How can I modify my boost::any like class

Question

I'm trying to implement boost::any like class:

struct any { private: struct holderBase { virtual ~holderBase(){} }; template<typename T> struct holder : public holderBase { T content; holder(const T& value) : content(value){} holder(const holder<T>& other) : content(other.content){} }; holderBase *hl; public: template<typename T> any(const T& data = T()) { hl = new holder<T>(data); } any(const any& other) { hl = other.hl; } template<typename T> T get() { if(holder<T>* p_hl = dynamic_cast<holder<T>*>(hl)) return p_hl->content; else throw std::runtime_error("std::runtime_error"); } };

I use a holder class (inherited by holderBase) to store the data. How can I modify the any::get() function (or even modify the whole code) so that it doesn't need a template parameter (the get() function)?

Richard J. Ross III · Accepted Answer · 2012-05-03 15:46:51Z

You could do it like this:

template<typename T> T get(T *ptr);

Similar to the C time function, you would return the result, as well as store it in ptr.

Edit: You could also override the casting operator:

template<typename T> operator T() { return get<T>(); }

Which will implicitly do what you want.

+1 for the first option, I would avoid the second at all costs. Implicit conversions are a bad idea in general, and implicit conversions to any type are the worst of them.

David · Accepted Answer · 2012-05-03 15:42:49Z

Stating the obvious: If you don't want to return 1 particular type to the user then it needs to be templated. There's nothing you can do about it.

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