Using vectors to store different objects inherited from the same parent class c++

Question

I have a class, let's call it Polynomials.

The number of needed Polynomials are not known in advance and are determined at run-time. My current implementation uses a container class, let's call it Solutions, in which there is a vector, let's call it polynomialVector, it is defined this way:

 std::vector<Polynomial> polynomialVector;

The vector is created and populated in this way (The constructor of Solutions do this job):

polynomialVector.reserve(numberofneededpolynomials);
for(int i = 0; i < numberofneededpolynomials ; i++) {
 polynomialVector.emplace_back(0) //Polynomial constructor takes an int parameter
}

Now I want to create specific type of polynomials that will inherit from the general polynomials class. The type of polynomial will be determined at run time by the user. For example if the user inputs 0, then the general Polynomial class is used, if he inputs 1, then a specific derived polynomial class is used, for type 2, a different specific type of polynomial is used, etc.

I've seen in examples that this can be done using regular arrays:

Polynomial * polynomial1 = new DerivedPolynomial1(parameters);
Polynomial * polynomial2 = new DerivedPolynomial2(parameters);
Polynomial * arrayPolynomial[2] = {polynomial1, polynomial2};

But I need to do it using vectors since the application creates a number of different polynomials (created randomly by the polynomial class but this is not relevant) that is not known in advance. And my entire code that wraps the Polynomial class (the Solutions class) is already coded for vectors.

I need to do something like this:

cin >> type;
if(type == 0) {
 polynomialVector.reserve(numberofneededpolynomials);
 for(int i = 0; i < numberofneededpolynomials ; i++) {
  *create polynomial of type 0 in a similar way that emplace_back does for the general case that I described*
 }
if(type == 1) {
  ... same thing but for type 1
}

How to do this?

One of the methods create a polynomial in a specific way by adding two different polynomials and doing some other stuff. In my regular application the new polynomial is created then pushed_back into the vector. But using pointers if I push back a pointer, the object will disappear when the function returns, thus the pointer will point to trash. What should I do? — jazzybazz
– jazzybazz, Commented Apr 15, 2013 at 18:42

jcm · Accepted Answer · 2013-04-15 18:25:18Z

4

To store a polymorphic datatype in a std::vector, you need to store pointers.

To make sure the code doesn't leak memory, the easiest is to use a smart pointer.

std::vector<std::unique_ptr<Polynomial>> polynomialVector;
polynomialVector.emplace_back(new DerivedPolynomial1(parameters));

I assume you use C++11, since your code uses emplace_back

answered Apr 15, 2013 at 18:25

jcm

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5 Comments

Praetorian Over a year ago

Or you could use boost::ptr_vector<Polynomial> if using Boost is an option.

jazzybazz Over a year ago

One of the methods create a polynomial in a specific way by adding two different polynomials and doing some other stuff. In my regular application the new polynomial is created then pushed_back into the vector. But using pointers if I push back a pointer, the object will disappear when the function returns, thus the pointer will point to trash. What should I do?

jcm Over a year ago

Objects created on the heap (eg. with new) are destructed when you call delete, not when the function returns. With unique_ptr, the polynomials will be destructed when the vector goes out of scope. As long as the vector is there, your polynomials are there.

jazzybazz Over a year ago

If I try polynomialVector.pushback(&createdPolynomial), intellisense gives me an error saying that the type of &createdPolynomial does not match the type of polynomialVector (vector of unique pointers)

jcm Over a year ago

push_back will not work as it needs to do a copy; unique_ptr is not copyable as the name implies. Try emplace_back instead.

masoud · Accepted Answer · 2013-04-15 18:24:35Z

0

You should store them as pointers, something like below:

std::vector<Polynomial *> polynomialVector;
                       ^

Also, you must handle memory (new/delete) and push derived objects as pointer to the vector.

Feel free to use std::unique_ptr or std::shared_ptr instead of simple pointers:

std::vector<std::unique_ptr<Polynomial>> polynomialVector;

or

std::vector<std::shared_ptr<Polynomial>> polynomialVector;

answered Apr 15, 2013 at 18:24

masoud

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