I recently came across the std::any class, introduced in C++17, based on boost::any. This class can "hold an instance of any type" and auto automatically deduces the data type of a variable.
So what is the main difference? What are the pros and cons?
std::any and auto are completely different constructs.
std::any is a container type that can hold an object of any type:
std::any a = 42; // a holds an int, but type is std::any
a = std::string{"hi"}; // ok, a holds a string now
The type of the object held by std::any can change during the execution of the program.
auto is a keyword that designates a placeholder type. The type of a variable with auto is the type of the value used to initialize the variable:
auto a = 42; // a is int, for the entirety of the program
a = std::string{"hi"}; // error, a has type int
This type is determined statically, i.e. at compile time, and can never change during the execution of the program.
These constructs are not interchangeable, and so they have different use cases, and you can't compare the pros and cons of one versus the other meaningfully.