exceptions and return statements in c++ - c++

Question

I am new in c++ programming and i am trying to understand exceptions in c++. I made a simple model situation that shows things, which I don't understand(I hope, I wont mess up code too much). I made 2 basic classes with few methods(classes CPerson are basically linked list). My answer is how to stop current task with an exception. I am able to call an exception, but task continues and makes some mess in program.
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
using namespace std;
class CPerson{
public:
CPerson(){
p_next_person = NULL;
}
CPerson* p_next_person; // pointer to next person in linked list
int Postcode(); // returns postcode of person
friend ostream& operator<<(ostream& stream, const CPerson& pers){
cout << pers.ID << pers.postcode;
return stream;
}
char* ID;
int postcode;
};
//---------------------------------------------------------------
class CPeople{
public:
CPeople(){
first_person = NULL;
}
CPerson Person( const char* personID); // finds person by ID and returns it
bool NewPerson( const char* personID, int person_postcode); // add new person
CPerson* first_person ; // start of linked list
};
//-----------------------------------------------------------------
int CPerson::Postcode(){
return postcode;
}
//-----------------------------------------------------------------
CPerson CPeople::Person( const char* personID){
CPerson* now;
now = first_person;
while(now != NULL){
if(strcmp(now->ID,personID)==0){
break;
}
now = now->p_next_person;
}
// our person is in now (or now is NULL - if person wasn't found).
try{
if(now == NULL ){
throw 0;
// I need to stop code here
}else return *now;
}
catch (int e)
{
cout << "bla bla " << e << '\n';
}
}
//----------------------------------------------------------
int main(){
CPeople people;
int i = 0;
people.NewPerson( "JAck", 100 );
people.NewPerson( "Josh", 100 );
// Bob is not in people right now.
i = people.Person("BOB").Postcode();
cout << i;
// gives exception, which is nice. but it also changes i to some nonsence .. how do I fix it ?
cout << people.Person ( "BOB" );
// gives exception, which is nice. but also gives segmentation fault. how do I fix it ?
}

Answer 1

You have got the try block around 'throw. The try block should be around where you called the function and it should be caught with a catch. Thus your function will change to:
CPerson CPeople::Person( const char* personID){
CPerson* now;
now = first_person;
while(now != NULL){
if(strcmp(now->ID,personID)==0){
break;
}
now = now->p_next_person;
}
// our person is in now (or now is NULL - if person wasn't found).
if (now == NULL ){
throw 0;
// I need to stop code here
}
else return *now;
}
and main will look like:
int main(){
try {
CPeople people;
int i = 0;
people.NewPerson( "JAck", 100 );
people.NewPerson( "Josh", 100 );
// Bob is not in people right now.
i = people.Person("BOB").Postcode();
cout << i;
// gives exception, which is nice. but it also changes i to some nonsence .. how do I fix it ?
cout << people.Person ( "BOB" );
// gives exception, which is nice. but also gives segmentation fault. how do I fix it ?
}
catch (int e)
{
cout << "bla bla " << e << '\n';
}
}
Notice that once a catch is encountered, the following statement after the catch will be executed. That is why you should have catch out of the function definition.

Answer 2

Code like this
try{
if( now == NULL ){
throw 0;
// I need to stop code here
} else return *now;
} catch (int e) {
cout << "bla bla " << e << '\n';
}
entirely misses the point. Continuing execution with a shrug ("bla bla") as if nothing had happened isn't possible. Either you make sure that all contingencies are met in the catch, or you should catch the exception at a higher level. Here: there is no definition of the function's return value, which causes trouble up there where you call CPeople::Person.
You can surround these calls with a try - catch; omit them in the function and just throw.
Don't throw 0. Use an object capable of holding some information. Throw by value, catch by reference.