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C++ Templates: Parametric Polymorphism and Function Templates in CSE333, Autumn 2018, Study notes of C programming

Eastern Maine Community College (EMCC)C programming

An outline for lecture 14 of the CSE333 course at the University of X, focusing on C++ templates. The instructor, Hal Perkins, discusses the concept of parametric polymorphism and function templates. Students are introduced to the idea of writing 'generic code' that is type-independent and compile-time polymorphic. examples of template functions and compiler inference, as well as non-type templates. The lecture also covers class templates and their use in creating a pair class.

Typology: Study notes

2021/2022

Uploaded on 09/12/2022

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CSE333, Autumn 2018L14: C++ Templates
C++ Templates
CSE 333 Autumn 2018
Instructor: Hal Perkins
Teaching Assistants:
Tarkan Al-Kazily Renshu Gu Travis McGaha
Harshita Neti Thai Pham Forrest Timour
Soumya Vasisht Yifan Xu
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Download C++ Templates: Parametric Polymorphism and Function Templates in CSE333, Autumn 2018 and more Study notes C programming in PDF only on Docsity!

C++ Templates

CSE 333 Autumn 2018

Instructor: Hal Perkins

Teaching Assistants:

Tarkan Al-Kazily Renshu Gu Travis McGaha

Harshita Neti Thai Pham Forrest Timour

Soumya Vasisht Yifan Xu

Administrivia v Homework 2 due tomorrow (10/25) § File system crawler, indexer, and search engine § Don’t forget to clone your repo to double-/triple-/quadruple-check

compilation, execution, and tests!

  • If your code won’t build or run when we clone it, well, you should have

caught that…

v Midterm: Friday, 11/2 in class § Closed book, no notes § Old exams and topic list on the course web now

  • Everything up through C++ classes, dynamic memory, templates & STL § Review in sections next week v No new exercises due until after hw1 due

Suppose that… v You want to write a function to compare two ints v You want to write a function to compare two strings § Function overloading! // returns 0 if equal, 1 if value1 is bigger, - 1 otherwise int compare (const int &value1, const int &value2) { if (value1 < value2) return - 1 ; if (value2 < value1) return 1 ; return 0 ; } // returns 0 if equal, 1 if value1 is bigger, - 1 otherwise int compare (const int &value1, const int &value2) { if (value1 < value2) return - 1 ; if (value2 < value1) return 1 ; return 0 ; } // returns 0 if equal, 1 if value1 is bigger, - 1 otherwise int compare (const string &value1, const string &value2) { if (value1 < value2) return - 1 ; if (value2 < value1) return 1 ; return 0 ; }

Hm… v The two implementations of compare are nearly identical! § What if we wanted a version of compare for every comparable type? § We could write (many) more functions, but that’s obviously wasteful and redundant v What we’d prefer to do is write “ generic code ” § Code that is type-independent § Code that is compile-type polymorphic across types

Function Templates v Template to compare two “things”: #include #include // returns 0 if equal, 1 if value1 is bigger, - 1 otherwise template // <...> can also be written int compare (const T &value1, const T &value2) { if (value1 < value2) return - 1 ; if (value2 < value1) return 1 ; return 0 ; } int main (int argc, char **argv) { std::string h("hello"), w("world"); std::cout << compare ( 10 , 20 ) << std::endl; std::cout << compare <std::string>(h, w) << std::endl; std::cout << compare (50.5, 50.6) << std::endl; return 0 ; }

functiontemplate.cc

Compiler Inference v Same thing, but letting the compiler infer the types: #include #include // returns 0 if equal, 1 if value1 is bigger, - 1 otherwise template int compare (const T &value1, const T &value2) { if (value1 < value2) return - 1 ; if (value2 < value1) return 1 ; return 0 ; } int main (int argc, char **argv) { std::string h("hello"), w("world"); std::cout << compare ( 10 , 20 ) << std::endl; // ok std::cout << compare (h, w) << std::endl; // ok std::cout << compare ("Hello", "World") << std::endl; // hm… return 0 ; }

functiontemplate_infer.cc

What’s Going On? v The compiler doesn’t generate any code when it sees the template function § It doesn’t know what code to generate yet, since it doesn’t know what types are involved v When the compiler sees the function being used, then it understands what types are involved § It generates the instantiation of the template and compiles it (kind of like macro expansion)

  • The compiler generates template instantiations for each type used as

a template parameter

This Creates a Problem #include #include "compare.h" using namespace std; int main (int argc, char **argv) { cout << comp ( 10 , 20 ); cout << endl; return 0 ; } #include "compare.h" template int comp (const T& a, const T& b) { if (a < b) return - 1 ; if (b < a) return 1 ; return 0 ; } #ifndef COMPARE_H #define COMPARE_H template int comp (const T& a, const T& b); #endif // COMPARE_H

compare.h

compare.cc

main.cc

Solution # #include #include "compare.h" using namespace std; int main (int argc, char **argv) { cout << comp ( 10 , 20 ); cout << endl; return 0 ; } template int comp (const T& a, const T& b) { if (a < b) return - 1 ; if (b < a) return 1 ; return 0 ; } #ifndef COMPARE_H #define COMPARE_H template int comp (const T& a, const T& b); #include "compare.cc" #endif // COMPARE_H

compare.h

compare.cc

main.cc

Class Templates v Templates are useful for classes as well § (In fact, that was one of the main motivations for templates!) v Imagine we want a class that holds a pair of things that we can: § Set the value of the first thing § Set the value of the second thing § Get the value of the first thing § Get the value of the second thing § Swap the values of the things § Print the pair of things

Pair Function Definitions template void Pair:: set_first (Thing &copyme) { first_ = copyme; } template void Pair:: set_second (Thing &copyme) { second_ = copyme; } template void Pair:: Swap () { Thing tmp = first_; first_ = second_; second_ = tmp; } template std::ostream &operator<<(std::ostream &out, const Pair& p) { return out << "Pair(" << p. get_first () << ", " << p. get_second () << ")"; }

Pair.cc

Using Pair #include #include #include "Pair.h" int main (int argc, char** argv) { Pair<std::string> ps; std::string x("foo"), y("bar"); ps. set_first (x); ps. set_second (y); ps. Swap (); std::cout << ps << std::endl; return 0 ; }

usepair.cc

Review Questions (both template and class issues) v Why are only get_first() and get_second() const? v Why do the accessor methods return Thing and not references? v Why is operator<< not a friend function? v What happens in the default constructor when Thing is a class? v In the execution of Swap(), how many times are each of the following invoked (assuming Thing is a class)?

ctor ____ cctor ____ op= ____ dtor ____