exceptions and return statements in 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.