CS11 Advanced C++ Fall Lecture 7

Transcription

1 CS11 Advanced C++ Fall Lecture 7

2 Today s Topics Explicit casting in C++ mutable keyword and const Template specialization Template subclassing

3 Explicit Casts in C and C++ C has one explicit cast operator for everything Can convert between related types e.g. double to int Can cast pointers to different types e.g. void* to float*, or float* to char* Can cast pointers to int, and vice versa Can cast away const-ness Many unsafe or potentially unsafe scenarios! C++ provides four explicit casting operators Breaks down different kinds of casts into explicit operations Provides more type-safety checks at compile-time, run-time Easier for programmers to understand, too!

4 C++ Cast Operations const_cast Casts away const-ness static_cast Performs safe conversions between related types, using static type information dynamic_cast Uses runtime type information to safely cast down or across class hierarchies reinterpret_cast For all the dangerous stuff.

5 Removing const Constraints const_cast removes const constraints T const_cast<t>(const T value) Not for changing the type of value! Using const_cast to change types will not compile Examples: void update(specialwidget *psw); const SpecialWidget csw; update(&csw); // COMPILE ERROR update(const_cast<specialwidget*>(&csw)); // OK Widget *pw = new SpecialWidget(); update(const_cast<specialwidget*>(pw)); // COMPILE ERROR

6 Cached Values and const const_cast sometimes used when objects cache temporary values that are expensive to compute class Date {... string cache; bool cachevalid; void updatecachevals(); // sets cache value public:... string stringrep() const; }; stringrep() returns a string version of the date value Cache the result, as long as date value doesn t change

7 Cached Values and const (2) Implementation of stringrep() string Date::stringRep() const { if (!cachevalid) { // this is const, so cast to non-const Date* Date *mut = const_cast<date*>(this); mut->updatecachevals(); mut->cachevalid = true; } return cache; } this is const because the member function is const Must cast away const to update cached values!

8 Cached Values and const (3) That solution isn t very elegant. Also, it might not actually work! If original variable is declared as const, casting away const is not guaranteed to work on all implementations. For example, system might apply access constraints to the memory used by the variable Example: Date d1; const Date d2; string s1 = d1.stringrep(); string s2 = d2.stringrep(); //??? s2 is not guaranteed to have a valid result across all implementations

9 Mutable Data-Members Better solution is to use the mutable keyword class Date {... mutable string cache; mutable bool cachevalid; void updatecachevals() const; public:... string stringrep() const; }; Compiler will ensure that cache can be changed, even on variables declared const mutable means can never be const

10 Mutable Data-Members (2) Implementation of stringrep() gets simpler: string Date::stringRep() const { if (!cachevalid) { // Only mutable data-members are changed. updatecachevals(); cachevalid = true; } return cache; } And this also works now: Date d1; const Date d2; string s1 = d1.stringrep(); string s2 = d2.stringrep(); // OK!

11 Proper Uses of const_cast const_cast is best for situations when a function doesn t use const, but it ought to! The function doesn t change the argument, but argument wasn t declared const (usually by mistake) Example: dealing with bad standard libraries size_t strlen(char *s); // should be const char * const char *prompt = "Shall we play a game?"; size_t length = strlen(const_cast<char*>(prompt)); Very uncommon situation! You shouldn t need const_cast very often

12 Static Casts static_cast converts between related types static_cast<t>(value) Converts value to type T Only uses static type information (compile-time check) Example use-cases: Convert from an integral type to an enumeration Convert from a floating-point type to an integral type Convert from one pointer-type to another Typically, within the same class hierarchy Example: int *p = static_cast<int*>(malloc(100 * sizeof(int))); malloc() returns a void* which needs to be casted

13 Dynamic Casts dynamic_cast converts between types in a class hierarchy, using run-time type information dynamic_cast<t>(value) T must be a pointer or reference to a polymorphic type (a class with virtual member-functions) Performs a run-time type-check to see if value can be cast to T If so, conversion takes place and result has type T If not, dynamic_cast evaluates to 0! Mainly used to cast a base-class pointer/reference to a derived class. Has very complex behavior with multiple inheritance!

14 Static and Dynamic Casts Static casts are faster than dynamic casts Compile-time check vs. a runtime check Static casts can t move down a class hierarchy Requires run-time information about objects Example: class Widget { }; class SpecialWidget : public Widget { }; Widget *pw = new SpecialWidget(); // COMPILE ERROR: static type of pw is Widget* SpecialWidget *spw = static_cast<specialwidget*>(pw); // OK: pw points to a SpecialWidget SpecialWidget *spw = dynamic_cast<specialwidget*>(pw);

15 Static and Dynamic Cast Examples Dialog *d = new Dialog(); Window *w = static_cast<window *>(d); // OK, compile-time check (fast) Button *b = new Button(); Component *c = dynamic_cast<component *>(b); // OK, runtime check (slower) Component Window Button Dialog Button *b2; b2 = static_cast<button *>(c); b2 = dynamic_cast<button *>(c); // COMPILE ERROR // OK, b2 == b Dialog *d2; d2 = static_cast<dialog *>(b); d2 = dynamic_cast<dialog *>(b); // COMPILE ERROR // OK, d2 == 0

16 Reinterpreting Values reinterpret_cast<t>(value) Converts between unrelated types Again, converts value to type T Example use-cases: Convert an integer to a pointer, or vice versa Convert a pointer to a completely unrelated pointer type If destination type is same bit-width, the result s bit-pattern is same as the source s bit-pattern Should need reinterpret_cast very rarely If you do need it, encapsulate this functionality inside a class, and don t expose it to others! (Make sure you really do need it!)

17 Reinterpreting Values (2) Example: An API that represents windows and other GUI components with handles /** Clears the specified window. **/ void clearwindow(int handle) { Window *pwnd = reinterpret_cast<window*>(handle);... } API users aren t exposed to implementation details A safer solution? The API could use an STL map (or hash_map) to associate integer handles with GUI components Slower, but less likely to fail spectacularly!

18 The Point Class-Template (a review) template<class T> class Point { T x_coord, y_coord; public: Point() : x_coord(0), y_coord(0) {} Point(T x, T y) : x_coord(x), y_coord(y) {} T getx() { return x_coord; } void setx(t x) { x_coord = x; }... }; Parameterized on coordinate-type, named T class T means any type, not just classes Can use typename instead of class means same thing

19 Other Template Paramters Can parameterize templates on constants template<class T, int dim> class SquareMatrix { protected: T values[dim * dim]; public: }; SquareMatrix() { setidentity(); } ~SquareMatrix() { }... Type and dimension are template-parameters Allocation is static, so no delete in destructor

20 A Vector Template Write a generic vector-template: template <class T> class Vector { T *vals; int size; public: Vector(); explicit Vector(int size); T& elem(int i) { return vals[i]; }... // etc. }; Can instantiate template for various types: Vector<int> vints; Vector<string> vstrs; Vector<Shape*> vshapeptrs; Vector<Node*> vnodeptrs;

21 Specialized Vectors! Could specialize Vector<bool> s storage Instead of one byte/word per bool, store each bool value in a bit (a bit-packed representation) Provide a specialization of Vector template template <> class Vector<bool> { int *packedvals; int size; public:... bool& elem(int i) {... } }; All Vector<bool> variables use this specialization Specialized version is a separate class-template Must implement all the members, etc.

22 More Vector Specialization Vector<T> typically used to store pointers When T is a class, avoids slicing and polymorphism issues Can lead to code-bloat too! C++ compilers typically replicate template-code for each instantiation Lots of different Vector<T> instantiations will mean larger binary-file size Solution: specialize Vector for pointer-types

23 Specializing Vector for Pointers (1) Implement a special version of Vector<T> template <> class Vector<void*> { // Array of void* values void **vals; int size; public:... void* & elem(int i) {... } }; Provides base implementation for all Vectors of pointers but is only used for Vector<void*> variables!

24 Specializing Vector for Pointers (2) Next, provide a partial specialization of Vector template <class T> class Vector<T*> : private Vector<void*> { // Vector<void*> provides data-members, etc. public: typedef Vector<void*> Base; Vector() { } explicit Vector(int size) : Base(size) { }... T* & elem(int i) { return reinterpret_cast<t* &>(Base::elem(i)); } }; User specifies T* (e.g. Vector<Shape*>) Compiler uses Vector<T*> with Shape for T

25 Specializing Vector for Pointers (3) Why subclass with private modifier? template <class T> class Vector<T*> : private Vector<void*> { Use Vector<void*> implementation in Vector<T*> Don t want to expose Vector<void*> interface! private allows a class to use parent-class functionality, without affecting its own interface Why reinterpret_cast? T* & elem(int i) { } return reinterpret_cast<t* &>(Base::elem(i)); The void* is reinterpreted as a completely different type

26 Analysis of Vector-Approach Vector<void*>, etc. is some nasty code void* is rarely a good idea: untyped pointers Same with reinterpret_cast ignoring types! There are definite, measurable benfits: Far less template code generated by this method Compiled programs are smaller, faster Compile times can also be dramatically reduced Needed infrequently, but a very effective technique!

27 Subclassing Templates You write a template for a base-class: template<typename T> class Base { public: void f() { } }; Then you write a template for a derived class: template<typename T> class Derived : public Base<T> { public: void g() { f(); } }; This code won t compile!

28 Subclassing Templates (2) Inside Derived<T>::g(), the name f doesn t depend on the template-parameter T template<typename T> class Derived : public Base<T> { public: void g() { f(); } }; Known as a nondependent name Compiler doesn t look in dependent base-classes when looking up nondependent names i.e. compiler doesn t check Base<T> when looking for f

29 Subclassing Templates (3) Use this->f() instead of just f() this is always implicitly dependent in a template e.g. this has a type of Derived<T>* in this example Use Base<T>::f() instead of just f() Don t do this if f is virtual! You might not get the right results.

30 More Template Subclassing Fun You write this: void f() { } // A global function f template<typename T> class Base { public: void f() { } }; template<typename T> class Derived : public Base<T> { public: void g() { f(); } }; This code does compile!

31 More Template Subclassing Fun (2) When compiler tries to resolve f, it searches enclosing scope of Derived<T> That scope contains the global function f Qualified name of f is ::f() The code compiles, and ::f() is called instead of Base<T>::f() Moral: Be very careful when subclassing templates! Often need this-> or Base<T>:: for base-class member access, when deriving from a template

32 This Week s Assignment Small project (emphasis on small ) Experiment with something interesting to you The Standard Template Library C++ multiple-inheritance Create some templates Write some interesting program or utility Progress check-up in a week