How to create unique pointers in a loop C++?

Question

I am reading a double from the standard input and saving it into a variable d. I want to be able to do this an unspecified amount of times. I use the following code to create a pointer to d.

double *pd = new double;
pd = &d;

I then push this pointer into a constructed stack (list) class. But whenever I push more than one double it changes all of them (the pointer is the same).

Ex. push 2 and get an array [2]. push 3 and get array [3, 3] instead of [3, 2].

You're losing a pointer to dynamically referenced memory. bad idea. — Linuxios
– Linuxios, Commented Dec 11, 2012 at 23:50

bames53 · Accepted Answer · 2012-12-12 04:29:09Z

7

Why are you using pointers at all?

std::vector<double> v;

double d;
while (std::cin >> d)
    v.push_back(d);

Or as chris points out:

std::copy(std::istream_iterator<double>(std::cin),
          std::istream_iterator<double>(),
          std::back_inserter(v));

edited Dec 12, 2012 at 4:29

answered Dec 11, 2012 at 23:56

bames53

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1 Comment

Qaz Over a year ago

You can also initialize the vector with istream_iterators for an identical effect.

David G · Accepted Answer · 2012-12-12 00:04:48Z

1

*pd = d

instead of

pd = &d;

What you do is:

you have double d variable on stack
you create double and save its pointer in pd variable
then you save address to d variable in pd
and then you save address from pd on list

This means you have list of addresses to variable d (every single object on list is pointer to d variable).

edited Dec 12, 2012 at 0:04

David G

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answered Dec 11, 2012 at 23:50

Yester

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4 Comments

Qaz Over a year ago

Or you could do double *pd = new double(d); Even better, whateverStack.push(d);.

Astra Meyers Over a year ago

The stack used pushes a pointer so I have to create a pointer to push, unfortunately. But yes, thank you for the help!

Qaz Over a year ago

@AstraMeyers, What does it do with the pointer? Perhaps you could push &d instead.

Chad Over a year ago

@AstraMeyers use a different stack then.

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