llvm/llvm/include/llvm/Support/ExtensibleRTTI.h

//===-- llvm/Support/ExtensibleRTTI.h - ExtensibleRTTI support --*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// \file
//
// Defines an extensible RTTI mechanism designed to work with Casting.h.
//
// Extensible RTTI differs from LLVM's primary RTTI mechanism (see
// llvm.org/docs/HowToSetUpLLVMStyleRTTI.html) by supporting open type
// hierarchies, where new types can be added from outside libraries without
// needing to change existing code. LLVM's primary RTTI mechanism should be
// preferred where possible, but where open hierarchies are needed this system
// can be used.
//
// The RTTIRoot class defines methods for comparing type ids. Implementations
// of these methods can be injected into new classes using the RTTIExtends
// class template.
//
// E.g.
//
//   @code{.cpp}
//   class MyBaseClass : public RTTIExtends<MyBaseClass, RTTIRoot> {
//   public:
//     static char ID;
//     virtual void foo() = 0;
//   };
//
//   class MyDerivedClass1 : public RTTIExtends<MyDerivedClass1, MyBaseClass> {
//   public:
//     static char ID;
//     void foo() override {}
//   };
//
//   class MyDerivedClass2 : public RTTIExtends<MyDerivedClass2, MyBaseClass> {
//   public:
//     static char ID;
//     void foo() override {}
//   };
//
//   char MyBaseClass::ID = 0;
//   char MyDerivedClass1::ID = 0;
//   char MyDerivedClass2:: ID = 0;
//
//   void fn() {
//     std::unique_ptr<MyBaseClass> B = llvm::make_unique<MyDerivedClass1>();
//     llvm::outs() << isa<MyBaseClass>(B) << "\n"; // Outputs "1".
//     llvm::outs() << isa<MyDerivedClass1>(B) << "\n"; // Outputs "1".
//     llvm::outs() << isa<MyDerivedClass2>(B) << "\n"; // Outputs "0'.
//   }
//
//   @endcode
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_SUPPORT_EXTENSIBLERTTI_H
#define LLVM_SUPPORT_EXTENSIBLERTTI_H

namespace llvm {

/// Base class for the extensible RTTI hierarchy.
///
/// This class defines virtual methods, dynamicClassID and isA, that enable
/// type comparisons.
class RTTIRoot { … };

/// Inheritance utility for extensible RTTI.
///
/// Supports single inheritance only: A class can only have one
/// ExtensibleRTTI-parent (i.e. a parent for which the isa<> test will work),
/// though it can have many non-ExtensibleRTTI parents.
///
/// RTTIExtents uses CRTP so the first template argument to RTTIExtends is the
/// newly introduced type, and the *second* argument is the parent class.
///
/// class MyType : public RTTIExtends<MyType, RTTIRoot> {
/// public:
///   static char ID;
/// };
///
/// class MyDerivedType : public RTTIExtends<MyDerivedType, MyType> {
/// public:
///   static char ID;
/// };
///
template <typename ThisT, typename ParentT>
class RTTIExtends : public ParentT { … };

} // end namespace llvm

#endif // LLVM_SUPPORT_EXTENSIBLERTTI_H