CSE 332: C++ template examples Concepts and Models Templates impose requirements on type parameters –Types that are plugged in must meet those requirements –Otherwise, the code won’t compile (and errors will say why) The set of requirements imposed is called a concept Any specific type that meets the requirements is a model of that concept What requirement(s) does the expression return first; impose? What about while(first != last && *first != value) { ++first; } const T & Iterator

CSE 332: C++ template examples Interface Polymorphism: Concept Refinement A concept C is a refinement of concept D if C imposes all of the requirements of D Modeling and refinement satisfy three formal properties –Reflexivity: A concept refines itself –Containment: if T models C and C refines D then T models D –Transitivity: If C refines D then C refines any concept D refines What in the OO paradigm is this like? How does it differ? C1 C2 T1T2 T3T4 C0 containment transitivity can substitute, e.g., T3 for T1

CSE 332: C++ template examples One Last Detail: Associated Types template class Array : public ArrayBase { public: typedef T* iterator; Array(const int size); ~Array(); iterator begin () { return m_values;} iterator end () { return m_values+m_size;} private: T * m_values; }; void foo { Array a; fill_array(a); for (Array ::iterator i = a.begin(); i != a.end(); ++i) { cout << *i << “ ” << endl; } A type parameter is not concrete until its instantiated The compiler can work around that in many cases But sometimes the compiler needs more information Additional information can be associated with a type –For example, a class template can define an iterator type –Available to the template and code that uses the template –More about how this is done (and why) in later lectures

CSE 332: C++ template examples C++ Templates and Interface Polymorphism Two examples –Function template for printing different types –Class template for a linked list First example will refine a program several times –Ending up with a specialized reusable function template –Shows how to write general and specialized templates Second example will walk through finished code –Look at how to declare and define a class template –Look at another data structure that uses dynamic memory –A few practicalities you may need to know about E.g., including template source, working around compiler issues

CSE 332: C++ template examples First Example: Printing Different Types #include using namespace std; int main (int, char *[]) { int i = 7; bool b = false; int * ip = &i; char * cp = "hello, world!"; void * vp = cp; cout << "i is " << i << endl; cout << "b is " << b << endl; cout << "ip is " << ip << endl; cout << "cp is " << cp << endl; cout << "vp is " << vp << endl; return 0; } Large number of types in C++ Each handled a bit differently For example, << with ostream Simply giving to cout gives i is 7 b is 0 ip is 0xfef69094 cp is hello, world! vp is 0x8048a3c Let’s make some improvements

CSE 332: C++ template examples Improve the Output for bool #include using namespace std; int main (int, char *[]) {... cout << "i is " << i << endl; cout << "b is " << boolalpha << b << endl; cout << "ip is " << ip << endl; cout << "cp is " << cp << endl; cout << "vp is " << vp << endl; return 0; } First, fix how bool is output Use an iostream manipulator std::boolalpha –prints true or false –Instead of 1 or 0 Now we get i is 7 b is false ip is 0xfef2aa04 cp is hello, world! vp is 0x8048b0c But what about the pointers?

CSE 332: C++ template examples Improve the Output for Pointers #include using namespace std; int main (int, char *[]) {... cout << "i is " << i << endl; cout << "b is " << boolalpha << b << endl; cout << "ip is " << ip << " (points to " << *ip << ")" << endl; cout (cp) << " (points to \"" << cp << "\")" << endl; cout << "vp is " << vp << endl; return 0; } Now, fix pointers Use reinterpret_cast –Convert char * to void * Use dereferencing –Convert int * to int Now we have what we want i is 7 b is false ip is 0xfef23584 (points to 7) cp is 0x8048b70 (points to "hello, world!") vp is 0x8048b70 But, don’t want to have to do all that over again –Next time we want to print

CSE 332: C++ template examples Refactor the Code with a Function Template #include using namespace std; #include "common_T.h" int main (int, char *[]) { int i = 7; bool b = false; int * ip = &i; char * cp = "hello, world!"; void * vp = cp; print(cout, "i is ", i); print(cout, "b is ", b); print(cout, "ip is ", ip); print(cout, "cp is ", cp); print(cout, "vp is ", vp); return 0; } Define print function template –Consistent interface across types –Just pass message, variable With this template we get i is 7 b is 0 ip is 0xfeea28f4 cp is hello, world! vp is 0x8048adc Right back where we started? template void print (ostream & os, const char * message, const T & t) { os << message << t << endl; }

CSE 332: C++ template examples Template Specialization: More Polymorphism typedef char * charptr; typedef int * intptr; template <> void print (ostream & os, const char * message, const bool & b) { os << message << std::boolalpha << b << endl; } template <> void print (ostream & os, const char * message, const charptr & s) { os << message (s); if (s != 0) { os << " (points to \"" << s << "\")"; } os << endl; } template <> void print (ostream & os, const char * message, const intptr & ip) { os << message << ip; if (ip != 0) { os << " (points to " << *ip << ")"; } os << endl; } Specialize on individual types bool char * int * –Notice the use of typedef With specialization, we get i is 7 b is false ip is 0xfeebf064 (points to 7) cp is 0x8048c30 (points to "hello, world!") vp is 0x8048c30 And, we can reuse the solution! template void print (ostream & os, const char * message, const T & t) { os << message << t << endl; }

CSE 332: C++ template examples Concluding Remarks (1 of 2) The set of requirements that a class template or function template places on its parameterized types is called a concept in generic programming terminology Any type that meets those requirements is said to model the concept, and can be used in that template Concept refinement supports interface polymorphism Push common code and variables up into non- template base classes –Can help avoid subtle type errors –Can help reduce the size of the executable program Associated types expand the power of templates –E.g., used extensively in the C++ Standard Template Library

CSE 332: C++ template examples Concluding Remarks (2 of 2) Summary of generic programming ideas –Can use templates to plug different types into function and class declarations/definitions –Can look at template code to come up with what the requirements are for its parameterized types Fortunately, someone has already done that for the STL See Matt Austern’s book (CSE 532) for more details