c c++ , came across nice book mike mcshaffry (game coding complete). , love of bios almighty cannot wrap head around 'optional template class', namely piece of code:

class optional_empty { };  template <unsigned long size> class optional_base { public:      // code here };  template <class t> class optional : public optional_base<sizeof(t)> { public:      // code here };

why 'sizeof(t)' used template parameter when inheriting, , not plain 't'? make possible have t's of dynamic size? there no 'sizeof...' operator, it's not related variadic templates. or it? thanks.

because optional_base not expecting template type parameter template value parameter (of type unsigned long).


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