"Default delete" errors when trying to use unique_ptr with vectors for polymorphism in C++

Question

I'm relatively new to C++, and I'm hoping someone can help me resolve an issue I'm having with unique_ptr and vectors. Essentially I'm trying to use polymorphism so that I have a vector of type "Base", which is an abstract class (pure virtual). I'm then attempting to fill this vector with derived classes. I've included a trivial example below, showcasing exactly what I'm trying to achieve. Please note that I need to use C++11, which is why I haven't made use of "std::make_unique". The code compiles fine, but I get run-time errors about "default_delete" in class Animal.

A related question is should I be using unique_ptrs for run-time polymorphism as below? Or should I be using raw pointers instead?

Header file and CPP files below. Error output from VS is included below this. Very many thanks in advance for any help with this.

HEADER FILE:

#ifndef START_H
#define START_H
#include <vector>
#include <memory>



class Animal
{
public:
    virtual ~Animal() = default;
    void run();
    void setNumLegs(int a) { numLegs = a; }
    const int getLegs() const { return numLegs; }
private:
    double numLegs;
    virtual void useLegs() = 0;
};

class Biped : public Animal
{
private:
    void useLegs();
};

class Multiped : public Animal
{
public:
    double costOfShoes{ 12.0 };
private:
    void useLegs();
    void payForShoes();
    void becomeDestitute();
};

class Farm
{
public:
    std::vector<std::unique_ptr<Animal>> animals;
};


class Countryside
{
public:
    std::vector<std::unique_ptr<Farm>> farms;

};
#endif // START_H

CPP FILE:

#include "start.h"
#include <iostream>

int main() {
    
    Countryside countryside;

    std::unique_ptr<Farm> f(new Farm);

    std::vector<int> legs = { 2,4,5,2,10 };
    for (auto& numLegs : legs) {
        if (numLegs == 2) {
            std::unique_ptr<Biped> biped(new Biped);
            biped->setNumLegs(numLegs);
            f->animals.push_back(std::move(biped));
        }
        else if (numLegs > 2) {
            std::unique_ptr<Multiped> multiped(new Multiped);
            multiped->setNumLegs(numLegs);
            f->animals.push_back(std::move(multiped));
        }
    }
    
    countryside.farms.push_back(std::move(f)); //THIS IS WHERE THE PROBLEM IS I THINK

    for (auto& animal : f->animals) {
        animal-> run();
    }
    return 0;
}

void Animal::run() 
{
    useLegs();
}

void Biped::useLegs()
{
    std::cout << "Running with: "<< getLegs() <<"legs\n";
}

void Multiped::useLegs()
{
    std::cout << "Running with many legs:" << getLegs() << "!!! legs\n";
    payForShoes();

}
void Multiped::payForShoes() 
{
    std::cout << "Paying for shoes...\n";
    becomeDestitute();
}

void Multiped::becomeDestitute() 
{
    std::cout << "I have no money left.\n";
}

DEBUGGER ERROR OUTPUT:

•   _Mypair <struct at NULL>    std::_Compressed_pair<std::allocator<std::unique_ptr<Animal,std::default_delete<Animal>>>,std::_Vector_val<std::_Simple_types<std::unique_ptr<Animal,std::default_delete<Animal>>>>,1>
  

•   _Mypair._Myval2 <struct at NULL>    std::_Vector_val<std::_Simple_types<std::unique_ptr<Animal,std::default_delete<Animal>>>>
  

•   this    0x00000000 <NULL>   std::vector<std::unique_ptr<Animal,std::default_delete<Animal>>,std::allocator<std::unique_ptr<Animal,std::default_delete<Animal>>>> *
  

Answer 1

Hi the problem is you are deferencing a null pointer in the for loop (f is set to nullptr). The move operation moves the ownership of the item pointed to and the pointer f is set to nullptr (move semantics) After a move the object can no longer be used