When should I use std::any

Question

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

Answer 1

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)

Answer 2

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.