Iterators question

[adam2016]

Hi guys I am making a live flights departure system and in the program I have a vector of Flight objects now I encountered a problem first when I tried this code below

  void cleanUpTimeTable(){

     for(int i = 0; i < flights.size(); i++){


     if(t->tm_hour == flights[i].hour && t->tm_min == flights[i].minute+1){

          flights[i].departed = true;
     }
     }
     for(vector<Flight>::iterator it = flights.begin(); it != flights.end();it++){

         if(it->departed == true){

            flights.erase(it);
         }
     }
   }

the program crashed so I read on the forum that you need to assign it tp flights.erase(it)

it = flights.erase(it),

so how come you have to do this,why can't you just erase the iterator like in my sample code?

second part of the question,I also read that you should increment the iterator outside of the for loop test block

like this

void cleanUpTimeTable(){

     for(int i = 0; i < flights.size(); i++){


     if(t->tm_hour == flights[i].hour && t->tm_min == flights[i].minute+1){

          flights[i].departed = true;
     }
     }
     for(vector<Flight>::iterator it = flights.begin(); it != flights.end();){

         if(it->departed == true){

             it = flights.erase(it);
         }else{
         it++;
         }
     }
   }
[code]

how come? why is it when you increment the iterator inside the test block it crashes the program

[code]


void cleanUpTimeTable(){

     for(int i = 0; i < flights.size(); i++){


     if(t->tm_hour == flights[i].hour && t->tm_min == flights[i].minute+1){

          flights[i].departed = true;
     }
     }
     for(vector<Flight>::iterator it = flights.begin(); it != flights.end();it++){

         if(it->departed == true){

             it = flights.erase(it);
         }
     }
   }

thanks

[Peter87]

When you erase an element all iterators to that element gets invalided and are no longer safe to use. That is why you need to assign it to another, valid, iterator before you can continue using it. The erase function returns an iterator to the element that comes after the erased element so if you were to increment the iterator (it++) afterwards you would skip the element that comes after the erased element. If the erased element is the last element it means it would equal flights.end() and incrementing that iterator is not allowed and is likely to crash the program.

[jlb]

So which snippet is the problem?

The first seems to be correct.

The second is possibly changing the iterator twice, once in the increment section and once in the loop body if the if() statement evaluates to true.

Edit: Why the global variables?

[adam2016]

hi Jlb the first snippet caused a crash

thanks guys I never knew that =)

also yeah I hear it's not good to use globals