Just like std::optional, std::any can store an optional single value, but with the difference that it can store any type at runtime, just like a dynamically typed}language. As the std::any can withhold any type, you need to explicitly specify the type using the global function std::any_cast when reading the held object.
If the std::any is empty or withholds another type than the specified type, an exception is thrown.
Here is an example of how it works:
// Initialize an empty std::any
auto a = std::any{};
// Put a string in it
a = std::string{"something"};
// Return a reference to the withheld string
auto& str_ref = std::any_cast<std::string&>(a);
// Copy the withheld string
auto str_copy = std::any_cast<std::string>(a);
// Put a float in the 'any' and read it back
a = 135.246f;
auto flt = std::any_cast...
