COMS W3156: Software Engineering, Fall 2001

COMS W3156:Software Engineering, Fall 2001 Lecture #21: C, C++ Janak J Parekh janak@cs.columbia.edu

Administrativia • “Where’s the !($#%@)! prototype???” • Wednesday! • Moved code review deadline • Interested in doing research? • Come see me, or consider the research fair happening next Friday, the 30th, 10am-4pm • http://acm.cs.columbia.edu/research • For the fair, you need to sign up, but free food!

Project update • Deformalize (demodalize) chat? • Needed for logging • Formal seems too cumbersome • Software Engineering problem: keep reqs or change them? • ChangeOther for bots • How to communicate to server? • Introduce RequestActorChange, or in MapDelta?

Project update (II) • Death sequence? • Attacked message, with death boolean set to true • Kick the user from the game • Delete the object from the map • Remove the player from LDAP: HP no longer make sense • WrappedOutputStream: need to use for final implementation • WebServer: to serve up Images correctly, need Content-Length • I’ve put up my WebServer

Next class • Grr, Phil has one more class than we do • You can go attend his class tomorrow if you like: heavy C++ treatment • My next class begins operating systems and “design patterns” • I’ll probably go over a bit more C++ • But also recitation next week

Today’s class • Finish C • Intro to C++

Pointer examples int x = 7; int *ip = &x; printf(“%d\n”, ip); // What does this print? printf(“%d\n”, *ip); *ip = 9; printf(“%d\n”, *ip); printf(“%d\n”, x);

Pointers: miscellany • If ip == 4, and ints are 4 bytes long, what does ip+1 equal? • This is why pointers are “typed” • C does not have real arrays • Basically consecutive blocks of memory pointed to int a[7]; a == &a[0]; ++a == &a[1]; • Nothing, even pointers, are null-initialized

C’s use of pointers • C does not have Strings either • Basically an array of chars (char *) • Basically a pointer to a block of memory • Char pointers are all over the place • C’s loose typing: the meaning of “0” • False • End-of-string character • Null pointer (#define NULL 0)

Using pointers void foo(int *i) { *i = 5; } int main(void) { int i = 0; foo(&i); }

Using pointers (II) • Note that * and & essentially “cancel each other out” • What if we didn’t have anything to point the pointer to, initially? • What if we want to allocate memory in general for a pointer/reference without a “static” variable?

This is bad int *foo(void) { int i = 5; return &i; } int main(void) { int *j = foo(); }

Huh? • I lied: C has one form of garbage collection • Local variables that are allocated get cleaned up • Well, not really cleaned up: just deallocated • Need to manually allocate memory and assign it to a pointer • The malloc() command • Think of it as a poor man’s new()

Much better int *foo(void) { int *i = (int *)malloc(sizeof(int)); *i = 5; return &i; } int main(void) { int *j = foo(); }

Oh dear… • Any memory you malloc(), you must then free() • Unless you want to wait till the program ends • You don’t want to • “Save the whales. Free the mallocs.” • Otherwise, you’ve got an (insidious) memory leak • Again, philosophy is that you do everything

Sidebar: C == evil void foo(char *dst, char *src) { while (*dst++ = *src++); } • Yes, this code actually compiles and runs • What does it do? • * has higher precedence than ++ • Strings terminated by a “\0”, or 0 character, which is false • Wacky, huh?

Passing multiple values • So, in Java, how do you pass multiple values? • Lots of parameters (bad) • Pass a complex Object into a method call, and have it modify the Object • Return a new Object • In C? • No objects! • But, there are structs…

Struct • Basically, a class without methods struct Point { int x; int y; }; /* note the semicolon */ int main(void) { struct Point myPoint; myPoint.x = 5; myPoint.y = 5; }

Now, how to pass this around? • You don’t want to pass it call-by-value • Slow • Can’t return results back all that easy • You want to return it via call-by-reference • Pass a pointer to the struct around

Back to foo() struct Point { int x; int y; }; void foo(struct Point *myP) { (*myP).x = 5; (*myP).y = 5; } int main(void) { struct Point myPoint; foo(&myPoint); }

Miscellany on structs • (*myP).x looks ugly • C has a bizarre shortcut: myP -> x • Yes, a dash followed by a greater than • Always remember that a -> b is equivalent to (*a).b • Precedence of operators • It’s cumbersome to have to say struct Point every time • Use typedefs: define “aliases” • typedef struct Point PointT; • or even typedef struct Point *PointP;

More miscellany on structs • Since not everything happens in main, malloc’ing structs is very common • The sizeof operator works on pretty much any datatype, be it structs or primitive datatypes • Guaranteed tip: you will come across PointP myP = (PointP)malloc(sizeof(PointP)); • and be glad that you were attending this class…

Function prototypes • As I said, C’s compiler is not all that smart • If you have: int main(void) { int i = foo(); } int foo(void) { return 5; } • It will not work

Function prototypes (II) • Need to clue in the compiler by putting the following prototype before main: int foo(void); • Looks like a Java interface construct • Often, you collect the prototypes into a header (.h) file, then #include it into your various source (.c) files

On various .c files… • A common way to build multi-source-file code in C is to • build a set of .h files that export functionality (i.e. “public” methods) • #include these .h files • Compile and link the .c files together • As long as you have a prototype, the compiler will not care about the definition • Like compiling against an Interface • The Linker will, though…

C++ • Not strictly a superset of C, but C++ compilers backwards-compatible with C • Certain subtle language differences • C with classes, but a whole lot more • Function, operator overloading • Powerful “template” functionality (Phil – tomorrow) • Exceptions

Quick primer to C++ • g++ is the GNU C++ compiler • Actually a C++ plugin to gcc • Can feed C code to it, will work fine • Can integrate C and C++ code pretty easily • However, ideally, you want to avoid using many C constructs

Hello, world in C++ #include <iostream> using namespace std; int main(void) { cout << “Hello world\n”; } • Note use of namespaces, streams • << operator is overloaded to allow combination/concatenation of strings with streams • Can do cout << “foo” << “bar” << endl;

Class declarations in C++ class Rectangle { private: double width; double height; public: Rectangle(int width, int height); ~Rectangle(); void setWidth(int width); int getArea(); // and so on… }

Implementing methods • Inline in the class declaration • Or, use the class as a prototype and implement elswhere void Rectangle::setWidth(int w) { width = w; }

Constructors and destructors • You have both, not just one • Motivation of destructor: cleanup the object, free any memory, etc. • Don’t use C++’s default denstructor if you new anything • You should delete anything you new • Don’t use malloc/free unless absolutely necessary

C++ miscellany • Operator overloading: nice, but must be careful • Often need to override assignment and copy operators if you have funky data structures (e.g, the “=” operators) • References in function prototypes (finally!) • Templates: “type” data structures • STL: Standard Template Library • You now have “string”s! • Lots of other data structures • Somewhere between C and Java API’s • Very, very fast if used correctly

COMS W3156: Software Engineering, Fall 2001