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?

Goings

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.) – Savick

Answer 1

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.

Savick

Answer 2

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>)} { }

    // ...
};

Freehold

Probably better to use std::optional<std::size_t> than std::optional<int> since sizeof gives a result of type std::size_t. – Laterite

Why do we need std::optional? Can we just use pure std::size_t? – Strangeness

@DeanSeo std::size_t seems a better alternative to me also. – Goings

@Dean what if the any is empty? – Freehold

Then the size can be 0 no? Saves a boolean, which after alignment is not nothing. – Caesar

@Caesar The size should be 1 instead? Not sure, but in case for my_any[]? – Strangeness

@VittorioRomeo Right but even if there's a std::optional used, you'd still need to branch its initialization? – Strangeness