Segmentation fault while accessing an object inside a vector of object - c++

Question

I am having a segmentation fault while trying to access an object stored inside a vector. I have a Survey which consists of Processes. Each process consists of questions. So the Survey Object contains a vector or Processes and each process object contains a vector of questions. The class definitions are as follows:
class Survey {
private:
...
vector <Process> survey_processes;
....
public:
......
vector<Process> getSurveyProcesses()
{ return survey_processes; }
void addProcessObj(Process obj)
{ survey_processes.push_back(obj);}
.....
};
class Process
{
private:
....
vector<Question> proc_questions;
....
public:
...
vector<Question> getProcessQuestions()
{ return proc_questions;}
void addQuestionObj(Question obj)
{ proc_questions.push_back(obj); }
.....
};
class Question {
private:
int quesnum;
int answer;
...
public:
Question (int c_ques, int c_ans)
{
quesnum = c_ques;
answer = c_ans;
}
int getQuestionID()
{
return quesnum;
}
int getAnswer()
{
return answer;
}
...
};
As the new process object is created, I store it in the vector of Processes and for each Process Object, I push the Question Object in the vector of Questions. For the survey Object, I want to get each Process from the Process Vector and for each Process Object, get each question object and print it.
I can access the Process Objects and print them successfully by using following code.
cout << ((survey_obj.getSurveyProcesses()).back()).getProcessID() << endl;
When I try to extract the question object from inside the Process vector, it gives me a segmentation error. I believe I am making some syntax error while trying to access the object. How do I access the question Object embedded inside a vector of Questions for a given Process Object which is inside a vector of process objects for a given survey object?
Here is the relevant part of the code where the segmentation fault occurs.
int procnum = 0;
for (unsigned i = 11; i < all_words.size()-1; ++i)
{
vector<string> v;
string s = all_words.at(i);
stringstream ques_stream(s);
int ques_num;
ques_stream >> ques_num;
ques_stream.ignore();
// if process object already exists, do nothing. Otherwise create a new process object and add it to the survey object
if (procnum == ques_num)
;
else
{
Process proc_obj(ques_num);
survey_obj.addProcessObj(proc_obj);
procnum = ques_num;
}
string ques_strng;
ques_stream >> ques_strng;
ques_stream.ignore();
Question ques_obj(ques_strng);
// objective: put the new question object in the question vector of the last process object from the process vector of the survey object
cout << ((survey_obj.getSurveyProcesses()).back()).getProcessID() << endl;
Process current_proc_obj = (survey_obj.getSurveyProcesses()).back();
cout << " Current Process : " << current_proc_obj.getProcessID() << endl;
Question current_question_obj = (current_proc_obj.getProcessQuestions()).back();
((survey_obj.getSurveyProcesses()).back()).addQuestionObj(ques_obj);
**// this is where the segmentation fault occurs when i try to get the last object from process vector of the survey and for that object get the last element of the question vector. print the question id of this question object
cout << " Current Question : " << ((((survey_obj.getSurveyProcesses()).back()).getProcessQuestions()).back()).getQuestionID() << endl;**
cout << " Current Process Question : " << ((current_proc_obj.getProcessQuestions()).back()).getQuestionID() << endl;
}
I tried running the gdb debugger but it only tells me that error occurs while trying to access the questionID. I still do not know what i am doing wrong and any help would be greatly appreciated.

Answer 1

((survey_obj.getSurveyProcesses()).back()).addQuestionObj(ques_obj);
_______________copy^, _copy^ - ^_______added to copy
Since you are returning vectors by value, not reference, you are adding ques_obj to a temporary local vector, not the one held by survey_obj.
The crash is occurring because you're accessing past the end of the (empty) vector, since nothing got added to it.
One way to fix this is to return the class member vector by reference, instead of by value. (Since by value can be though of as a copy of the variable.)
class Survey {
private:
vector <Process> survey_processes;
public:
vector<Process>& getSurveyProcesses()//<--ampersand indicates return by reference
{ return survey_processes; }
};
Now, when you try this:
((survey_obj.getSurveyProcesses()).back()).addQuestionObj(ques_obj);
______^ this is the vector inside the Survey class,
not a copy
Fortunately, std::vector::back also returns by reference, so the ques_obj gets added to the object held by the vector, held by the Survey object - no local, temporary copies involved! Later, when you go to query that question, you'll find it in the place you expected it.
Two final notes: 1) You should decide whether you should also return the process_questions by reference; 2) If you had used the ability for vector to tell you how many elements it contains, instead of just assuming that back() would work, you would have found this problem earlier. :)