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I know there is a question about std::any vs std::variant, but std::optional is also about a same thing. C++ std::variant vs std::any
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The class template std::variant represents a type-safe union. An instance of std::variant at any given time either holds a value of one of its alternative types, or in the case of error - no value (this state is hard to achieve, see valueless_by_exception).
std::optional contains the object within itself, depending on where it is stored (stack/data/heap) std::optional makes a copy of the contained object. Monadic functions will be added in C++23 to improve the abstraction in our code by removing the needs of writing boilerplate code.
(since C++17) The class template std::optional manages an optional contained value, i.e. a value that may or may not be present. A common use case for optional is the return value of a function that may fail.
The class any describes a type-safe container for single values of any copy constructible type. 1) An object of class any stores an instance of any type that satisfies the constructor requirements or is empty, and this is referred to as the state of the class any object.
I have a very general rule:
Every time you want to use a union use std::variant.
Every time you want to use a void* use std::any.
Every time you want to return nullptr as an indication of an error use std::optional.
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