C++ Array Pointer In Struct Retaining Memory location but loses all values

Question

I'm fairly-new to C++, but have experience in higher-level languages. Just trying to self-teach. I've been working on this issue for a few days (off and on) and cannot seem to figure out what is happening here. I know it is something fundamental about C/C++ that I should understand. The problem:

I am storing a pointer to an array in a struct (stripped down):

namespace TestSpace {
    struct ArrayData {
        double *data;
        int count;
    };
}

In the same file, I have a function:

struct ArrayData *unsorted_5count_duplicates() {
    struct ArrayData *arr;
    double data[] = { 3.5, 1.2, 1.2, 4.0, 3.5 };

    arr = (ArrayData *)calloc(1,sizeof(*arr));

    //I added this line in attempt to fix the problem... no such luck.
    arr->data = (double *)calloc(5,sizeof(double));

    arr->data = data;
    arr->count = 5;

    return arr;
}

In main.cpp, I call unsorted_5count_duplicates() and everything checks out. As soon as I pass the object to AssertCountCorrect(arr), I get the exact same issue each time:

main.cpp

TestSpace::ArrayData *arr = TestSpace::unsorted_5count_duplicates();
//During debug, I can check and see *arr has all values set correctly, including the array.
AssertSpace::AssertCountCorrect(arr);

AssertSpace.h

//snippet:
void AssertCountCorrect(struct TestSpace::ArrayData *arr) {
    //I get -9.2559631349317831e+061 every time, for each value of arr->data.
    //   but, the memory reference is correct.
    double *arrdata = arr->data;
    for(int i = 0; i < arr->count; i++) {
        std::cout << *(arrdata + i) << "\n";
    }

}

I've tried passing by reference, making everything const, assigning data via pointers to the array... it all always yields the same result... everything checks out until I pass it to the AssertCountCorrect() function.

Solution I implemented

It was a simple, fundamental issue. I needed to copy all values of the array into the struct, as demonstrated below:

arr->data = (double *)calloc(5,sizeof(double));
for(int i = 0; i < 5; i++) {
    *(arr->data + i) = *(data + i);
}

double data[] = { 3.5, 1.2, 1.2, 4.0, 3.5 }; What do you expect this to do? This statement has no effect on the data member. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Feb 26, 2014 at 7:55
There is just so much here... You aren't allocating enough space for arr, you are allocating space for arr->data then orphaning it by assigning arr->data = data. Try addressing those issues first and then see what happens. — Jeremy West
– Jeremy West, Commented Feb 26, 2014 at 8:03

Benjamin Lindley · Accepted Answer · 2014-02-26 08:30:48Z

5

In addition to not allocating the correct amount of memory, as pointed out in Sakthi's answer, you have another problem. Two, actually (at least).

arr->data = (double *)calloc(5,sizeof(double));

This assigns arr->data, which is a pointer, to point to a dynamically allocated region of memory. Then, the next line:

arr->data = data;

This is two problems. First, it is a memory leak. You no longer have access to the memory you just allocated with calloc. Second, data is a local array, so when you return arr from the function, arr->data is a dangling pointer. You need to copy the data over. There are functions for that in the standard library, such as std::copy or std::memcpy, or you can simply use a for loop.

As a side note, if you need a dynamic array, the correct structure is generally std::vector. I realize you're just trying to learn pointers, but once you've got that situation figured out, stop using them (for the most part).

edited Feb 26, 2014 at 8:30

answered Feb 26, 2014 at 8:04

Benjamin Lindley

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7 Comments

Thumper Over a year ago

I updated my question with why that line is there. My apologies.

Benjamin Lindley Over a year ago

@Thumper: Okay, so you don't have the first problem (a memory leak), but you still have the second problem (a dangling pointer). You were correct to add the calloc line, you just needed to go further and delete the other line (and add the copying code).

nonsensickle Over a year ago

I didn't see your answer there... Seems you wrote it before me too... +1

Thumper Over a year ago

Well, I knew it was something fundamental. Thank you. @nonsensickle, I actually used your answer with the example code to answer my question. I'll update my question with the solution.

Thumper Over a year ago

I need much more than a dynamic array. I stripped out all the members of the struct to simplify the code. I originally wanted to do this in C, but I don't yet understand the languages well enough to fully-understand object orientation in C, nor am I satisfied with my ability to implement it.

|

Sakthi Kumar · Accepted Answer · 2014-02-26 07:55:43Z

5

arr = (ArrayData *)calloc(1,sizeof(arr));

to be of

arr = (ArrayData *)calloc(1,sizeof(*arr));

or

arr = (ArrayData *)calloc(1,sizeof(struct ArrayData));

sizeof(arr) where arr is just a pointer would return the size of the pointer, not the struct type.

answered Feb 26, 2014 at 7:55

Sakthi Kumar

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3 Comments

Thumper Over a year ago

That doesn't appear to make a difference in the results.

Benjamin Lindley Over a year ago

@Thumper: None the less, it is vitally important that you fix this. Even if you get your code appearing to work otherwise, it is still wrong if you don't fix this. See: undefined behavior

Thumper Over a year ago

I understand the issue with this, and it has been addressed. (and will be in the future). One hurdle at a time! Thank you!

BlackMamba · Accepted Answer · 2014-02-26 08:47:43Z

1

you can change this line arr->data = data; to memcpy(arr->data, data, sizeof(data));

answered Feb 26, 2014 at 8:47

BlackMamba

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