std::any vs std::variant

std::any and std::variant are both ways to create a variable that can hold different types, but they work quite differently:

std::any can hold a value of any single type, and the type can be decided at runtime. It's like a type-safe void*.

1std::any a = 42;
2a = "hello";// OK, a now holds a string

std::variant holds a value that can be one of a fixed set of types, which must be specified at compile-time.

1std::variant<int, string> v = 42;
2v = "hello"; // OK, string is one of the types
3v = 3.14; // Error: double is not one of the types

Use std::any when:

• The type really can't be known at compile-time
• You need to store any single value, but the type might change

Use std::variant when:

• The type can be one of a limited set of known types
• You need to store multiple types in the same variable or container
• You want the type-safety and performance benefits of knowing the types at compile-time

Generally, prefer std::variant where possible. It offers better performance and type-safety. Only use std::any when the flexibility of runtime typing is truly necessary.