Since C++17 std::any is introduced. One can now write code like this
#include <iostream>
#include <any>
#include <string>
int main () {
const double d = 1.2;
std::any var = d;
const std::string str = "Hello World";
var = str;
}
A double is assigned to the variable var and than a std::string was assigned to it.
Why has std::any been introduced?
I think this is violating the least astonishment rule, because I find it hard to think of a situation, where this can be used to express more clearly, what I like to express.
Can somebody give me a good example, when std::any is beneficial.
https://gcc.godbolt.org/z/-kepOD
When to Use
void* as an extremely unsafe pattern with some limited use cases, std::any adds type-safety, and that’s why it has some real use cases.
Some possibilities:
- In Libraries - when a library type has to hold or pass anything without knowing the
set of available types.
- Parsing files - if you really cannot specify what are the supported
types.
- Message passing.
- Bindings with a scripting language.
- Implementing an interpreter for a scripting language
- User Interface - controls might hold anything
- Entities in an editor
(ref)
I would summarize it as classic "use when you cannot avoid it".
I can only think of non-performance-critical implementations of dynamically typed scripting languages to represent variables from the scripting world, but even that with a stretch (Boost.Spirit/example/qi/compiler_tutorial does it without, for both the parser and the runtime).
For everything else from parsers (e.g. Boost.Spirit.X3) to library APIs (e.g. ASIO) there would usually be a faster/better/more-specific alternative, as very few things are really "anything", most are more specific than that.
std::variant and/or std::optional for "almost any value"std::packaged_task / std::function + lambdas for "callback with arguments", which would be a case of void* in C APIs.- etc.
Specifically, I wouldn't blindly plug it as a replacement for a void*, as it may allocate memory on the heap, which can be deadly for high performance code.
It's used in Wt, to provide a non-template interface for tabular data.
There are conversions to string for builtin and Wt types, and you can register additional conversions by specialising Wt::any_traits. This allows anything to be displayed as an entry in a table, the view classes don't have to know anything about the types they are displaying.
std::any is a vocabulary type. When you need to store, well, some bit of anything, as a value you can use it.
There are a number of "first level" uses of it:
When interacting with scripting languages which themselves have such types, it is a natural fit.
When you have a property tree with highly polymorphic content, and the structure of the tree is decoupled from the producer and consumer of the tree.
When replacing the equivalent of a void* chunk of data being passed through an intermediate layer who really doesn't care what it is carrying.
It can also be used as a building block in other cases. For example, std::function could choose to store its value in the std::any:
template<class R, class...Args>
struct func<R(Args...)> {
mutable std::any state;
R(*f)(std::any& state, Args&&...) = nullptr;
template<class T>
void bind(T&& t) {
state = std::forward<T>(t);
f = [](std::any& state, Args&&...args)->R {
return std::any_cast<T&>(state)(std::forward<Args>(args)...);
};
}
R operator()(Args...args)const {
return f(state, std::forward<Args>(args)...);
}
};
that is a pretty small implementation of (most of) std::function. Basically I've used any to type erase copy/move/destroy.
You can using this elsewhere for similar problems (where you are type-erasing some operation and also want to type erase copy/move/destroy), or generalize it.
