Get the size of std::any

Question

Is there any way to get the size (in bytes) of the data stored by std::any? The only workaround I came up with is querying the type of its value by std::any::type and comparing the result to a list of known types like my_any.type() == typeid(T), then the size is sizeof(T). Unfortunately, this solution only works when the types are known beforehand.

Do you know any solution?

This question is clear and well asked, regardless of the comments, and does not deserve such down voting. (Giving a need for `any::size()` does not much improve the question, tough it may help to expose [XY problems](https://en.wikipedia.org/wiki/XY_problem).) — Walter, Apr 09 '18 at 10:44

Vittorio Romeo · Answer 1 · 2018-04-09T22:24:39.553

6

std::any does not provide any way of performing an action on the underlying stored type T. However, information like sizeof(T) could be available when initializing/assigning to std::any.

One possible solution is creating your own wrapper around std::any that keeps track of the size. E.g.

class my_any : std::any
{ 
    std::size_t _stored_size = 0;

public:
    template <typename T>
    my_any(T&&) : _stored_size{sizeof(std::decay_t<T>)} { }

    // ...
};

edited Apr 09 '18 at 22:24

answered Apr 09 '18 at 10:34

Vittorio Romeo

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Probably better to use `std::optional` than `std::optional` since `sizeof` gives a result of type `std::size_t`. – Peter Apr 09 '18 at 10:39
1

Why do we need `std::optional`? Can we just use pure `std::size_t`? – Dean Seo Apr 09 '18 at 10:41
@DeanSeo `std::size_t` seems a better alternative to me also. – plasmacel Apr 09 '18 at 10:44
@Dean what if the any is empty? – Vittorio Romeo Apr 09 '18 at 14:16
Then the size can be 0 no? Saves a boolean, which after alignment is not nothing. – rustyx Apr 09 '18 at 21:47
@rustyx The size should be `1` instead? Not sure, but in case for `my_any[]`? – Dean Seo Apr 10 '18 at 05:13
@VittorioRomeo Right but even if there's a `std::optional` used, you'd still need to branch its initialization? – Dean Seo Apr 10 '18 at 05:15

score 6 · Accepted Answer · edited Apr 09 '18 at 11:31

You cannot get the size of the object held by std::any (other than the workaround you've mentioned). std::any is a minimal implementation of type erasure.

If you want something more powerful, write it yourself (it's not hard, just model it on std::any or boost::any and add the size() functionality). There are many other things you may want to add to any, such as I/O, storing multiple data (or the contents of any container) as arrays, etc..

At the expense of an additional data member (to store the size), you may also extend std::any by writing a wrapper, as suggested in Vittorio's answer.

Get the size of std::any

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