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COMP2004
Programming Practice
2002 Summer School


Kevin Pulo
School of Information Technologies
University of Sydney

Unix development tools

• make
  • Automatic code compilation
• gdb
  • Debugging
• shell
  • Simple text-manipulation programs
  • Bourne shell vs bash
  • Many other utilities

make

OBJS = main.o student.o course.o
CXXFLAGS = -Wall -g
LDFLAGS = -lm

all: prog
prog: $(OBJS)
$(CXX) $(CXXFLAGS) $(LDFLAGS) \
-o prog $(OBJS)
clean:
rm -f $(OBJS)

gdb

• Compiled with -g
• Crashed with

Segmentation fault (core dumped)

• Run: gdb prog core
• Common commands
  • bt
    • Get a stack backtrace
    • Shows where the crash happened
  • p head
    • Print value of variable head

Simple shell

• Simple shell commands
  • cd, cp, mv, rm
  • echo - Prints parameters to stdout
  • cat - Prints files (or stdin) to stdout
• Redirection
  • echo "foo" > foo.txt
  • echo "bar" >> foo.txt
  • cat < hello.txt
• Redirection caveat
  • prog file.txt > file.txt

Shell / environment variables

• Assign:
  • variable_name="value of variable"
• Use:
  • echo "$variable_name"
• Shell variable -> environment variable:
  • export variable_name

If

• Exit status:
  • 0 is true, everything else is false
  • exit n command in shell

if cmp file1.txt file2.txt; then
echo "files same"
elif diff -i file1.txt file2.txt; then
echo "files same except case"
else
echo "files different"
fi

Alternate if

• if [ "$somevar" = "hello" ]; then
• if [ "$somevar" != "hello" ]; then
• if [ "$num" -lt 42 ]; then
• if [ "$num" -ge 42 ]; then
• if [ -r "$fname" ]; then
• if [ "$fname" -nt "file.txt" ]; then
• and so on...

stdout -> shell variable

• Use backticks or $() in bash:

echo "f1.h f2.h f3.h" > file.lst
filelist=`cat file.lst`
cat $filelist
cat `cat file.lst`
cat $(cat file.lst)

• Shell arithmetic (expr in Bourne, $(()) in bash):
  • result=`expr 3 + 8 \* 2`
  • result=$((expr 3 + 8 * 2))

while

count=1
while [ $count -le 10 ]; do
echo "$count"
count=$(($count + 1))
done

while :; do
if [ -e "file.txt" ]; then
break
fi
sleep 1
done

Command line parameters

• The shell script sample:

#!/gnu/usr/bin/bash
while [ $# -ge 2 ]; do
echo "$1" "$2"
shift
done

bash$ sample a bc defgh a bc
bc defgh

case

case "$1" in
-d*)
cmd="diff"
;;
-c*)
cmd="cmp"
;;
*)
exit 1
;;
esac
$cmd file1.txt file2.txt

for and pipes

#!/gnu/usr/bin/bash
for infile in tests/*.in; do
name=`basename $infile .in`
if prog < $infile | cmp - \
tests/$name.out; then
echo "$name : passed"
else
echo "$name : failed"
fi
done

Useful Unix utilities

• sed- stream editor
• awk- processing columnar data
• sort- sorts lines in a file
• uniq- removes duplicate lines
• head- output first n lines
• tail- output last n lines
• cut- extracts columns of characters
• join, paste - merges files by columns

Course survey

• Voluntary and anonymous
  • So don't write your name on it
• 15 minutes
  • I won't be here
  • Student representative collects and seals forms
• Comments are useful for improving all courses
• Unit of Study = Programming Practice
• Unit of Study Code = COMP2004

Advanced C++

• Generic code
  • Template functions
  • Template classes
• Exceptions
• Exception Safety
• String streams

Template Functions

template
typename vector::size_type
find(const vector &v,
const T &value) {
for (typename vector::size_type
i = 0; i < v.size(); ++i)
if (v[i] == value) return i;
return v.size();
}
vector vi;
find(vi, 42);

Template Classes

template
class Node {
T value;
Node *next;
...
};

• Heavy reliance on operators
• All template code in .h files
  • Only time this is allowed

Exceptions

• Seperate error detection from handling
• Any object can be exception
• First catch type matched is used (including inheritance)

• void func() throw (e1, e2)
• void func() throw ()
• void func()

• Constructors can throw exception
• Copy constructors and destructors shouldn't

Exceptions

struct Error { int i;
Error(int i) : i(i) {} };
struct OtherError { };

try {
throw Error(42);
} catch (Error e) {
cout << e.i << endl;
throw OtherError();
} catch (...) {
throw;
}

Exception Safety

• Each function must:
• Basic Guarantee
  • Resources not leaked, objects still usable
• Strong Guarantee
  • Program state is as before the call
• Nothrow
  • The function will never throw

Exception Unsafe Code

void some_function(string name) {
Person *fred = new Person();

fred->setName(name);

delete fred;
}

Fixing with try/catch

void some_function(string name) {
Person *fred = new Person(name);
try {
fred->setName(name);
} catch (...) {
delete fred;
throw;
}
delete fred;
}

Fixing with auto_ptr

void some_function(string name) {
Person *fredp = new Person(name);
auto_ptr fred(fredp);

fred->setName(name);
}

String streams

• Old - istrstream / ostrstream
• New - istringstream / ostringstream
• Allow input / output to / from strings using normal << and >>

New style

int main() {
string s = "42 15";
istringstream is(s);
int i, j;
is >> i >> j;

ostringstream os;
os << i << "." << j;
s = os.str();
cout << s << endl;
}


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