How do I support any_cast to cast a custom class to a string?

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This question is related to Type erasing type erasure, `any` questions?

Given the class wstr below (just a string wrapper)

class wstr { public: std::string text; };

I have an any...

std::any myAny({myWstr});

..and I want to cast (convert) it to a string..

std::string myString = std::any_cast<std::string>(myAny);

... Is there a means of doing this by using template specialization, or (as I suspect) is this missing the point of using std::any?

This question is not about implicit conversion. I was thinking that I may need to write a friend function/overloading the cast, similar to writing ostream operator extensions.

Another way of asking this would be: Am I correct in thinking an std::any_cast does NOT CAST to anything, but rather ONLY CASTS a std::any back to it's original form, and therefore one cannot overload a function that supports the cast to eg, std::string, and is not available (for some reason) for friend function overloading / template specialization?

wstr myWstr("foo"); std::any myAny({myWstr}); wstr myWstr = std::any_cast<wstr>(myAny); //is okay. std::string mMytr = std::any_cast<std::string>(myAny); //No overloads!

See my question on the subject a while back. You cannot any_cast to another type. — Timo
– Timo, Commented Jan 22, 2019 at 12:17
std::any<wstr> is wrong, because std::any isn't a template. Are you perhaps thinking about std::variant or std::optional? — Some programmer dude
– Some programmer dude, Commented Jan 22, 2019 at 12:19
afaik any_cast is not really to cast between different types but merely to access the value stored in the any in a typesafe manner — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Jan 22, 2019 at 12:26
It does indeed look like you are missing the point of std::any. What are you trying to achieve exactly? And what do you think std::any does? — Dan M.
– Dan M., Commented Jan 22, 2019 at 13:10
@Konchog: The thing is, you shouldn't want to "overload/specialize" any_cast. If that's what you want to do, then you are probably using any outside of its intended use case and purpose. — Nicol Bolas
– Nicol Bolas, Commented Jan 22, 2019 at 14:24

P.W · Accepted Answer · 2019-01-22 12:36:53Z

The example given for any_cast at cppreference shows that casting an int to a std::string results in bad_any_cast exception being thrown.

The result will be the same if any_cast is used to convert wstr to std::string.

And BTW, std::any<wstr> myAny; does not work, since std::any is a class and not a template.

Instead you can test it this way:

#include <string> #include <iostream> #include <any> class wstr { public: std::string text; }; int main() { wstr ws; auto a = std::any(ws); try { std::cout << std::any_cast<std::string>(a) << '\n'; } catch(const std::bad_any_cast& e) { std::cout << e.what() << '\n'; } }

See demo here.

Thanks, but due to the poverty of my initial question, this misses the point.. the point I was making was concerning extending/overloading std::any_cast, not expecting it to do the cast as it stands.
@Konchog: then you will have to write your own conversion function, constructor or operator. std::any_cast is not the way to do it.

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