Create a vector of sub classes?

Question

I have been using a dynamic array but I had to add and remove items. I've read it's not recommended to use realloc or to resize the arrays when one can simply use std::vector however I'm having problems in changing my array to a vector.

This is my current code:

int main(){
    // This is what I'm doing now
    State*arr[3];
    int pos = 0;
    arr[0] = new Menu();
    // How do I change it to a vector? This is what I'm trying:
    std::vector<State> vec;
    vec.push_back(Menu());
    ...
}

However I keep getting error: "Cannot allocate an object of abstract type 'State'" What am I doing wrong?

These are class State and Menu:

class State
{
public:
    virtual ~State() {};
    virtual void capture_events() = 0;
    virtual void logic() = 0;
    virtual void render() = 0;
};

Menu : public State
{
public:
    Menu();
    ~Menu();
    void capture_events();
    void logic();
    void render();
};

Even if you could do this, it wouldn't be very sensible. There is no polymorphism on objects in C++ (they are subject to slicing). — us2012, Oct 12 '13 at 16:58
I added info about the subclass, I named it Menu to prevent confusions. — lisovaccaro, Oct 12 '13 at 17:09

score 3 · Accepted Answer · edited May 23 '17 at 12:14

3

You need extra indirection because State is a polymorphic base class. You can do this using std::unique_ptr from <memory>.

#include <memory>

std::vector<std::unique_ptr<State>> states;
states.emplace_back(new Menu());

It is very important to use std::unique_ptr<State> and not State*, because of exception safety. Consider the following:

std::vector<State*> states;
states.push_back(new Menu());
foo(); // what if foo throws an exception?
       // the next line wouldn’t get executed!
for (auto ptr : states) delete ptr;

In contrast, std::unique_ptr uses RAII to make sure the objects are always deleted if the vector goes out of scope, even in the case of early returns or exceptions. For further reference, see The Definitive C++ Book Guide and List.

edited May 23 '17 at 12:14

Community

1
1

answered Oct 12 '13 at 17:13

thanks, it seems it works fine however I don't know how to get an item from the states vector. I tried getting it like a regular array with `states[pos-1];` but I cannot access it's methods – lisovaccaro Oct 12 '13 at 18:12
@Liso22 `states[pos-1]->somefunction();` – us2012 Oct 12 '13 at 18:23
Thanks and how do you return an item from the Vector in a function? I tried: `State* get_top(){ return states[pos-1]; }` which produced a very long error – lisovaccaro Oct 12 '13 at 18:44
@Liso22 `State& get_top() { return *states[pos-1]; }` or `State* get_top() { return states[pos-1].get(); }`. – Oct 12 '13 at 19:01

score 0 · Answer 2 · answered Oct 12 '13 at 17:02

0

You cannot instantiate a class which has pure virtual methods, i.e. a method like

virtual void func() = 0;

The =0means that you must derive the class and implement this method there. This derived class can be instantiated.

As long as a class contains pure virtual methods you cannot instantiate it and you will get your error.

answered Oct 12 '13 at 17:02

chrizke

458
3
10

This answer is valid, it's just not the OP's problem. – Puppy Oct 12 '13 at 17:23

Klaus · Answer 3 · 2013-10-12T17:17:42.383

If you use a vector you objects must be default constructible. You can not construct a object of your class State if one of the virtual methods is marked with =0 to set the class abstract. The idea to have a abstract class is that you can't create objects of that class.

What you maybe want to do:

Create a class family with a abstract base like your State. Create a vector of pointers to instances( objects) of your class. Create the objects with new and push_back the pointer to your vector. If you erase some elements don't forget to delete the objects with delete.

Don't forget to make the destructor of your family also virtual!

Something like that:

class Base
{
     virtual void Do() =0;
     virtual ~Base();
};

class A: public Base
{
     void Do() { ... }
}

class B ...
class C ...

// Create your vector somewhere...
vector<Base*> myVect;


void AddObject()
{
    // It is not important which kind of object you create. Base* obj can "hold" every
    // object which is build from a class which is derived from Base!
    Base* obj=new A();
    myVect.push_back(obj); 
}

void DeleteObj( Base* obj )
{
    myVect.erase( ...find the object...);
    delete obj;
}

void PopBack()  
{
     Base* ptr = myVect.back(); // get last object pointer 
     delete ptr;                // delete the object
     myVect.pop_back();         // remove pointer to object from vector
}

I was trying to do just this, I added more info to the question and I think it's clear now. What I don't understand is how to declare myVect, so it works with the multiple subclasses. Also why do I need to erase? what happens if I use pop_back? — lisovaccaro, Oct 12 '13 at 17:12
I added the answers to your question. If it is still not clear to you, feel free to ask / comment again! — Klaus, Oct 12 '13 at 17:18

Create a vector of sub classes?

3 Answers3