CS 242 History C++ � C++ is an object-oriented extension of C � C was designed by Dennis Ritchie at Bell Labs • used to write Unix • based on BCPL John Mitchell � C++ designed by Bjarne Stroustrup at Bell Labs • His original interest at Bell was research on simulation • Early extensions to C are based primarily on Simula • Called “C with classes” in early 1980’s • Popularity increased in late 1980’s and early 1990’s • Features were added incrementally Classes, templates, exceptions, multiple inheritance, type tests... Design Goals How successful? � Provide object-oriented features in C-based � Given the design goals and constraints, language, without compromising efficiency • this is a very well-designed language • Backwards compatibility with C � Many users -- tremendous popular success • Better static type checking � However, very complicated design • Data abstraction • Many specific properties with complex behavior • Objects and classes • Difficult to predict from basic principles • Prefer efficiency of compiled code where possible • Most serious users chose subset of language � Important principle – Full language is complex and unpredictable • If you do not use a feature, your compiled code • Many implementation-dependent properties should be as efficient as if the language did not • Language for adventure game fans include the feature. (compare to Smalltalk) Significant constraints Overview of C++ � C has specific machine model � Additions and changes not related to objects • Access to underlying architecture • type bool � No garbage collection • pass-by-reference • user-defined overloading • Consistent with goal of efficiency • function templates • Need to manage object memory explicitly • … � Local variables stored in activation records • Objects treated as generalization of structs, so some objects may be allocated on stack • Stack/heap difference is visible to programmer 1
C++ Object System Some good decisions � Object-oriented features � Public, private, protected levels of visibility • Classes • Public: visible everywhere • Objects, with dynamic lookup of virtual functions • Protected: within class and subclass declarations • Inheritance • Private: visible only in class where declared – Single and multiple inheritance � Friend functions and classes – Public and private base classes • Careful attention to visibility and data abstraction • Subtyping � Allow inheritance without subtyping – Tied to inheritance mechanism • Better control of subtyping than without private base • Encapsulation classes Some problem areas Sample class: one-dimen. points � Casts class Pt { • Sometimes no-op, sometimes not (esp multiple inher) public: � Lack of garbage collection Pt(int xv); Overloaded constructor • Memory management is error prone Pt(Pt* pv); – Constructors, destructors are helpful though � Objects allocated on stack int getX(); Public read access to private data • Better efficiency, interaction with exceptions virtual void move(int dx); Virtual function • BUT assignment works badly, possible dangling ptrs protected: � Overloading void setX(int xv); Protected write access • Too many code selection mechanisms private: � Multiple inheritance int x; Private member data • Efforts at efficiency lead to complicated behavior }; Virtual functions Sample derived class class ColorPt: public Pt { Public base class gives supertype � Member functions are either public: • Virtual, if explicitly declared or inherited as virtual ColorPt(int xv,int cv); • Non-virtual otherwise ColorPt(Pt* pv,int cv); Overloaded constructor � Virtual functions ColorPt(ColorPt* cp); • Accessed by indirection through ptr in object int getColor(); Non-virtual function • May be redefined in derived (sub) classes virtual void move(int dx); Virtual functions � Non-virtual functions virtual void darken(int tint); protected: • Are called in the usual way. Just ordinary functions . void setColor(int cv); Protected write access • Cannot redefine in derived classes (except overloading) private: � Pay overhead only if you use virtual functions int color; Private member data }; 2
Run-time representation Compare to Smalltalk Point class Template Point object Point vtable Code for move Point object Method dictionary vptr x y newX:Y: 3 x 2 ... ... 3 move ColorPoint object ColorPoint vtable Code for move ColorPoint class Template ColorPoint object vptr Method dictionary x x 5 Code for darken y newX:Y:C: c blue 4 color color 5 move red Data at same offset Function pointers at same offset Why is C++ lookup simpler? Calls to virtual functions � Smalltalk has no static type system � One member function may call another • Code p message:pars could refer to any object class A { public: • Need to find method using pointer from object virtual int f (int x); • Different classes will put methods at different place in virtual int g(int y); method dictionary }; � C++ type gives compiler some superclass int A::f(int x) { … g(i) …;} int A::g(int y) { … f(j) …;} • Offset of data, fctn ptr same in subclass and superclass � How does body of f call the right g? • Offset of data and function ptr known at compile time • Code p->move(x) compiles to equivalent of • If g is redefined in derived class B, then inherited f must call B::g (*(p->vptr[1]))(p,x) if move is first function in vtable data passed to member function; see next slide “This” pointer (analogous to self in Smalltalk) Non-virtual functions � Code is compiled so that member function takes � How is code for non-virtual function found? “object itself” as first argument � Same way as ordinary “non-member” functions: Code int A::f(int x) { … g(i) …;} • Compiler generates function code and assigns address compiled as int A::f(A *this, int x) { … this->g(i) …;} • Address of code is placed in symbol table • At call site, address is taken from symbol table and placed in compiled code � “this” pointer may be used in member function • But some special scoping rules for classes • Can be used to return pointer to object itself, pass � Overloading pointer to object itself to another function, ... • Remember: overloading is resolved at compile time • This is different from run-time lookup of virtual function 3
Scope rules in C++ Virtual vs Overloaded Functions � Scope qualifiers class parent { public: void printclass() {printf("p ");}; • binary :: operator, ->, and . virtual void printvirtual() {printf("p ");}; }; • class::member, ptr->member, object.member class child : public parent { public: � A name outside a function or class, void printclass() {printf("c ");}; • not prefixed by unary :: and not qualified refers to virtual void printvirtual() {printf("c ");}; }; global object, function, enumerator or type. main() { � A name after X::, ptr-> or obj. parent p; child c; parent *q; p.printclass(); p.printvirtual(); c.printclass(); c.printvirtual(); • where we assume ptr is pointer to class X and obj is an object of class X q = &p; q->printclass(); q->printvirtual(); q = &c; q->printclass(); q->printvirtual(); • refers to a member of class X or a base class of X } Output: p p c c p p p c Subtyping Independent classes not subtypes � Subtyping in principle class Point { class ColorPoint { public: public: • A <: B if every A object can be used without type int getX(); int getX(); error whenever a B object is required void move(int); void move(int); • Example: int getColor(); protected: ... Point: int getX(); void darken(int); Public members private: ... void move(int); protected: ... }; ColorPoint: int getX(); private: ... int getColor(); }; Public members void move(int); � C++ does not treat ColorPoint <: Point as written void darken(int tint); � C++: A <: B if class A has public base class B • Need public inheritance ColorPoint : public Pt • Why?? • This is weaker than necessary Why? Why C++ design? Function subtyping � Client code depends only on public interface � Subtyping principle • In principle, if ColorPoint interface contains Point • A <: B if an A expression can be safely used in any interface, then any client could use ColorPoint in context where a B expression is required place of point � Subtyping for function results • However -- offset in virtual function table may differ • If A <: B, then C → A <: C → B • Lose implementation efficiency (like Smalltalk) � Subtyping for function arguments � Without link to inheritance • If A <: B, then B → C <: A → C • subtyping leads to loss of implementation efficiency � Terminology � Also encapsulation issue: • Covariance: A <: B implies F(A) <: F(B) • Subtyping based on inheritance is preserved under • Contravariance: A <: B implies F(B) <: F(A) modifications to base class … 4
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