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

I should mention that http://llvm.org/docs/ProgrammersManual.html#isa - this document have some additional description.

The source code of isa, cast and dyn_cast is located in single file, and commented a lot.

http://llvm.org/doxygen/Casting_8h_source.html

00047 // isa - Return true if the parameter to the template is an instance of the
00048 // template type argument.  Used like this:
00049 //
00050 //  if (isa(myVal)) { ... }
00051 //
00052 template 
00053 struct isa_impl {
00054   static inline bool doit(const From &Val) {
00055     return To::classof(&Val);
00056   }
00057 };

00193 // cast - Return the argument parameter cast to the specified type.  This
00194 // casting operator asserts that the type is correct, so it does not return null
00195 // on failure.  It does not allow a null argument (use cast_or_null for that).
00196 // It is typically used like this:
00197 //
00198 //  cast(myVal)->getParent()
00199 //
00200 template 
00201 inline typename cast_retty::ret_type cast(const Y &Val) {
00202   assert(isa(Val) && "cast() argument of incompatible type!");
00203   return cast_convert_val::SimpleType>::doit(Val);
00205 }

00218 // dyn_cast - Return the argument parameter cast to the specified type.  This
00219 // casting operator returns null if the argument is of the wrong type, so it can
00220 // be used to test for a type as well as cast if successful.  This should be
00221 // used in the context of an if statement like this:
00222 //
00223 //  if (const Instruction *I = dyn_cast(myVal)) { ... }
00224 //
00225 
00226 template 
00227 inline typename cast_retty::ret_type dyn_cast(const Y &Val) {
00228   return isa(Val) ? cast(Val) : 0;
00229 }