Today USING POINTERS. Functions: parameters and arguments. Todaywewilllookattopicsrelatingtotheuseofpointers

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1 Today Todaywewilllookattopicsrelatingtotheuseofpointers USING POINTERS Callbyvalue Call by reference Copy constructors Dynamic memory allocation Thelastoftheseisn tdirectlytodowithpointers,butweneedto coverittounderstandsomeofthestuffthatistodowith pointers. ThismaterialiskindofcoveredinChapter3byPohl. Alltheexamplesinthesenotesareontheclasswebsite. Wewillstartbyrecallingtheuseoffunctions. cis15-fall2007-parsons-lectv.3 2 Functions: parameters and arguments Function header declaration: type name ( parameters ); Function definition: type name ( parameters ) { statements Function invocation: or name ( arguments ); variable_of_type = name ( arguments ): Functionshavetobedeclaredbeforetheycanbecalled Thebookusestheword parameters whenafunctionis declared and arguments when a function is invoked(or called ) Whenafunctioniscalled,theprogramcontrolshiftsfrom wherever the function call originates to the body of the function The function arguments get initialized as local variables within the function. Now, parameters can be either: callbyvalueor call by reference cis15-fall2007-parsons-lectv.3 3 cis15-fall2007-parsons-lectv.3 4

2 Call by value Withcallbyvalue,thevalueofeachargumentiscopiedtoalocal variable within the function When the function ends, the program control returns to wherever the function was called from, and the memory allocated within the function returns to the program s memory stack Evenifthevaluesofthelocalargumentswithinthefunction changed during the execution of the function, the values that wereusedtoinvokethefunctiondonotchange Example: void myfun( int a ) { a++; cout << "inside myfun, a=" << a << endl; // end of myfun() int a = 7; cout << "before calling myfun, a=" << a << endl; myfun( a ); cout << "after calling myfun, a=" << a << endl; // end of main() cis15-fall2007-parsons-lectv.3 5 cis15-fall2007-parsons-lectv.3 6 Call by reference Theoutputis: before calling myfun, a=7 inside myfun, a=8 after calling myfun, a=7 Withcallbyreference,theaddressofeachargumentiscopiedtoa local variable within the function When the function ends, the program control returns to wherever the function was called from, and the memory allocated within the function returns to the program s memory stack Because the local arguments are addresses, any changes that were made to the values stored at these address locations during the execution of the function are retained when the function ends inc++,therearetwowaystoimplementcallbyreference: using pointers; and using references. cis15-fall2007-parsons-lectv.3 7 cis15-fall2007-parsons-lectv.3 8

3 Example of call by reference using pointers: void myfun( int *a ) { (*a)++; cout << "inside myfun, *a=" << *a << endl; // end of myfun() int a = 7; cout << "before calling myfun, a=" << a << endl; myfun( &a ); cout << "after calling myfun, a=" << a << endl; Andtheoutputis: before calling myfun, a=7 inside myfun, *a=8 after calling myfun, a=8 cis15-fall2007-parsons-lectv.3 9 cis15-fall2007-parsons-lectv.3 10 Example of call by reference using references: void myfun( int &a ) { a++; cout << "inside myfun, a=" << a << endl; // end of myfun() int a = 7; cout << "before calling myfun, a=" << a << endl; myfun( a ); cout << "after calling myfun, a=" << a << endl; Why use call-by-reference? We use call-be-reference for efficiency. Call-by-value requires the computer to copy the parameters before passing them to the function. Thisisfineiftheparametersareafew charsor doubles. ButinC++wemightcallafunctiononacomplexobjectthat holdsmanymanybytesofdata. Itisfarmoreefficient,inbothmemoryandtime,topassa pointertosuchanobjectthantocopyit. cis15-fall2007-parsons-lectv.3 11 cis15-fall2007-parsons-lectv.3 12

4 Copy constructors Ifyoudodecidetopassacomplexobjectbycall-by-value,you needtodefineacopyconstructorforit. TheproblemisthatC++onitsdoesn tknowhowtocopy complex objects. Soyouhavetodescribeexactlyhowtomakeacopy. Here s a copy constructor for the point object: point::point(const point& p) { x = p.x; y = p.y; (point is not complex enough to require a copy constructor, but itmakesagoodexamplesinceweknowitsowellbynow). C++knowsthisisacopyconstructorbythesignature. Thereisnoreturntype(justlikeaconstructor). Theonlyargumentisareferencetoanobjectofthesameclassas the constructor is defined for. Usingacopyconstructorwegetadeepcopyoftheoriginalobject. Thisisincontrasttotheshallowcopythatwegetifwedon t define a copy constructor. cis15-fall2007-parsons-lectv.3 13 cis15-fall2007-parsons-lectv.3 14 Passing arrays to functions Given the following example: int sum( int A[], int n ) { int s=0; for ( int i=0; i<n; i++ ) s += A[i]; return( s ); // end of sum() Whenthearray Aispassedtothefunction sum(),itispassed using call-by-value on it s base address(i.e., the address of A[0] However, passing an address call-by-value is the same as passing the thing that is addressed call-by-reference. Thus within the context of a function header definition, the following two statements are equivalent: int sum( int A[],... ) {... and int sum( int *A,... ) {... but not in other contexts! ThisexplainsthefunctionheadersyouseeinsomeoftheC++ libraries. cis15-fall2007-parsons-lectv.3 15 cis15-fall2007-parsons-lectv.3 16

5 Generic pointers Last class, we talked about pointers to specific data types, e.g.,: Andtheoutputis: before calling myfun, a=7 inside myfun, a=8 after calling myfun, a=8 int *pi; char *pc; Youcanalsohaveapointertoavoid: void *pv; Clearlythisisnotapointer*to*anything(whatisavoid?). A pointertoavoid isagenericpointer. Youcanuseittopointtodifferentkindsofobject. Whenyoudereferencethepointer,itislikeconvertingittothat data type cis15-fall2007-parsons-lectv.3 17 cis15-fall2007-parsons-lectv.3 18 Below are all legal statements, given the definitions above: pv = pi; pv = pc; pi = reinterpret_cast<int*>(pv); *pi = 12; *pc = A ; Youcanuseagenericpointer,forexample,asanargumenttoa functiontowhichoumightneedtopassdifferentkindsofobject. Dynamic memory allocation InC++,therearetwofunctionsthathandledynamicmemory allocation: new and delete Thesyntaxfor newis: newtype-name newtype-nameinitializer newtype-name[expression] For example: int *p, *q, *r; p = new int(5); // allocation and initialization q = new int[10]; // allocation, but uninitialized r = new int; // allocation, but uninitialized Some compilers initialize values to 0 by default, but not all that isnotpartofthelanguagespecification,sodon trelyonit! cis15-fall2007-parsons-lectv.3 19 cis15-fall2007-parsons-lectv.3 20

6 The syntax for delete is: deleteexpression delete[ ]expression Thefirstformisfornon-arrays;thesecondformisforarrays Thepointofdynamicmemoryallocationistoallowyurprogram todecide,whilerunning,howmuchdataitneedstostore. Youcan,therefore,tailorthesizeofanarraytotheproblemyou are trying to solve. Thenextslidegivesandexample(frombook,p137): int *data; int size; cout << "\nenter array size: "; cin >> size; assert( size > 0 ); data = new int[size]; // allocate array of ints assert( data!= 0 ); for ( int j=0; j<size; j++ ) { cout << (data[j]=j) << \t ; cout << "\n\n"; delete[] data; // deallocate space for array // end of main() cis15-fall2007-parsons-lectv.3 21 cis15-fall2007-parsons-lectv.3 22 Namespaces Youhavealreadybeenusingnamespacesasin: The std namespace is the standard C/C++ namespace that comes with the language Anamespaceisawayofgroupingclassestoavoidnameconflict Thatis,youcouldhavetwothingswiththesamename,butin different name spaces, and then there would be no conflict cis15-fall2007-parsons-lectv.3 23 Declaration of classes within a namespace looks like this: namespace myspace { class myclass1 {... ; class myclass2 {... ; \\ end of namespace Notethatwhenyoudefineanamespaceinaheaderfile,youdo notneedtousethe.hintheincludestatement: versus #include <time.h> Thefirstincludestatementispartofanamespace;thesecondis not cis15-fall2007-parsons-lectv.3 24

7 Summary Thislecturehaslookedatusesofpointers: We mainly discussed the use of pointers(and references) for call-by-reference. We also described copy constructors; Generic pointers; and Dynamic memory allocation. Finally, we briefly discussed namespaces. cis15-fall2007-parsons-lectv.3 25

pointers + memory double x; string a; int x; main overhead int y; main overhead

pointers + memory double x; string a; int x; main overhead int y; main overhead pointers + memory computer have memory to store data. every program gets a piece of it to use as we create and use more variables, more space is allocated to a program memory int x; double x; string a;