[Extended Abstract] Kurt Stephens ION, Inc.

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kurtstephens@acm.org May 22, 2001 (revised May 16, 2003) Object serialization and object inspector user interfaces are concerns that can be orthogonally implemented using introspection via meta-object protocols (MOP). The C++ language lacks a formal meta-object protocol, although some are available as source pre-processors. A full MOP is not necessary for many classes of problems where basic introspection is useful. This paper describes a technique: extensible visitation of objects as a basic introspection primitive. Introspection, the access of information about a program's construction during run-time, is valuable. Meta-object protocols (MOP) KICZALES91 allow object-oriented systems to reify their own definition. Some C++ MOP implementations (IGUANA , OPENCXX ) use pre-compilation. For common concerns, such as object serialization and object inspector user interfaces, a complete MOP is useful but not required. Extensible visitation based on the Visitor design pattern GAMMA95 is a mechanism, similar to XDR XDR , for interfacing different introspections with little impact to an existing C++ framework. Generic programming techniques aim to eliminate boilerplate code LAMMEL03 . Traversal code is often the largest part of many computations. Standard C++ ostream classes are visitors that produce output as a side-effect of visiting typed data. Developers must create ostream visitation methods for each new application class. Developers must create similar visitation methods for other visitors. These methods are specialized for different visitors, yet do mostly the same thing; recursively visit all data members while side-effecting the visitor. For example: class Foo { private: double x, y; // data members int i3[3]; public: Foo() { ... } Foo(double _x, double _y) { ... } // Visitation Methods, one for each type of visitor. // ostream dump method. ostream &print(ostream &os) { os << "Foo(" << x ", " << y << ", "; os << "{ " << i3[0] << ", " << i3[1] << ", " << i3[2] << " }"; return os << ")"; } // XML output method. ostream &print_xml(ostream &os) { os << "<Foo>"; os << "<x>"; print_xml(x, os); os << "</x>"; os << "<y>"; print_xml(y, os); os << "</y>"; os << "<i3>"; print_xml(i3[0], os); print_xml(i3[1], os); print_xml(i3[2], os); os << "</i3>"; return os << "</Foo>"; } // inspector generation method. inspector &generate_inspector(inspector &is) { is.begin_frame("Foo", this); is.generate_inspector("x", &x); is.generate_inspector("y", &y); is.begin_frame("i3[]", &i3); is.generate_inspector("0", &i3[0]); is.generate_inspector("1", &i3[1]); is.generate_inspector("2", &i3[2]); is.end_frame(&i3); is.end_frame(this); return is; } // XDR support method int xdr(XDR *xdr) { return xdr_double(xdr, &x) && xdr_double(xdr, &y) && xdr_int(xdr, &i3[0]) && xdr_int(xdr, &i3[1]) && xdr_int(xdr, &i3[2]); } // ... other visitor types } In the example above, more visitation methods are needed as the application classes (visitees) interface with more visitors. Adding, removing or renaming data members becomes a maintenance problem, since each visitation method must be updated. Likewise, adding new visitations becomes difficult as the application grows, since each application class must be updated. If the number of application classes in a system is N and the number of visitor classes is M , the complexity of the application-visitor interfaces will be N * M , if visitor-specific methods are implemented for each application class. If a single generic visitation method is used for each visitor class, the complexity of the application-visitor interfaces will be N + M . Ideally a single visitation method suffices for most types of visitation. Essentially we want to move the complexity from the visited classes to the visitors, since the number of visitor classes will be small compared to the visited classes. Visitor classes tend to interface auxiliary systems and protocols, such as XML and other I/O systems, and change less frequently than the application classes. XVF (eXtensible Visitation Framework) is a C++ template-based framework for developing visitors and generic visitation methods. In XVF, a generic introspection visitor is messaged with type, naming and layout information about the visited objects. The visitor consumes the introspective messages and controls the recursion during visitation. The visitor's introspection methods are virtual, such that any visitor can be used in any visitation traversal. The set of introspection methods in the visitor closely follows a decomposition of the data types in C++: structures (classes), pointers, arrays and intrinsic types (char , int , double ). In XVF, Visitors are sub-classes of XVFVisitor . XVFVisitor has methods for keeping track of what object is being visited and what members are being visited. Each intrinsic type (e.g. char , int , double ) has a visitation method specialized for it in the visitor class. Visitors are messaged with information about the visitee during visitation traversal. In XVF, the XVFType class provides basic type introspection: type name, type size and allocation and assignment methods. XVFType objects are constructed dynamically by class templates instantiated by template functions. There are five XVFType templates: intrinsic types (char , int , unsigned long ), abstract classes (non-instantiable classes), concrete classes (instantiable classes), pointers, and arrays. The XVFTypeIntrinsic<T> template creates XVFType classes for intrinsic types. The templates XVFTypeClass<T> and XVFTypeAbstract<T> create type objects for instantiable and abstract classes, respectively. Type objects for intrinsic types are created "by hand" in XVF. Pointer and array type objects are constructed by XVFTypePtr<T> and XVFTypeArray<T> , respectively. XVFType templates are never instantiated by the user of XVF, but are created automatically by the instantiation of xvf_type(T*) template functions based on pointers to type T . Each type object implements a virtual visit(void *data, XVFVisitor *V) method that is specialized by the type object template to call a template function xvf_visit((T *) data, V) ; For example, if aFoo is an instance of class Foo , the template function call, xvf_type(&aFoo) , returns the XVFType type object for the object instance aFoo . Likewise, the static method, XVFType::type("Foo") , returns the same XVFtype type object for the class named "Foo" . The type objects are used to aid the visitors when visiting each class. For example: a XML input visitor may need to construct a new object based on a XML tag name. Visitee classes are not sub-classes of a special "Visitee" class. They are "glued" to the XVF library by static type polymorphic functions: XVFType *xvf_type(T *x) returns the XVFType object for type T . T *xvf_alloc(T *dummy) allocates an object of type T . void xvf_dealloc(T *x) deallocates an object of type T . void xvf_visit(T *x, XVFVisitor *V, int opts) recursively visits an object of type T with visitor V . Note: XVF does not dispatch using const &T because then void could not be handled in a unified manner and because data are visited by address. Any type T that implements a xvf_type(T *x) and xvf_visit(T *x, XVFVisitor *V, int opts) (the opts argument is explained later) functions can be visited by any XVFVisitor object. XVF supplies macros for constructing a class' visitation method and type object accessor functions. For example: // Declare xvf_type(Foo *) XVF_CLASS_BEGIN(Foo); class Foo { double x, y; int i3[3]; public: Foo() { ... } Foo(double x, double y) { ... } // xvf_type() support XVF_CLASS_METHODS(XVF_); // xvf_visit() support XVF_CLASS_VISIT_BEGIN_INLINE(Foo) { XVF_MEMB(x); XVF_MEMB(y); XVF_MEMB(i3); } XVF_CLASS_VISIT_END() }; // Define xvf_visit(Foo *) XVF_CLASS_END(Foo); // Define xvf_type(Foo *) XVF_CLASS_INSTANCE(Foo); The visitor is messaged with information about the visited object during traversal. The visitor can dynamically control recursion for any visitation member. The XVF_MEMB() macro above expands into messages to the visitor. For example: any visitor V visiting Foo F will have the following methods called on it: xvf_visit(&F, V); is equivalent to: V->this_begin(xvf_type(&F), &F, opts); if ( V->memb(xvf_type(&F.x), "x", &F.x, opts) ) { V->memb_begin(xvf_type(&F.x), "x", &F.x, opts); xvf_visit(&F.x, V, opts); V->memb_end(xvf_type(&F.x), "x", &F.x, opts); } if ( V->memb(xvf_type(&F.y), "y", &F.y, opts) ) { V->memb_begin(xvf_type(&F.y), "y", &F.y, opts); xvf_visit(&F.y, V, opts); V->memb_end(xvf_type(&F.y), "y" &F.y, opts); } if ( V->memb(xvf_type(&F.i3), "i3", &F.y, opts) ) { V->memb_begin(xvf_type(&F.i3), "i3", &F.y, opts); V->arry_begin(xvf_type(&F.i3), xvf_type(&F.i3[0]), &F.i3, 3, opts); V->elem_begin(0, opts); xvf_visit(&F.i3[0], V, opts); V->elem_end(0, opts); V->elem_begin(1, opts); xvf_visit(&F.i3[1], V, opts); V->elem_end(1, opts); V->elem_begin(2, opts); xvf_visit(&F.i3[2], V, opts); V->elem_end(3, opts); V->arry_end(xvf_type(&F.i3), xvf_type(&F.i3[0]), &F.i3, 3, opts); V->memb_end(xvf_type(&F.i3), "i3" &F.y, opts); } V->this_end(); The opts argument to the visit functions relays const -ness to the visitor without requiring different routines and type objects for const and non-const visitation. If opts == XVF_NON_CONST in V->visit(T *data, int opts) , V->visit() has license to modify *data . For example: an inspector user-interface visitor may generate an editable or non-editable user-interface field depending if opts == XVF_NON_CONST or opts == XVF_CONST . Sometimes the visitation semantics of a class' members should be through a pair of accessor methods or functions, rather than directly on an object data member. For example: XVF_CLASS_BEGIN(Temperature); class Temperature { private: double _f; // Fahrenheit: f = (c * 1.8) + 32 double _c; // Celsius: c = (f - 32) / 1.8 public: Temperature() { celsius(0); } // Getter, setter. double fahrenheit() const { return _f; } void fahrenheit(double x) { _f = x; _c = (_f - 32) / 1.8; } // Getter, setter. double celsius() const { return _c; } void celsius(double x) { _c = x; _f = _c * 1.8 + 32; } // Getter, setter. // Note: different setter name style double kelvin() const { return _c + 273.15; } void set_kelvin(double x) { celsius(x - 273.15); } XVF_CLASS_METHODS(XVF_); XVF_CLASS_VISIT_BEGIN_INLINE(Temperature) { // Visit by accessor code XVF_ACC_T("fahrenheit", // name double, // type _xvfv_this->fahrenheit(), // getter _xvfv_this->fahrenheit(XVF_TEMP) // setter ); // Visit by accessor names using // shorthand for: // getter: _xvfv_this->celsius() // setter: _xvfv_this->celsius(XVF_TEMP) XVF_ACC_T_M("celsius", double, celsius, celsius ); XVF_ACC_T_M("kelvin", double, kelvin, set_kelvin ); } XVF_CLASS_VISIT_END() }; XVF_CLASS_END(Temperature); XVF_CLASS_INSTANCE(Temperature); If any visitor updates the "fahrenheit" , "celsius" or "kelvin" members of a Temperature object, the Fahrenheit, Celsius and Kelvin values will be synchronized through the getter and setter methods in Temperature . A visitation method can relay visitor-specific information using visitation attributes. Imagine a visitor that needs to know how each member is protected in a C++ class: ... class Bar { private: int a; // a is private. protected: float b; // a is protected. public: char *c; // c is public. Bar(...) XVF_CLASS_VISIT_BEGIN_INLINE(Bar) { // Save and set attribute value. XVF_ATTR_BEGIN("C++:protection", "private"); XVF_MEMB(a); // Set attribute value. XVF_ATTR("C++:protection", "protected"); XVF_MEMB(b); XVF_ATTR("C++:protection", "public"); XVF_MEMB(c); // Restore attribute value. XVF_ATTR_END("C++:protection"); } XVF_CLASS_VISIT_END(); }; ... The specialized visitor will check the attribute named "C++:protection" for its value and produce results accordingly. Below are sample uses of visitor objects using the same visit methods for class Foo . The visitor code is not presented here. A structured data visitor writes a human-readable representation to an ostream . XVF_Dump_Out<ostream> xvf(cout); Foo p(1, 2); xvfv << p; An archiver visitor writes a machine-readable representation to an ostream . XVF_Archive_Out<ostream> xvfv(cout); Foo p(1, 2); xvfv << p; An XML output visitor writes objects as an XML stream. XVF_XML_Out<ostream> xvfv(cout); Foo p(1, 2); xvfv << p; An XML input visitor reads an XML stream into a object. XVFV_XML_In<istream> xvfv(cin); Foo p; xvfv >> p; An inspector visitor generates TCL/TK code to create an inspector GUI. Tcl_Interp *interp; XVF_TK_Inspector xvfv(interp); Foo p(1, 2); xvfv << p; TK widget generated by XVF_TK_Inspector . TK widget generated by XVF_TK_Inspector . These accessor visitors apply getter and setter methods by name. XVF_Getter xvfg; Foo p(1, 2); double py; xvfg(&p, xvf_type(&p), "y", &py); // py = p.y XVF_Setter xvfs; xvfs(&p, xvf_type(&p), "x", &py); // p.x = py This visitor provides XDR services. XDR *xdr; XVF_XDR xvfv(xdr); xvfv << p; External Data Representation (XDR) XDR uses generic visitation for encoding, decoding and freeing C objects. XDR visitors cannot be sub-classed directly and do not provide any mechanisms for XDR visitors to determine member names. XDR is used primarily for Remote Procedure Calls (RPC) RPC . OpenC++ OPENCXX provides introspection using pre-compiler techniques. Iguana IGUANA seems to be based on pre-compiler techniques as well, but no public implementations are offered. LAMMEL03 discusses generic traversal combinator generation for generic programming support. Namespaces, enumerations, bit-fields, unions, function pointers and methods are not handled. Class versioning for backward compatibility is not handled. A pre-processor to parse members from class definitions to generate and insert xvf_visit() methods would be helpful; OpenC++ or SWIG SWIG might be applicable. A C interface for legacy code may also be useful. This paper describes a method for basic introspection in C++ using the Visitor design pattern. A prototype implementation of XVF is available at URL: http://kurtstephens.com/pub/xvf . SWIG Dave Beazley URL: http://www.swig.org 2003 IGUANA Vinny Cahill URL: http://www.dsg.cs.tcd.ie/~coyote 2003 OPENCXX Shigeru Chiba URL: http://www.csg.is.titech.ac.jp/~chiba/openc++.html 2003 GAMMA95 Erich Gamma Richard Helm Ralph Johnson John Vlissides 0-201-63361-2 Addison-Wesley 1995 KICZALES91 Gregor Kiczales 0262610744 MIT Press 1991 LAMMEL03 Ralf Lammel Simon Jones Scrap Your Boilerplate: A Practical Design Pattern for Generic Programming 26-37 ACM 2003 RPC R. Srinivasan URL: http://www.ietf.org/rfc/rfc1831.txt 1995 XVF Kurt Stephens URL: http://kurtstephens.com/pub/xvf ION, Inc. 2001 XDR Sun Microsystems, Inc URL: http://www.ietf.org/rfc/rfc1014.txt 1987