.. sectionauthor:: Sean Silva <[email protected]>
LLVM avoids using C++'s built in RTTI. Instead, it pervasively uses its own hand-rolled form of RTTI which is much more efficient and flexible, although it requires a bit more work from you as a class author.
A description of how to use LLVM-style RTTI from a client's perspective is given in the Programmer's Manual. This document, in contrast, discusses the steps you need to take as a class hierarchy author to make LLVM-style RTTI available to your clients.
Before diving in, make sure that you are familiar with the Object Oriented Programming concept of "is-a".
This section describes how to set up the most basic form of LLVM-style RTTI (which is sufficient for 99.9% of the cases). We will set up LLVM-style RTTI for this class hierarchy:
class Shape {
public:
Shape() {}
virtual double computeArea() = 0;
};
class Square : public Shape {
double SideLength;
public:
Square(double S) : SideLength(S) {}
double computeArea() /* override */;
};
class Circle : public Shape {
double Radius;
public:
Circle(double R) : Radius(R) {}
double computeArea() /* override */;
};
The most basic working setup for LLVM-style RTTI requires the following steps:
In the header where you declare
Shape
, you will want to#include "llvm/Support/Casting.h"
, which declares LLVM's RTTI templates. That way your clients don't even have to think about it.#include "llvm/Support/Casting.h"
In the base class, introduce an enum which discriminates all of the different classes in the hierarchy, and stash the enum value somewhere in the base class.
Here is the code after introducing this change:
class Shape { public: + /// Discriminator for LLVM-style RTTI (dyn_cast<> et al.) + enum ShapeKind { + SquareKind, + CircleKind + }; +private: + const ShapeKind Kind; +public: + ShapeKind getKind() const { return Kind; } + Shape() {} virtual double computeArea() = 0; };
You will usually want to keep the
Kind
member encapsulated and private, but let the enumShapeKind
be public along with providing agetKind()
method. This is convenient for clients so that they can do aswitch
over the enum.A common naming convention is that these enums are "kind"s, to avoid ambiguity with the words "type" or "class" which have overloaded meanings in many contexts within LLVM. Sometimes there will be a natural name for it, like "opcode". Don't bikeshed over this; when in doubt use
Kind
.You might wonder why the
Kind
enum doesn't have an entry forShape
. The reason for this is that sinceShape
is abstract (computeArea() = 0;
), you will never actually have non-derived instances of exactly that class (only subclasses). See Concrete Bases and Deeper Hierarchies for information on how to deal with non-abstract bases. It's worth mentioning here that unlikedynamic_cast<>
, LLVM-style RTTI can be used (and is often used) for classes that don't have v-tables.Next, you need to make sure that the
Kind
gets initialized to the value corresponding to the dynamic type of the class. Typically, you will want to have it be an argument to the constructor of the base class, and then pass in the respectiveXXXKind
from subclass constructors.Here is the code after that change:
class Shape { public: /// Discriminator for LLVM-style RTTI (dyn_cast<> et al.) enum ShapeKind { SquareKind, CircleKind }; private: const ShapeKind Kind; public: ShapeKind getKind() const { return Kind; } - Shape() {} + Shape(ShapeKind K) : Kind(K) {} virtual double computeArea() = 0; }; class Square : public Shape { double SideLength; public: - Square(double S) : SideLength(S) {} + Square(double S) : Shape(SquareKind), SideLength(S) {} double computeArea() /* override */; }; class Circle : public Shape { double Radius; public: - Circle(double R) : Radius(R) {} + Circle(double R) : Shape(CircleKind), Radius(R) {} double computeArea() /* override */; };
Finally, you need to inform LLVM's RTTI templates how to dynamically determine the type of a class (i.e. whether the
isa<>
/dyn_cast<>
should succeed). The default "99.9% of use cases" way to accomplish this is through a small static member functionclassof
. In order to have proper context for an explanation, we will display this code first, and then below describe each part:class Shape { public: /// Discriminator for LLVM-style RTTI (dyn_cast<> et al.) enum ShapeKind { SquareKind, CircleKind }; private: const ShapeKind Kind; public: ShapeKind getKind() const { return Kind; } Shape(ShapeKind K) : Kind(K) {} virtual double computeArea() = 0; + + static bool classof(const Shape *) { return true; } }; class Square : public Shape { double SideLength; public: Square(double S) : Shape(SquareKind), SideLength(S) {} double computeArea() /* override */; + + static bool classof(const Square *) { return true; } + static bool classof(const Shape *S) { + return S->getKind() == SquareKind; + } }; class Circle : public Shape { double Radius; public: Circle(double R) : Shape(CircleKind), Radius(R) {} double computeArea() /* override */; + + static bool classof(const Circle *) { return true; } + static bool classof(const Shape *S) { + return S->getKind() == CircleKind; + } };
Basically, the job of
classof
is to returntrue
if its argument is of the enclosing class's type. As you can see, there are two general overloads ofclassof
in use here.- The first, which just returns
true
, means that if we know that the argument of the cast is of the enclosing type at compile time, then we don't need to bother to check anything since we already know that the type is convertible. This is an optimization for the case that we statically know the conversion is OK. - The other overload takes a pointer to an object of the base of the
class hierarchy: this is the "general case" of the cast. We need to
check the
Kind
to dynamically decide if the argument is of (or derived from) the enclosing type.
To be more precise, let
classof
be inside a classC
. Then the contract forclassof
is "returntrue
if the argument is-aC
". As long as your implementation fulfills this contract, you can tweak and optimize it as much as you want.- The first, which just returns
Although for this small example setting up LLVM-style RTTI seems like a lot of "boilerplate", if your classes are doing anything interesting then this will end up being a tiny fraction of the code.
For concrete bases (i.e. non-abstract interior nodes of the inheritance
tree), the Kind
check inside classof
needs to be a bit more
complicated. Say that SpecialSquare
and OtherSpecialSquare
derive
from Square
, and so ShapeKind
becomes:
enum ShapeKind {
SquareKind,
+ SpecialSquareKind,
+ OtherSpecialSquareKind,
CircleKind
}
Then in Square
, we would need to modify the classof
like so:
static bool classof(const Square *) { return true; }
- static bool classof(const Shape *S) {
- return S->getKind() == SquareKind;
- }
+ static bool classof(const Shape *S) {
+ return S->getKind() >= SquareKind &&
+ S->getKind() <= OtherSpecialSquareKind;
+ }
The reason that we need to test a range like this instead of just equality
is that both SpecialSquare
and OtherSpecialSquare
"is-a"
Square
, and so classof
needs to return true
for them.
This approach can be made to scale to arbitrarily deep hierarchies. The trick is that you arrange the enum values so that they correspond to a preorder traversal of the class hierarchy tree. With that arrangement, all subclass tests can be done with two comparisons as shown above. If you just list the class hierarchy like a list of bullet points, you'll get the ordering right:
| Shape | Square | SpecialSquare | OtherSpecialSquare | Circle
.. TODO:: Touch on some of the more advanced features, like ``isa_impl`` and ``simplify_type``. However, those two need reference documentation in the form of doxygen comments as well. We need the doxygen so that we can say "for full details, see http://llvm.org/doxygen/..."