CSE 303 Concepts and Tools for Software Development Hal Perkins Winter 2009 Lecture 24 Introduction to C CSE 303 Winter 2009 Lecture 24 1 C C is an enormous language All of C Classes and objects kind of like Java some crucial differences Many more little conveniences I O new delete function overloading pass by reference bigger standard library Namespaces kind of like Java packages Stuff we won t do const different kinds of casts exceptions templates multiple inheritance We will focus on a couple themes rather than just a big bag of new features to memorize CSE 303 Winter 2009 Lecture 24 2 Our focus OOP in a C like language may help you understand C and Java better We can put objects on the stack or the heap an object is not a pointer to an object Still have to manage memory manually Still lots of ways to HCBWKMSCODa Still distinguish header files from implementation files Allocation and initialization still separate concepts but easier to construct and destruct Programmer has more control on how method calls work different defaults from Java a hopefully crash but who knows might silently corrupt other data CSE 303 Winter 2009 Lecture 24 3 Hello World include iostream int main Use standard output stream cout and operator to send Hello World and an end line to stdout std cout Hello World std endl return 0 Differences from C new style headers no h namespace access I O via stream operators Differences from Java not everything is in a class any code can go in any file CSE 303 Winter 2009 Lecture 24 4 Compiling Need a different compiler than for C use g on attu Example g Wall o hello hello cc The cc extension is a convention just like c for C but less universal also see cpp cxx C Uses the C preprocessor no change there Now A few niceties before our real focus classes and objects CSE 303 Winter 2009 Lecture 24 5 I O Operator takes a ostream and various things and outputs it returns the stream which is why this works std cout 3 hi f x n Easier and safer than printf Operator takes istream and various things and inputs into it Easier and safer than scanf Do not use pointers e g int x std cin x Can think of and as keywords but they are not Operator overloading redefines them for different pairs of types In C they mean left shift and right shift of bits undefined for non numeric types Lack of address of for input done with call by reference later CSE 303 Winter 2009 Lecture 24 6 Namespaces In C all non static functions in the program need different names Even operating systems with tens of millions of lines Namespaces cf Java packages let you group top level names namespace myspace definitions Of course then different namespaces can have the same function names and they are totally different functions Can nest them Can reuse the same namespace in multiple places Pariticularly common in the h and the cc For example the whole C standard library is in namespace std To use a function variable etc in another namespace do thespace someFun not like in Java CSE 303 Winter 2009 Lecture 24 7 Using To avoid having to write namespaces and constantly use a using declaration Example include iostream using namespace std int main cout Hello World endl return 0 CSE 303 Winter 2009 Lecture 24 8 Onto OOP Like Java Fields vs methods static vs instance constructors Method overloading functions operators and constructors too Not quite like Java access modifiers e g private syntax and default declaration separate from implementation like C funny constructor syntax default parameters e g 0 Nothing like Java Objects vs pointers to objects Destructors and copy constructors virtual vs non virtual to be discussed CSE 303 Winter 2009 Lecture 24 9 Stack vs heap Java cannot stack allocate an object only a pointer to one C can stack allocate a struct then initialize it C stack allocate and call a constructor where this is the object s address as always Property p1 10000 Java new Property calls constructor returns heap allocated pointer C Use malloc and then initialized must free exactly once later C Like Java but can also do new int 42 Like C must deallocate but must use delete instead of free CSE 303 Winter 2009 Lecture 24 10 Destructors An object s destructor is called just before the space for it is reclaimed A common use Reclaim space for heap allocated things pointed to first calling their destructors But not if there are other pointers to it aliases Meaning of delete x call the destructor of pointed to heap object then reclaim space Destructors also get called for stack objects when they leave scope Advice Always make destructors virtual learn why soon CSE 303 Winter 2009 Lecture 24 11 Arrays Create a heap allocated array of objects new A 10 Calls default zero argument constructor for each element Convenient if there s a good default initialization Create a heap allocated array of pointers to objects new A 10 More like Java but not initialized As in C new A and new A 10 have type A new A and new A 10 both have type A Unlike C to delete a non array you must write delete e Unlike C to delete an array you must write delete e Else HYCSBWK the deleter must know somehow what is an array CSE 303 Winter 2009 Lecture 24 12 Digression Call by reference In C we know function arguments are copies But copying a pointer means you still point to the same uncopied thing Same in C but a reference parameter the character after it is different Callee writes void f int x x x 1 Caller writes f y But it s as though the caller wrote f y and everywhere the callee said x they really said x So that little has a big meaning CSE 303 Winter 2009 Lecture 24 13 Copy Constructors In C we know x y or f y copies y if a struct then member wise copy Same in C unless a copy constructor is defined then do whatever it says A copy constructor by definition takes a reference parameter else we d need to copy but that s what we re defining of the same type Let s not talk about the const CSE 303 Winter 2009 Lecture 24 14 Now more OOP Subclassing In many ways OOP is all about subclasses overriding methods Often not what you want but what makes OOP fundamentally different from say functional programming CSE341 C gives you lots more options than Java with different defaults so it s easy to scream compiler bug when you mean I m using the wrong feature Basic subclassing class D public C This is public inheritance C has other kinds too won t cover Differences affect visibility and issues when you have multiple superclasses won t cover So do not forget the public
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