How is LLVM isa<> implemented?

Question

From http://llvm.org/docs/CodingStandards.html#ci_rtti_exceptions

LLVM does make extensive use of a hand-rolled form of RTTI that use templates li

Answer 1

Just adding stuff to osgx's answer: basically each class should implement classof() method which does all the necessary stuff. For example, the Value's classof() routine looks like this:

  // Methods for support type inquiry through isa, cast, and dyn_cast:
  static inline bool classof(const Value *) {
    return true; // Values are always values.
  }

To check whether we have a class of the appropriate type, each class has it's unique ValueID. You can check the full list of ValueID's inside the include/llvm/Value.h file. This ValueID is used as follows (excerpt from Function.h):

  /// Methods for support type inquiry through isa, cast, and dyn_cast:
  static inline bool classof(const Function *) { return true; }
  static inline bool classof(const Value *V) {
    return V->getValueID() == Value::FunctionVal;
  }

So, in short: every class should implement classof() method which performs the necessary decision. The implementation in question consists of the set of unique ValueIDs. Thus in order to implement classof() one should just compare the ValueID of the argument with own ValueID.

If I remember correctly, the first implementation of isa<> and friends were adopted from boost ~10 years ago. Right now the implementations diverge significantly :)