2CPP15 - Templates


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This is an intermediate conversion course for C++, suitable for second year computing students who may have learned Java or another language in first year.

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2CPP15 - Templates

  1. 1. TEMPLATES Michael Heron
  2. 2. Introduction • In a previous lecture we talked very briefly about the idea of parametric polymorphism. • Being able to treat specific types of object independent of what they actually are. • In this lecture we are going to talk about C++’s templating system. • This lets us incorporate parametric polymorphism correctly in our programs.
  3. 3. The Problem • Say we want to create a basic kind of data structure. • A queue • A stack • A hashmap • How do we do that and still be able to deal with object orientation? • Answer is not immediately straightforward.
  4. 4. Java, Pre 1.5 • Many kinds of standard data structures exist as the standard libraries in Java. • Such as ArrayLists. • Before version 1.5, the following syntax was used. • Requires explicit casting of objects as they come off of the structure. • Why? • Polymorphism Arraylist list = new ArrayList(); String str; list.add (“Bing”); list.add (“Bong”); str = (String)list.get(0);
  5. 5. Java, 1.5+ • New versions of Java work with a different, more demonic syntax. • Explicit type information recorded along with the structure. • No need for casting as things come off of the structure. • How is such a thing achieved?? Arraylist<String> list = new ArrayList<String>(); String str; list.add (“Bing”); list.add (“Bong”); str = list.get(0);
  6. 6. Generics/Templates • The system in Java and C# is known as the generics system. • In C++, it’s called Templating. • Both systems are roughly equivalent. • Used for the same purpose, with various technical differences. • Templates more powerful than the Java/C# implementation. • Provides a method for type-casting structures to permit more elegant syntactic representation. • Permits compile time checking of homogenous consistency.
  7. 7. How Do They Work? • Code we provide serves as a template for C++ to generate specific instances of the code. • Works like a structure that can deal with data at a family level of granularity. • Exists in two kinds: • Function templates • Class templates
  8. 8. Function Templates • Function templates mimic a more flexible form of method overloading. • Overloading requires a method to be implemented for all possible types of data. • Function templates allow us to resolve that down to a single template definition. • Compiler can analyse provided parameters to assess the function that must be created. • Within limits… cannot interpret functionality where it isn’t syntactically valid.
  9. 9. Function Templates template <class T> T add_nums(T a, T b); #include <iostream> #include "TemplateTest.h" using namespace std; template <typename T> T add_nums(T num1, T num2) { return num1+num2; } int main() { cout << add_nums ('a', 1) << endl; return 0; }
  10. 10. Ambiguity • Because it is the compiler doing the work of interpreting parameters, it cannot deal well with ambiguity. • Where we say T, we must have it apply consistently across a function call. • We can ensure a certain kind of template being called by type-casting the function call.
  11. 11. Ambiguous Function Call #include <iostream> #include "TemplateTest.h" using namespace std; template <typename T> T add_nums(T num1, T num2) { return num1+num2; } int main() { cout << add_nums (2.0f, 1) << endl; return 0; }
  12. 12. Non-Ambiguous Function Call #include <iostream> #include "TemplateTest.h" using namespace std; template <typename T> T add_nums(T num1, T num2) { return num1+num2; } int main() { cout << add_nums<float> (2.0f, 1) << endl; return 0; }
  13. 13. Multi-Type Functions #include <iostream> #include "TemplateTest.h" using namespace std; template <typename T, typename S> T add_nums(T num1, S num2) { return num1+num2; } int main() { float f; f = add_nums<float, int> (5.0f, 3); cout << f << endl; return 0; }
  14. 14. Class Templates • In a similar way, we can create templates for C++ classes that are type agnostic. • Again, the compiler will infer typing from context where it possibility can. • Good design in C++ separates out definition and implementation. • Into .h and .cpp files. • Slight problem with doing this the way we have done previously. • It won’t work.
  15. 15. Class Templates • Templates are not real code. • Not as we understand it. • It’s like a template letter in a word processor. • Until the details are filled in, it’s not an actual letter. • When separating out the definitions, the compiler can’t deal with the T until it knows what types it’s working with. • The code doesn’t exist until you use it. • This causes linking problems in the compiler.
  16. 16. Class Template Problems • The complete definition for a function must be available at the point of use. • If the full definition isn’t available, the compiler assumes it has been defined elsewhere. • In the main program, it knows the type of the data type we want to use. • In the definition file, it doesn’t. • So it never generates the appropriate code. • Solution – inline functions.
  17. 17. A Stack Template template <typename T> class Stack { private: T *stack; int current_size; public: Stack() : stack (new T[100]), current_size (0) { } void push(T thing) { if (current_size == 100) { return; } stack[current_size] = thing; current_size += 1; }
  18. 18. A Stack Template T pop() { T tmp; current_size -= 1; tmp = stack[current_size]; return tmp; } void clear() { current_size = 0; } int query_size() { return current_size; } ~Stack() { delete[] stack; } };
  19. 19. Using The Stack #include <iostream> #include <String> #include "Stack.h" using namespace std; int main() { Stack<int> *myStack; myStack = new Stack<int>(); myStack->push (100); cout << myStack->pop() << endl; return 0; } #include <iostream> #include <String> #include "Stack.h" using namespace std; int main() { Stack<string> *myStack; myStack = new Stack<string>(); myStack->push ("Hello"); cout << myStack->pop() << endl; return 0; }
  20. 20. Benefits of Templates • Templates allow us to have classes and functions that work independently of the data they use. • No need to write a hundred different stacks, just write one using a template. • No significant performance overhead. • It takes a little time for the compiler to generate the actionable code, but it is not significant.
  21. 21. Qualified and Unqualified Names • What does the typename keyword mean in C++? • Have to go back quite a ways to explain this! • Remember in our first header files, we’d often tell them to use a namspace? • To get access to the string class for one example. • This is not good practice. • We tell every class that uses the header to make use of that namespace.
  22. 22. Qualified Names • Qualified names are those that specifically note a scope in a reference. • For example, string is defined in std, thus: • std::string • std::cout • Scope is independent of using a namespace #include <iostream> int main() { std::cout << "Bing!" << std::endl; return 0; }
  23. 23. The Problem template<class S> void do_something() { S::x1 * x2; } What are we doing here? Accessing a member variable called X1 in the parametrized class S? Or… Creating a pointer of type S::x1, with the name x2? Typename resolves this problem.
  24. 24. One Last Thing… • It’s often important to handle deep copies in templates. • For the same reason it’s important in normal classes. • Must include copy constructors and assignment operators here. • Remember the rules regarding these with reference to pointers and others.
  25. 25. Deep Copies on Templates Stack<T>(const Stack<T> &s) { current_size = s.current_size; stack = new T[100]; for (int i = 0; i < 100; i++) { stack[i] = s.stack[i]; } } Remember these are designed to work on value objects and must be explicitly de-referenced when working with pointers, like so: Stack<string> *myStack, *stack2; myStack = new Stack<string>; stack2 = new Stack<string>(*myStack);
  26. 26. Deep Copies on Templates Stack<T>& operator= (const Stack<T> &s) { current_size = s.current_size; delete[] stack; stack = new T[100]; for (int i = 0; i < 100; i++) { stack[i] = s.stack[i]; } return (*this); }
  27. 27. Summary • Method overloading offers a degree of ad hoc polymorphism. • Combinations must be specified initially. • Templates permit for parametric polymorphism. • More complex, but a powerful way of creating generic data types. • We’ll see the power of this in the next lecture.