Metaclasses and Reflection in C++

Metaclasses and Reflection in C++

By Detlef Vollmann

Overload, 9(45):, October 2001

Over the last two years I've held several tutorials on meta-classes and reflection in C++. I use the term reflection here in its original sense, "looking back to oneself", nowadays sometimes called introspection. The more general process to allow modifications at class level at run-time is the old task to provide a meta-level, but is today sometimes (mis-)called "behavioural reflection" or "structural reflection".

In some sense this article presents work in progress, though it's been going on for nearly ten years now. It's not the definitive meta-object protocol for C++, but more a presentation of lesser-known C++ techniques to solve some specific design problems.


C++ is a strongly typed compiler language. Though not as strongly typed as ADA, a C++ compiler will complain if you try to assign an object of one type to an object of another type (if there is no acceptable conversion). Obviously, this requires that the compiler know all available types. More specifically, all classes must be known at compile-time [ 1 ] . But sometimes, it would be quite handy to add new classes at runtime. And in some application domains, this is absolutely necessary.

A simple story

Let's look at a simple example: Susan, the manager of a local bookstore, wants to expand into the Internet. So she asks you to write a simple program for an Internet bookshop. No problem for you. Part of your solution will probably look like the class model in Figure 1.

Figure 1. Simple Shop Model

Simple Shop Model

The implementation of this in C++ is straightforward. Here is the Book class:

class Book
  Book(const string & author_,
    const string & title_,
    const string & publisher_,
    double price_,
    double weight_);
  string getName()
    string name;
    name = author + ": " + title;
    return name.substr(0, 40);
  double getPrice();
  double getWeight();
  string author, title, publisher;
  double price, weight;

Your solution works, Susan is happy, and all is fine for a while... But changes come on the Web in Internet time: the bookshop is a success and Susan decides to sell CDs as well. So you have to change your program. With object orientation, you can do this quite easily and your modified class model will look like Figure. 2.

Figure 2. Product Model

Product Model

As you probably guessed, this was only the beginning. Some time later, Susan wants to sell Pop music accessories like T-shirts, posters, etc. as well.

Now it is clear that it is not acceptable to modify the source code of your program every time a new product category is introduced. So you start to think about the actual requirements, and find that you need to provide different interfaces for your Product class (Figure. 3): A simple interface for ShoppingCart providing getName() , getPrice() , and getWeight() . This is what you already have. Then you need a different interface for a general search machine [ 2 ] , which must provide information like:

  • what is the actual class of the object

  • what attributes does that class have

  • what are the actual values of these attributes for the object.

This is a classic reflection interface that gives you information about the properties of classes and objects. But you also need a third interface for product maintenance that allows you to define new product classes, specify the attributes for them, create instances of these classes, and set the attribute values of these instances. Such an interface is called a "Meta-Object Protocol (MOP)" and thoroughly discussed in [ Kiczales- ]. The reflection protocol is a subset of such a MOP.

Figure 3. A Better Model

A Better Model

Meta Classes for C++

What is the meaning of "Meta-Object Protocol"? Well, meta-information is information about something else seen from a level beyond - a meta-level. So, information about the attribute values of an object, say, is information on the object level. But information about the properties of the object itself, about its attributes, its structure, etc. is meta-information.

In C++, this information is captured in the class definition for the object, so the class is a meta-object. And in C++, you have all the functionality of a MOP at class level - which is at development time. But that level is not available at runtime: You cannot manipulate classes like objects, you cannot add new classes at runtime. The idea of a MOP is to collapse the meta-level (classes) and the object level (objects); i.e. make the class definitions normal objects and the object properties are normal attribute values of the class definitions that can be manipulated at runtime.

While languages like CLOS or Smalltalk provide this combined level directly, C++ as a strongly typed compiled language has no such features. So, what can you do about it? The typical solution is to provide a MOP yourself, as proposed e.g. in [ Coplien ] or [ Buschmann- ].

MOP Overview

For simplicity, we ignore methods for now, so our MOP must provide:

  • definition of new classes

  • adding attributes to classes

  • querying attributes of classes

  • creating objects

  • querying the class of an object

  • setting attribute values of an object

  • querying attribute values of an object

  • deleting objects

If you use an old rule of OO design, you take all the nouns of the above requirements and make classes out of them. When you think about the "attributes" and "values" you have to decide whether they are typed. As the underlying language C++ is typed this should be mirrored in your design.

Another question is about inheritance support. For our example with the Internet shop and a product hierarchy this would probably quite useful. So, a first class model is shown in Figure 4.

Figure 4. MOP Class Model

MOP Class Model


While it is useful to go top-down for a general overview, it's easier to start with the simple basic things for the details. So we'll first look at Type .

The main purpose of Type is to distinguish different kinds of Attribute s. For this, a simple enum would suffice. But the idea of types is to have different kind of Values for different Type s, so the Type should create new Values . So we put the enum inside the Type class, provide the newValue() method, and get the interface shown in Figure. 4.

Now for implementation. Although we don't look closely at Value right now, if we have different kinds of values we probably need some base class for them. Let's call it " BaseValue ", and newValue() can just return a pointer to BaseValue .

Now we know what to create, but how? While there are several patterns to implement polymorphic creation [4], the simplest one for our purposes is probably the Prototype, which can be easily implemented with a simple static vector [ 3 ] .

Now we have everything to implement the entire class:

class Type { public: enum TypeT {stringT, intT, doubleT, unknownT}; explicit Type(TypeT typeId_) : typeId(typeId_) {} BaseValue * newValue() const { return prototypes[typeId]->clone(); } TypeT getType() const { return typeId; } static void init(); { prototypes[stringT] = new Value<string>(""); prototypes[intT] = new Value<int>(0); prototypes[doubleT] = new Value<double>(0); } private: TypeT typeId; static vectorBaseValue * prototypes; }; vector<BaseValue *> Type::prototypes(Type::unknownT);


As we have decided to create new Value s through Type , Attribute contains only a name and type, so here is its implementation:

class Attribute { public: Attribute(const string & name_, Type::TypeT typeId) : name(name_), type_(typeId) {} const string & getName() const { return name; } Type getType() const { return type_; } private: string name; Type type_; };


We can now finish the class (meta-) level of our model by looking at Class itself. As multiple inheritance is probably not an issue for our purposes, we have just one pointer to a base class (which can be 0). The more important question is the attribute list: Should it hold only the attributes defined for this class or should it include all inherited attributes? While for the actual object value access a complete list is more useful (and much faster), for class maintenance it might be important to know which attribute was defined in which class. So we just keep both. For the complete list, the order might be significant: should the own attributes come first or the inherited ones? In most illustrations the inherited attributes come first, so we keep this order as well.

What happens if the name of an attribute is the same as the name of an inherited attribute? As C++ allows it, we can allow it as well (saving us some extra effort to check this), but in this case we must guarantee that on lookup of an attribute by name we get the most derived attribute. So, findAttribute() must do a reverse search. What shall we return from findAttribute() ? The STL way would be to return an iterator, but for applications with GUIs (to create new objects and assign values to its attributes based on selection lists) an index-based access to the attributes will be more appropriate. So findAttribute() returns an index and getAttribute() takes an index and returns an Attribute. So the Attribute lists need to be indexed containers, so we choose vectors for them.

A major purpose of Class is to create Objects from it, so it has a method newObject() which returns a pointer to an Object. Do we need to keep a repository with references to all created objects? For a full reflection interface we should do this. But for actual applications this is almost never useful, as objects of the same class are created for completely different purposes. But do we need the repository for internal use? It depends on what we want to do with objects after they were created. This leads directly to another important decision: What do we do with already existing objects if we add a new attribute to a class? One option is to add this attribute to all existing objects and assign it a default value. The other option is to leave these existing objects and add the new attribute only to new objects. This leads to differently structured objects of the same class at the same time, and then we must add some version information to the objects. But there is a third option: To forbid the modification of a class definition once an instance of that class was created. This is the easiest option, so we adopt it for our MOP and add a flag definitionFix . With that flag, we can skip the object repository.

A last design question is when to add the attributes: At creation time of a class definition (through the constructor) or later (with a member function)? For different applications both options might be useful, so we'll provide two constructors and addAttribute() .

Now you can implement this [ 4 ] :

class ClassDef { //typedefs Container, Iterator for attributes public: ClassDef(ClassDef const * base, const string & name_) : baseClass(base), name(name_), definitionFix(false) { baseInit(); effectiveAttributes.insert(effectiveAttributes.end(), ownAttributes.begin(), ownAttributes.end()); } template <typename iterator> ClassDef(ClassDef const * base, const string & name_, iterator attribBegin, iterator attribEnd) : baseClass(base), name(name_), ownAttributes(attribBegin, attribEnd), definitionFix(false) { baseInit(); effectiveAttributes.insert(effectiveAttributes.end(), ownAttributes.begin(), ownAttributes.end()); } string getName() const; Object * newObject() const { definitionFix = true; return new Object(this); } AttrIterator attribBegin() const; AttrIterator attribEnd() const; Attribute const & getAttribute(size_t idx) const; void addAttribute(const Attribute &); size_t getAttributeCount() const; size_t findAttribute(string const & name) const { // this does a reverse search to find the most derived AttributeContainer::const_reverse_iterator i; for (i = effectiveAttributes.rbegin(); i != effectiveAttributes.rend(); ++i) { if (i-getName() == name) { return distance(i, effectiveAttributes.rend()) - 1; } } return getAttributeCount(); } private: void baseInit() { if (baseClass) { baseClass-definitionFix = true; copy(baseClass->attribBegin(), baseClass->attribEnd(), back_inserter<AttributeContainer>(effectiveAttributes)); } } ClassDef const * const baseClass; string name; AttributeContainer ownAttributes, effectiveAttributes; mutable bool definitionFix; };

Figure 5. Value Model

Value Model


Before we can design Object , we have to think about Value . We need a common interface to manage them, which we already called BaseValue . But what interface do we need? The whole idea of Value is to store values, so we need a set() function. What parameter? The only thing we have is BaseValue , so that's the parameter type. Pass by value, by reference, or by pointer? Definitely not by value, as BaseValue is only an interface. On the other hand, what you pass is a value, so the parameter passing should be by value to let you pass temporaries. So one option would be to pass by const reference. But though this helps for the problem at hand, it doesn't cure the fundamental problem: you should have a value, but all you have is a polymorphic interface. The real solution here is the pimpl idiom, also known as Cheshire Cat , Envelope/Letter , or more generally Handle/Body . So we add a handle class, name it Value , and look at it later again. For now we're still at BaseValue .

We now have set(Value) , so what about get() ? The return type of get() would be Value , and the implementation would look like:

Value BaseValue::get() { return *this; // calls Value(BaseValue const &) }

But that we can do directly, so get() doesn't make much sense. What other BaseValue functions do we need? Values must be copied, so we add clone() . That's all what we really need from a value, but we add asString() for convenience [ 5 ] .

class BaseValue { public: virtual ~BaseValue(){} virtual BaseValue * clone() const = 0; virtual string asString() const = 0; // fromString() virtual void set(Value const & v) = 0; // no get()! private: // Type info };


Now, as we have the interface, what about the implementation? We need values for int , double , string , etc. And an int value must hold an int , a double value a double , etc. This looks like an opportunity for a template. So, let's define RealValue<T >, derive it from BaseValue , implement the inherited interface, and we're nearly done. But as RealValue<T> is just a wrapper around T with some additional functionality, but essentially still a T , we should provide conversion in both directions, by providing a converting constructor and a conversion operator.

template <typename PlainT> class RealValue : public BaseValue { public: RealValue(PlainT v) : val(v) {} RealValue * clone() const { return new RealValue(*this); } string asString() const { ostringstream os; os << val; return os.str(); } operator PlainT() const // conversion to plain type { return val; } RealValue<PlainT>::set(Value const & v) { val = v.get<PlainT>(); } private: PlainT val; };

A note about RealValue : As we have conversion in both directions, we can use RealValue<T> like T :

RealValueint i = 1; int j = i; RealValuedouble d = i + 5.2 / (i*2); cout << d << endl;

Nearly: the following doesn't work:

RealValue<string> name, author = "Bjarne", title = "The C++ PL"; name = author + ": " + title; cout << name << endl;

The reason is that the compiler only applies one user-defined conversion, but for string literals, you need two: from char const * to string, and from string to RealValue<string> . If you want to work with RealValue<string> outside the MOP, you should define a specialization:

template <> class RealValue<string> : public BaseValue, public string { public: RealValue(string const & s) : string(s) {} RealValue(char const * s) : string(s) {} RealValue() {} RealValue * clone() const { return new RealValue(*this); } string asString() const { return static_cast<string>(*this); } // no operator string(), conversion to base automatically void set(Value const & v) { string::operator=(v.get<string>()); } };

Note: Actually, its not really clean to derive RealValue<string> from std::string , but as long as you don't delete a RealValue<string> through a pointer to string , it will work.

Value handle

Now back to the handle class Value . As a handle class, it contains its body and cares for it. Its main job is to adopt/create and to delete its body. And it mirrors the interface of the body and forwards all messages. But it should also be a real value class, thus providing default and copy constructor and assignment. But how do we implement the default constructor? As we don't know what type to create, we must create an empty handle without a body and check before forwarding if we actually have something to forward to. The assignment is essentially the set() , so we skip the set() .

Now let's come back to the get() . Of course, to return a Value or BaseValue doesn't make sense. What about returning the RealValue or even the wrapped underlying value? That would be really useful, but for that we have to tell get() what we want as return type. So get() becomes a member template and so can return whatever is inside the RealValue .

class Value // Value handle { public: Value(BaseValue const & bv) : v(bv.clone()) {} Value(Value const & rhs) : v(rhs.v ? rhs.v-clone() : 0) {} explicit Value(BaseValue * bv = 0) : v(bv) {} ~Value() { delete v; } Value & operator=(const Value & rhs) { // this is not a typical pimpl assignment, but a set() if (v) { if (rhs.v) { // fine, all v's exist v->set(rhs); } else { // the other v doesn't exist, so we must delete our own :-( BaseValue * old = v; v = 0; delete old; } } else { // we don't have a v, so just copy the other v = (rhs.v ? rhs.v-clone() : 0); } return *this; } template <typename PlainT> PlainT get() const { if (v) { RealValue<PlainT> const & rv = dynamic_castRealValuePlainT const &(*v); return rv; // uses conversion operator } else { return PlainT(); } } std::string asString() const { if (v) { return v->asString(); } else { return string(); } } private: BaseValue * v; }; 


Finally we come to Object . Now, as we have everything else, an Object is mainly a container for its attribute values. To ease implementation, we will structurally mirror the attribute container in the class definition, so we use a vector. As we have so much effort invested in our Value handle, it would make sense to store that in the vector. But for future extensions it will be easier to have the BaseValue pointers directly available. The constructor will create the values through the types of the attributes, so the only constructor takes a ClassDef* . To set and get the values for the attributes, we provide two options: to specify the attribute by name and also by index. For reflection purposes (as well as for internal implementation) we need a pointer to the class definition, but then we have it all:

class Object { public: explicit Object(ClassDef const * class_) : myClass(class_), values(class_->getAttributeCount()) { buildValueList(); } ClassDef const & instanceOf() const { return *myClass; } Value getValue(size_t attribIdx) const { return *values[attribIdx]; // calls Value(BaseValue &) } Value getValue(string const & attribName) const { size_t idx = instanceOf()->findAttribute(attribName); // should check for not found return getValue(idx); } void setValue(size_t idx, Value const & v) { values[idx]->set(v); } void setValue(string const & attribName, Value const &v) { size_t idx = instanceOf()->findAttribute(attribName); // should check for not found setValue(idx, v); } private: typedef vector<BaseValue *> ValueContainer; void buildValueList() { ClassDef::AttrIterator a; ValueContainer::iterator i = values.begin(); for (a = instanceOf()->attribBegin(); a != instanceOf()->attribEnd(); ++a, ++i) { *i = a->getType().newValue(); } } ClassDef const * const myClass; ValueContainer values; };

Next Article

This article has described the design and implementation of a meta-object layer for C++. A subsequent article will continue this discussion by demonstrating how these techniques can be used to solve the design problems found in the Internet bookshop example.

If you have comments on these techniques, or proposals how the problems could be solved completely differently, or if you find errors in this article, I would really appreciate your feedback to .

Further source code to illustrate the implementation of the ideas in this article can be found at .


[Kiczales-] Gregor Kiczales, Jim des Rivières, Daniel G. Bobrow: "The Art of the Metaobject Protocol", MIT Press 1991, ISBN 0-262-61074-4

[Coplien] James O. Coplien: "Advanced C++ Programming Styles And Idioms", Chapters 8-10, Addison-Wesley, 1992, ISBN 0-201-54855-0

[Buschmann-] Frank Buschmann, Regine Meunier, Hans Rohnert, Peter Sommerlad, Michael Stal: "Pattern-Oriented Software Architecture: A System of Patterns", Wiley 1996, ISBN 0-471-95869-7

[Gamma-] Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides: "Design Patterns", Addison-Wesley 1994, ISBN 0-201-63361-2

[ 1 ] I will not elaborate on differences between type and class.

[ 2 ] The idea of the search machine is to have a batch process that queries all objects about its attribute values and create an own internal index from that information.

[ 3 ] actually, a simple array would suffice, as its size is fix and known at compile time. But for future extensions a vector is more flexible.

[ 4 ] As class is a reserved word in C++, and I don't like identifiers to differ only in case from others I have chosen ClassDef here.

[ 5 ] fromString() would also be useful, but we omit it here.

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