C++: Access violation writing location

Question

Using: MSVS2012

Code

elemalg.h

#include <vector>
#include <string>
#include <fstream>

class ElemAlg
{
private:
std::string difficultlyLevel, question, answerToRead;
std::vector<std::string> questions, answers;

std::vector<std::string> GetQuiz(int);
};

elemalg.cpp

#include "elemalg.h"

std::vector<std::string> ElemAlg::GetQuiz(int difficulty)
{
if (difficulty == 1) { difficultyLevel = "algE"; }
if (difficulty == 2) { difficultyLevel = "algM"; }  
if (difficulty == 3) { difficultyLevel = "algH"; }
if (difficulty == 4) { difficultyLevel = "algVH"; }

std::ifstream fin(difficultyLevel + ".txt");
while (std::getline(fin, question)) { questions.push_back(question); }
fin.close();

std::ifstream fin2(difficultyLevel + "Answers.txt");
while (std::getline(fin2, answerToRead))    { answers.push_back(answerToRead); }
fin2.close();

return questions;
}

MathTutor.cpp

#includes etc
ElemAlg *ea;
ea->GetQuiz(1);

GetQuiz is definitely passed an integer between 1 and 4, this is verified before the method is called

difficultyLevel is a string defined in the header file.

The compiler throws an Unhandled exception and Access violation writing location ... as soon as it hits the first if function.

If I remove the if functions and define difficultyLevel as algE just for testing the same problem.

If I remove difficultyLevel entirely and just open the file as "algE.txt" and "algEAnswers" then I get the same problem but at a different memory location once the code hits the while loop.

The parameter should be verified inside the function with an assert and not outside. If questions is a global variable, I don't know why you try to return it. — Neil Kirk, Oct 20 '14 at 14:49
[MCVE](http://stackoverflow.com/help/mcve) or it didn't happen. — Angew is no longer proud of SO, Oct 20 '14 at 14:49
Your ElemAlg object is not valid. I think you need to send us the surrounding the code. — ChrisWard1000, Oct 20 '14 at 14:50
difficultyLevel is just a string used to determine which file to open. i.e. if the user has decided upon difficulty of 2 then the file "algM.txt" is opened — user3001499, Oct 20 '14 at 14:53
Please edit your question and show at least the exact types. If you say _"string"_, I suppose you actually mean a `std::string`? — πάντα ῥεῖ, Oct 20 '14 at 15:01
added what I think is relevant to be added. Sorry for original incomplete question. — user3001499, Oct 20 '14 at 15:12
We need to COMPILE it ourselves or we are just shooting in the dark here. — Neil Kirk, Oct 20 '14 at 15:15
If you have it like this: `ElemAlg *ea; ea->GetQuiz(1);` that's blatantly wrong! Change to `ElemAlg ea; ea.GetQuiz(1);` — πάντα ῥεῖ, Oct 20 '14 at 15:19
@user3001499 - `If I remove the if functions and define difficultyLevel as algE just for testing the same problem` You should stop removing code, and instead keep the original faulty code and learn why it is giving you an issue and fix it. Removing code potentially may then hide the bug, giving you the false sense that you've fixed the problem "by magic". — PaulMcKenzie, Oct 20 '14 at 15:21

score 24 · Accepted Answer · edited Oct 20 '14 at 15:18

Your problem is here:

ElemAlg *ea;
ea->GetQuiz(1);

You're not creating an instance of ElemAlg, so you're calling a member function on an uninitialized pointer.

Because the member function you are calling isn't virtual the compiler won't have to do any runtime lookup, which is why the call goes to GetQuiz. However, the this pointer will be garbage (as ea is uninitialized), so the moment you access a member variable (such as difficultyLevel) you'll have undefined behaviour. In your case the undefined behaviour leads to an access violation.

Either initialize ea:

ElemAlg *ea=new ElemAlg;
ea->GetQuiz(1)

or, if you don't need to allocate it on the heap just do:

ElemAlg ea;
ea.GetQuiz(1)

Thank you so much...my program is different but it helped me to solve my problem... I just change the memory allocation statement with new keyword Getting Same Error with statement : `MyQueue *newnode = (MyQueue*)malloc(sizeof(MyQueue));` Change above statement to : `MyQueue *newnode = new MyQueue;` — RAVI VAGHELA, Jun 29 '16 at 12:12

C++: Access violation writing location

1 Answers1