Pointer to a vector

Question

What is wrong with the following code

vector < double >* a;
a->push_back(25);
a->push_back(30);
a->push_back(15);
a->push_back(40);
cout << a->at(2) << endl;

It should print 15, but it prints nothing

Obligatory: you should wrap a pointer like that as `std::unique_ptr`. In general, if you have `new` and `delete` in modern C++ application code, you'd better have a good reason for it. Otherwise, use smart pointers! — hyde, Nov 28 '18 at 20:17
Obligatory - there are almost no reasons to use dynamic storage to store a vector. — SergeyA, Nov 28 '18 at 20:19

ΦXocę 웃 Пepeúpa ツ · Answer 1 · 2018-11-28T20:19:53.713

1

a is a pointer but is not properly initialized... it must be like:

int main()
{
    std::vector<double>* a = new std::vector<double>;
    a->push_back(25);
    a->push_back(30);
    a->push_back(15);
    a->push_back(40);
    std::cout << a->at(2) << std::endl;
    delete a;
    return 0;
}

edited Nov 28 '18 at 20:19

answered Nov 28 '18 at 20:14

ΦXocę 웃 Пepeúpa ツ

47,427
17
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score 1 · Answer 2 · answered Nov 29 '18 at 11:22

1

What is wrong ...?

You are using a pointer where an automatic duration value is more appropriate.

std::vector < double > a;
a.push_back(25);
a.push_back(30);
a.push_back(15);
a.push_back(40);
std::cout << a.at(2) << std::endl;

answered Nov 29 '18 at 11:22

Caleth

52,200
2
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score -1 · Answer 3 · answered Nov 28 '18 at 20:33

-1

Your pointer wasn't initialized. Additionally, based on the answer by ΦXocę 웃 Пepeúpa ツ, make sure when you are working with pointers to use new/delete correctly. For every new keyword, there should be a delete. It is also good practice to avoid dangling pointers (pointers that do not reference anything) by setting them to NULL (as of C++11, it's recommended to use nullptr: NULL vs nullptr (Why was it replaced?) and What exactly is nullptr? questions offer some good explanations) when you are done.

However, in this case, I agree with Acorn. You shouldn't have to point to a vectorobject because of how it allocates memory. It should be sufficient to just use a vector for your solution.

answered Nov 28 '18 at 20:33

jameyb

37
7

`It is also good practice to avoid dangling pointers (pointers that do not reference anything) by setting them to NULL` - nothing good about this anti-pattern. – SergeyA Nov 28 '18 at 20:35
Please clarify? I specifically stated that `nullptr` is better than using `NULL`. What is not good here? – jameyb Nov 28 '18 at 20:39
It is not about nullptr vs null. It is about that setting pointer to nullptr after deleting is is a false safety and antipattern. – SergeyA Nov 28 '18 at 20:42
Well, it depends doesn't it? Perhaps if you want to discuss this later it'd be more appropriate to talk in the chat or something. I am confused as to exactly how you can claim it is false. Reading through the question and answers below, it seems that it really does depend: https://stackoverflow.com/questions/1931126/is-it-good-practice-to-null-a-pointer-after-deleting-it – jameyb Nov 28 '18 at 20:48
1

See https://stackoverflow.com/questions/704466/why-doesnt-delete-set-the-pointer-to-null (some answers after the top one). The main problem with that is false sense of safety. Even on your own link, it is mentioned that it is a bandaid over fundamental problem. WHY your application deletes the pointer twice? It could be argued that it is better to let it crash rather than mask the problem, until it pops somewhere else. – SergeyA Nov 28 '18 at 20:53

Pointer to a vector

3 Answers3