'&$%CSE 303:Concepts and Tools for Software DevelopmentDan GrossmanSpring 2005Lecture 13— C: post-overview, function pointers, coding up objectsDan Grossman CSE303 Spring 2005, Lecture 13 1'&$%Where are We“Official Notice”:• Homework 4 will be posted today, due a week from Thursday• Midterm Friday, “first page” will be posted today or tomorrow– Covers through today• Homework 2 will be available tomorrow (thanks Ben!)Today:• Top-down view of C• Function pointers• Coding up objects (more later?)Dan Grossman CSE303 Spring 2005, Lecture 13 2'&$%Top-down post-overviewNow that we have seen m ost of C, le t’s summ arize/organize:• Preprocessing– #include for declarations defined elsewhere– #ifdef for conditional compilation– #define for token-based textual substitution• Compiling (type-checking and code-generating)– A sequence of declarations– Each C file becomes a .o file• Linking– Take .o and .a files and make a program– libc.a in by default, has printf, malloc, ...– More laterDan Grossman CSE303 Spring 2005, Lecture 13 3'&$%• Executing– O/S maintains the “big array” address-space illusion– Execution starts at main– Library manages the heap via malloc/free.Dan Grossman CSE303 Spring 2005, Lecture 13 4'&$%C, the language• A file is a sequence of declarations:– Global variables (t x; or t x = e;)– struct (and union and enum definitions)– Function prototypes (t f(t1,...,tn))– Function definitions– typedefs• A function body is a statement– Statements are sim ilar to in Java (+ goto, –exception-handling, ints for bools)– Local declarations have local scope.• Left-expressions (locations) and right-expressions (values,including pointers-to-locations)– * for pointer dereference, & for address-of, . for field acces sDan Grossman CSE303 Spring 2005, Lecture 13 5'&$%C language continued“Convenient” expression forms:• e->f means (*e).f• e1[e2] means *(e1 + e2)– But + for pointer arithmetic takes the size of the pointed toelement into account!– That is, if e1 has type t* and e2 has type int, then , then(e1 + c) == (((int)e1) + (sizeof(t) * c))– The compiler “does the sizeof for you” – don’t double-do it!“Size is exposed”: In Java, “(just about) everything is 32 bits”. In C,pointers are usually the same size as other pointers, but not everythingis a pointer.Dan Grossman CSE303 Spring 2005, Lecture 13 6'&$%C is unsafeThe following is allowed to set your computer on fire:array-bounds violation (bad pointer arithmetic), dangling-pointerdereferences, dereferencing NULL, using results of wrong casts, usingcontents of uninitialized locations, linking errors (inconsistentassumptions), ...Casts are not chec ked (no secret fields at run-time; all bits look thesame)Dan Grossman CSE303 Spring 2005, Lecture 13 7'&$%Function pointers“Pointers to code” are almost as useful as “pointers to data”.(But the syntax is more painful.)(Somewhat silly) example:void app_arr(int len, int * arr, int (*f)(int)) {for(; len > 0; --len)arr[len-1] = (*f)(arr[len-1]);}int twoX(int i) { return 2*i; }int sq(int i) { return i*i; }void twoXarr(int len, int* arr) { app_arr(len,arr,&twoX); }void sq_arr(int len, int* arr) { app_arr(len,arr,&sq); }CSE 341 spends a week on why function pointers are so useful; todayis mostly just how in C.Dan Grossman CSE303 Spring 2005, Lecture 13 8'&$%Function pointers, cont’dKey computer-science idea: You can pass what code to execute as anargument, just like you pass what data to process as an argument.Java: An object is (a pointer to) code and data, so you’re doing bothall the time.// Javainterface I { int m(int i); }void f(int arr[], I obj) {for(int len=arr.length; len > 0; --len)arr[len-1] = obj.m(arr[len-1]);}C separates the concepts of code, data, and pointers.Dan Grossman CSE303 Spring 2005, Lecture 13 9'&$%C function-pointer syntaxC syntax: painful and confusing. Rough idea: The compiler “knows”what is code and what is a pointer to code, so you can write less thanwe did on the last s lide:arr[len-1] = (*f)(arr[len-1]);→ arr[len-1] = f(arr[len-1]);app_arr(len,arr,&twoX);→ app_arr(len,arr,twoX);For types, let’s pretend you always have to write the “pointer to code”part (i.e., t0 (*)(t1,t2,...,tn)) and for declarations the variableor field name goes after the *.Sigh.Dan Grossman CSE303 Spring 2005, Lecture 13 10'&$%Toward objectsIf you want a pointer to code and data, like in Java, then DIY:struct MyPoint {// dataint x;int y;// codeint (*getX)(struct MyPoint*);void (*setX)(struct MyPoint*,int);int (*getY)(struct MyPoint*);void (*setY)(struct MyPoint*,int);double (*distance2origin)(struct MyPoint*);};“extra argument” is Java’s this, e lse c ode has no access to the other(data and code) fields.When this “coding pattern” became common, C++ was born (sorta).Dan Grossman CSE303 Spring 2005, Lecture 13 11'&$%A much bigger storyWe have just s cratched the s urface of “C-level O OP”.Food for thought (not on the exam!):• How c ould a subclass override methods, add methods, or addfields? (you need casts in a couple different places!)• What is the difference between calling this->getX and callingMyPoint_getX?• Aren’t struct MyPoint objects awfully large – how could wesave space?Dan Grossman CSE303 Spring 2005, Lecture 13
View Full Document
Unlocking...