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CS3101-2, Lecture 5 CS3101-2 Programming Languages C++ Lecture 5 Matthew P. Johnson Columbia University Fall 2003

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CS3101-2, Lecture 5 Agenda hw3 was due last night Today: Templates Exceptions Other odds and ends The STL Grading and the final hw4 TBA tonight

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CS3101-2, Lecture 5 Templates Often want to do basically the same thing with different things functions work on variables only types specified algorithmic thinking computer science functionalism in phil. of mind abstraction human = the rational animal (Aristotle) Sometimes want to do basically the same thing with different types of things Queue of ints, queue of widgets abstract data types

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CS3101-2, Lecture 5 max functions Suppose want the max of two numbers What kind of numbers? ints chars floats doubles All! How?

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CS3101-2, Lecture 5 max functions Soln 1: Write one, maxly general function double max(double a, double b) { return a > b ? a : b; } double x = max(2.5, 3.5); char c = (char)max(A,B); This works but its not nice All four types can widen to doubles but must be cast back

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CS3101-2, Lecture 5 max functions Soln 2: Write one function for each type int max(int a, int b) { return a > b ? a : b; } double max( etc. Is allowed in C++ (though not in C) But manually duplicating code for nontrivial ftns bad hard to maintain

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CS3101-2, Lecture 5 max functions Soln 3: Use the C preprocessor macros #define max(a,b) (a > b ? a : b) C source code is preprocessed #include s replaced with header files #ifndef, etc. macro calls replaced with macro content int c = max(2,3); int c = (2 > 3 ? 2 : 3); Works too, but complications, e.g.: z = max(x++, y++) z = (x++ > y++ ? x++ : y++) x, y inc-ed twice Need many parens sq(a+b), etc.

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CS3101-2, Lecture 5 max functions Soln 4: Use the CPP in a more sophisticated way Dont use the CPP to generate expressions but to generate functions #define define_max(t)\ t max(t a, t b) {\ return a > b ? a : b;\ } define_max(char) define_max(int) etc. no ; Avoids prev. CPP problems But reqs code for all poss types Done manually

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CS3101-2, Lecture 5 Templates template result ftn(param-list) {} The place-holder for the substituted type is t template and class are used as keywords can use typename in place of class T is a type Primitive or class All occurrences in ftn replaced with real type

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CS3101-2, Lecture 5 max functions Soln 5: use templates parameterized function expands per type as necessary template T max(T a, T b) { return a > b ? a : b; } Now can simply call the ftn: x = max(2.5,3.5); Compiler autoly creates only the ftn specializations needed

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CS3101-2, Lecture 5 Sorting things Consider problem of sorting: Sorting ints Sorting doubles Sorting strings Sorting widgets Point of sorting: put list in order Q: What does in order mean? A: Given an ordering relation < on the members For x and y, tells whether x < y Reorder s.t. x is before y iff x < y

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CS3101-2, Lecture 5 Generic sorting Sort alg doesnt depend of element type Merge sort, quick sort, etc. Need only give means to compare to elms How? In C we pass in a pointer to a compare ftn: void qsort(void *base, int n, int size, int (*cmp)(const void *, void *)); Pass in pointer to ftn: int cmp(const void *a, void *b) { Widget* w1 = (Widget*)a; } Works, but very awkward

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CS3101-2, Lecture 5 Generic sorting In Java, we pass a Comparable implementer In the sort ftn we say if (a.compareTo(b) < 0) // means a < b Objects must implement this interface compare with ftn call Primitives cant implement Compared with ops could put in wrappers

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CS3101-2, Lecture 5 Generic sorting C++ soln 1: Define our own Comparable analog: abstract class Has virtual compareTo Or, better: has virtual operators Any class extending our class can now be sorted Pass in array of Comparable-extending objects Sort uses polymorphism to treat as (mere) Comparables Downside: can only sort objects if they extend Comparable Mult inher: can always add Comp parent, but must do so To sort primitives must create wrapper classes

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CS3101-2, Lecture 5 Generic sorting C++ soln 2: use templates! Let sort take an array of some arb. kind Dont need Comparable Dont need compareTo In sort, just say if (a < b) If these are numbers, this works If these are objects that overload ops, this works Only requirement: kind supports ops

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CS3101-2, Lecture 5 Templates: swapping Remember our swap-with-ptrs ftn? void swap(int &a, int &b) { int temp c = a; a = b; b = a; } Suppose we want to swap other types templates

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CS3101-2, Lecture 5 Generic swapping template void swap(T &a, T &b) { T temp c = a; a = b; b = a; } Now can swap any prim Can also swap any objects As long as = op is public

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CS3101-2, Lecture 5 Fancier swapping Remember our fancier swap ftn? void swap(int &a, int &b) { a ^= b ^= a ^= b; } Fancier template function: template void swap(T &a, T &b) { a ^= b ^= a ^= b; } Now can swap ints, chars, longs But: cannot swap objects Unless their ^= is overloaded unlikely

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CS3101-2, Lecture 5 Template specialization string s,t max(s,t); works But max(hi,there) doesnt: if (hi < there) compares two pointers - where the char[] s start Not what we mean Soln: create a specialization special version for this case We check for spec. before template char *max(char *a, char *b) { return strcmp(a,b) > 0 ? a : b; }

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CS3101-2, Lecture 5 Class templates Couple weeks ago: wrote a stack class supported only integers Well abstract element type away Abstract data types Only changes to declar: 1. prepend on class dfn: template class className {} 2.Replace int T For ftn implems, we 1. prepend the same 2. replace className with className 3. Replace int T template void Stack ::push(const T elm){} To instantiate: Stack strStack;

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CS3101-2, Lecture 5 Class specialization Similarly, can specialize member functions of class templates: void stack ::push(const char *const item) { data[count++] = item; }

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CS3101-2, Lecture 5 Templates & statics Review: static data members one inst shared by all class insts What about statics in templates classes? Q: Could one inst be shared by all insts? A: No consider: template class C { static T mem; } mem couldnt me shared by all insts shared by all insts But: for C, mem shared by all C insts

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CS3101-2, Lecture 5 Templates & friends Given class, can declare some outside ftn or class its friend We have a stack class Suppose: want to declare external sort ftn its friend Before: had stack with ints could use sort ftn based on ints Now: have Stack friend is template too template class Stack { friend void C ::f5(X );

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CS3101-2, Lecture 5 Odds and ends: Forward declarations Suppose classes Cat and Dog each depend on each other class Cat { void look(Dog d) { cout ioiter Enter two nums: 5 6 T">

CS3101-2, Lecture 5 I/O iterators ioiter.cpp Code: int x = *intIn; intIn++; x += *intIn; Output: C:\3101-2\lec5>ioiter Enter two nums: 5 6 The sum is: 11 But if code: int x = *intIn; /*intIn++;*/ x += *intIn; Then output: C:\3101-2\lec5>ioiter Enter two nums: 5 6 The sum is: 10

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CS3101-2, Lecture 5 copy function vect2.cpp Another way to print container: use copy function if (!vect.empty()) { ostream_iterator out(cout, " "); copy(vect.begin(), vect.end(), out); } copy(src begin it, src end it, dest it); src begin it: vect.begin() src end it: vect.end() dest it: ostream_iterator out(cout, " ") its an ostream_iterator its wrapping around cout its outputting T s its printing between the T s

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CS3101-2, Lecture 5 shuffle, sort, search, min vect2.cpp void sort(begin it, end it) it-s must be random-access members must support ==, < void random_shuffle(begin it, end it) same reqs bool binary_search(begin, end, target) same reqs also: assumes sorted min_element(v.begin(), v.end()) returns iterator

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CS3101-2, Lecture 5 shuffle, sort, search, min vect3.cpp All ftns translate automatically to strings transform ftn vect4.cpp transform(begin it, end it, dest it, ftn) transform(v.begin(), v.end(), v.begin(), square); cout