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COMP2004
Programming Practice
2002 Summer School


Kevin Pulo
School of Information Technologies
University of Sydney

Associative Containers

• Associate values with keys
  • eg. hashtable
• Provide efficient insertion/retrieval
• Maintain an internal ordering
  • May differ with library implementations
  • ie. you can't trust the internal order

Maps

• Maps are the most common associative container
• Correspond to hashtables/hashmaps
• Operator overloading makes them easy to use

Simple Map Example

#include

Accessing map items

• map[key]
  • Each key has only one value
  • Returns reference to value for key
  • Can modify the value
  • Creates key and value if needed

Map Iterator Example

#include

Pair

• Dereferencing a map iterator gives a pair object
• A pair is a simple data structure
• The first member gives the key
• The second member gives the value
• You can use pairs directly in your code
  • ie. first doesn't have to be a key, second doesn't have to be a value
  • You can use anywhere you need to store two things together

Manually inserting into a map

• Given

map m;

• then can insert a pair with

pair p("test", 42);
m.insert(p);

• or more simply

m.insert(pair("test", 42));

Set

• Similar to a map
• Has only a key, with no value
• Useful when you only care about existence

set example

#include
using namespace std;
int main() {
set nums;
int val;
while (cin >> val)
nums.insert(val);
for (set::const_iterator i =
nums.begin(); i != nums.end(); ++i)
cout << *i << endl;
}

Testing Existance

• How do you check if a particular key exists?
• Iterating over the set/map too be slow
• Both set and map provide a find() method
• It returns an iterator
  • To the entry with that key, if found
  • end() if key not found

set find() example

set s;
set::const_iterator si = s.find("it");
if (si != s.end())
cout << "Found it" << endl;
else
cout << "Didn't find it" << endl;

Another set find()

int main() {
set nums;
int val;
while (cin >> val)
nums.insert(val);

if (nums.find(42) != nums.end())
cout << "42 entered sometime";
else
cout << "42 not entered";
}

map find() example

map m;
map::const_iterator mi
= m.find("it");
if (mi != m.end())
cout << "It is : " << mi->second
<< endl;
else
cout << "Didn't find it" << endl;

Another map find()

int main() {
map count;
string s;
while (cin >> s) count[s]++;
if (count.find("hello") != count.end())
cout << "You said hello";
else
cout << "You never said hello";
}

Removing from a map/set

• Use the erase() method
• Can take a key:

m.erase(key);

• or iterator:

m.erase(m.find(key));

• Some STL implementations have a buggy key version
  • If so, just use the iterator version

multimap / multiset

• There are also 'multi' versions of map and set
• They allow multiple identical keys
• multimaps are not used too often
  • map > used instead

multiset Example

#include
int main() {
multiset nums;
int val;
while (cin >> val)
nums.insert(val);
for (multiset::const_iterator i =
nums.begin(); i != nums.end(); ++i)
cout << *i << endl;
}

Exceptions

• Allows seperation of error handling
• Detect errors locally
• Handle errors where appropriate
• Has a performance impact with most compilers

C++ Exceptions

• Exceptions can be of any type
• Could throw a vector, int, string, Person, or whatever
• Usually it is best to use specific exception classes

Throwing An Exception

struct Domain_Error {
int number;
Domain_Error(int d) : number(d) { }
};

double square_root(int n) {
if (n < 0) {
throw Domain_Error(n);
}
return sqrt(n);
}

Catching An Exception

int main() {
int num;
while (cin >> num) {
try {
cout << square_root(num) << '\n';
} catch(Domain_Error e) {
cerr << e.number
<< " is not valid\n";
}
}
}

Uncaught exceptions

• If an exception is thrown in a function but not caught, it goes to the calling function
• This continues all the way to main()
• If it's still not caught by main(), the program exits
• Usually with the message

Aborted (core dumped)

• Can then use gdb as normal

Multiple Catchers

try {
// do something...
} catch (SomeType st) {
// handle SomeType exception
} catch (SomeOtherType sot) {
// handle SomeOtherType exception
} catch (...) {
// handle anything else
}

Multiple Catchers II

• First catch type that matches is used
• So with inheritance
  • Derived classes must come first
  • Otherwise the base will match

• Inherited exceptions can be useful

class IOError { };
class NoFileError : public IOError { };
try { } catch (IOError e) { }

Rethrowing Exceptions

• A catch block can throw an exception
• All remaining catch blocks on the same level are ignored
• The original exception can be rethrown
  • throw;
  • Even in a catch(...) block

try {
try {
throw 10;
} catch (int i) {
cerr << "Caught : " << i << endl;
throw 20.0;
} catch (...) {
cerr << "Won't reach" << endl;;
}
} catch (int i) {
cerr << "int : " << i << endl;
} catch (double d) {
cerr << "double : " << d << endl;
throw;
}

Exception Specifications

• Functions can declare what exceptions they throw
• void func() throw (e1, e2);
  • func can throw types e1 and e2
  • Or classes inheriting from them
• void func() throw ();
  • func can not throw any exceptions
• void func();
  • func can throw any exceptions

Function Pointers

• The exception specifications of a function pointer must match the function

void foo() throw (X);
void foo2();

void (*pf1)() throw (X,Y) = &foo; // OK
void (*pf2)() throw () = &foo; // error
void (*pf3)() throw (X) = &foo2; // error



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