Can anybody help me to do my own Vector class?

Question

I have problem only with the push_back function, the compiler said:

CRT detected that the application wrote to memory after end of heap buffer

I want to make a push_back function, that adds a new element to the vector's end.

#pragma once
#include <cstdio>
#include <cmath>
#include <iostream>
#include <cstdlib>

class tomb {
private:
    double *adat;
    int szam;
public:
    tomb(){
        adat = NULL;
        szam = 0;
    }
    int meret()const {
        return szam;
    }
    ~tomb() {
        delete[] adat;
    }
    double & operator[](int n) {
        return adat[n];
    }
    const double & operator[](int n)const {
        return adat[n];
    }
    void push_back(const double &a) {
        double *tmp;
        int pos = szam + 1;
        tmp = new double[szam+1];

        for (int i = 0; i < szam; i++)
        {
            tmp[i] = adat[i];
        }

        tmp[pos] = a;
        delete[] adat;
        adat = tmp;
        ++szam;
    }
    void Kiir()const {
        for (int i = 0; i < szam; i++)
        {
            std::cout << adat[i] << "\n";
        }
    }
};

`tmp[pos] = a` writes to one-past-the-end of your newly allocated block -- `pos` is `szam+1` at that point. — G.M., May 07 '17 at 14:59
At the very least you should use `std::unique_ptr` to manage the lifetime of `adat` instead of doing it yourself. It'll also make it easier to update your code to be exception safe. — Captain Obvlious, May 07 '17 at 15:05
@Zsembes -- `{ tomb t1; t1.push_back(10.0); tomb t2 = t1;}` -- That code is faulty, even if you fix your `push_back` function. It is faulty due to the violation of the rule of 3. You will be deleting the same pointer value twice on destruction of `t1` and `t2`. — PaulMcKenzie, May 07 '17 at 15:07
As a side not, you shouldn't pass primitive types (such as double) by const reference, as it might as well be more expensive than a copy. — DeiDei, May 07 '17 at 15:10
@Zsembes This is where I think it is better to see how a simple vector class is actually implemented instead of making all of the mistakes that can only be corrected by rewriting your entire attempt. — PaulMcKenzie, May 07 '17 at 15:10
@PaulMcKenzie and how can I correct is? I really never heard about the rule of 3, I'm a beginner — Zsembes, May 07 '17 at 15:11
@Zsembes -- All of your questions have lengthy answers. So [here is a simple implementation](http://coliru.stacked-crooked.com/a/0b1eaa1bfd7c6c52). — PaulMcKenzie, May 07 '17 at 15:17

score 0 · Answer 1 · answered May 07 '17 at 14:59

0

pos should be szam not szam+1. You are willing to insert at the last position, which in 0-based indexing is n-1.

answered May 07 '17 at 14:59

cbuchart

10,847
9
53
93

score 0 · Answer 2 · answered May 07 '17 at 15:00

The problem is in this line:

tmp[pos] = a;

Since pos is initialized to szam + 1, that is equivalent to:

tmp[szam + 1] = a;

which is one out of the array limit.

The solution is to get rid of pos altogether and just do:

tmp[szam] = a;

BTW, your class is using the default copy constructor and assignment operator, and those will not work properly. You should really do something about that.

Can anybody help me to do my own Vector class?

2 Answers2