Dynamic array and pointer in a class trouble finding the right way to define them

Question

I want the constructor to create a dynamic array and assign arguments passed to the constructor to the members of the array.

A simplified version of what I have in the header:

//in classes.h
#ifndef classes_h
#define classes_h
#include <iostream>
class Base{
        int a,*var;
public:
    Base();
    Base(int);
    ~Base();
    int func(int);
};
#endif

And in the *.cpp:

//In classes.cpp
#incldue "classes.h"
Base::Base(int a){
    var=new int[2];
    var[0]=a;
    var[1]=func(a);
}
Base::~Base(){
        delete var;
}
int Base::func(int b){
    return b++;
}
int main(){
Base obj(1);
return 0;}

I need to be able to pass that array to a function which will modify it in some way, but I'm actually having more trouble defining the array...

I don't get any errors from the compiler or linker, but by debugging I arrived at the conclusion the problem is around the creation of the array.

I went over the basics of pointers, dynamic arrays and classes 3 (or maybe more) times, but to no avail. I'm hoping what I'm trying to do is actually possible. If not, what's the closest thing?

Please don't fix one error at a time in your code, rendering existing answers obsolete. — Luchian Grigore, Nov 01 '12 at 06:52
The statement `return b++;` return the _current_ value of `b` and _then_ increase the _local_ variable. So if you pass the value `1` to `func` it will return `1`. Maybe you want `++b`? — Some programmer dude, Nov 01 '12 at 07:05
@JoachimPileborg Definitely! Order of operation, much easier with a+=1... :) — wizzard, Nov 01 '12 at 07:12

score 1 · Answer 1 · answered Nov 01 '12 at 06:47

1

You're running into undefined behavior because of

var=new int[2];

being freed with

delete var;

instead of delete[] var;.

You should use a std::vector<int> instead. That way, you needn't worry about a destructor/copy constructor/assignment operator.

Also, note that

int Base::func(int b){
    return b++;
}

is basically a no-op, as the previous value of b is returned. Did you mean return b+1?

answered Nov 01 '12 at 06:47

Luchian Grigore

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is there really no other way to do it without ::vector? – wizzard Nov 01 '12 at 06:51
2

@wizzard sure there is, it's called "the hard way". – Luchian Grigore Nov 01 '12 at 06:51
There is a multitude of reasons as to why `std::vector` makes dynamics arrays far less painful than hand-coding. Unless you have a solid reason for avoiding it, **don't**. – moshbear Nov 01 '12 at 06:55
My reason is to understand why it's not working the way it is right now. I decided to 'refresh' my memories on c++, but it turned out they were all long gone... (the language has become prettier last 4-5 years). Decided to make a simple 3D array, turned out it's not simple at all, looked around, found boost, turns up I have a problem defining the arguments I pass to boost, came here to ask after being unable to find anything related to my problem. Turns out My problem is even more basic than I thought :D – wizzard Nov 01 '12 at 07:00
@wizzard 1. you don't tell us exactly where the problem is. 2. Did you notice that `func` does nothing? – Luchian Grigore Nov 01 '12 at 07:02
@LuchianGrigore 2 was supposed to be just some function a `def f():pass` in Python if you will. Anyway, It must be what Als suggested, gonna look up the Copy constructor and Copy assignment operator edit my script a bit and report back with the good news or any further problems I encounter :) Thanks for the timely responses :) – wizzard Nov 01 '12 at 07:09

score 1 · Accepted Answer · edited May 23 '17 at 12:12

1

You have two problems:

You are not following the Rule of Three.
You have an Undefined Behavior.

Details:

delete var;

should be:

delete []var;

new requires delete and new[] requires delete [], a mismtach results in Undefined Behavior.

Suggestion:

You should be using a std::vector or std::array instead of an dynamic c-style array. It will save you all the efforts and they are less error prone.

edited May 23 '17 at 12:12

Community

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answered Nov 01 '12 at 06:47

Alok Save

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So my problem lies in the shallow copies? – wizzard Nov 01 '12 at 06:50
@wizzard: Yes, Indeed. Technically, Your code has Undefined Behavior, but usually that will result in memory leak not the said behavior. – Alok Save Nov 01 '12 at 06:52
Gonna educate myself further into c++ by reading more on the Rule of Three and return shortly. – wizzard Nov 01 '12 at 06:56

score 0 · Answer 3 · answered Nov 01 '12 at 09:39

0

If you don't like vectors in this example you could use a std::pair but a vector is the way to go. I haven't used an array in months.

answered Nov 01 '12 at 09:39

James

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Dynamic array and pointer in a class trouble finding the right way to define them

3 Answers3