Cannot instantiate abstract class using c++11 - c++

Question

Why can't I use an abstract class like an interface at runtime.
I get the output:
1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\xmemory0(615): error C2259: 'Creature' : cannot instantiate abstract class
1> due to following members:
1> 'std::string Creature::Move(std::vector<std::string,std::allocator<_Ty>> &)' : is abstract
1> with
1> [
1> _Ty=std::string
1> ]
1> visual studio 2013\projects\cpp_demo\cpp_demo\creature.h(9) : see declaration of 'Creature::Move'
1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\xmemory0(614) : while compiling class template member function 'void std::allocator<_Ty>::construct(_Ty *,const _Ty &)'
1> with
1> [
1> _Ty=Creature
1> ]
1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\xmemory0(752) : see reference to function template instantiation 'void std::allocator<_Ty>::construct(_Ty *,const _Ty &)' being compiled
1> with
1> [
1> _Ty=Creature
1> ]
1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\type_traits(580) : see reference to class template instantiation 'std::allocator<_Ty>' being compiled
1> with
1> [
1> _Ty=Creature
1> ]
1> c:\program files (x86)\microsoft visual studio 12.0\vc\include\vector(650) : see reference to class template instantiation 'std::is_empty<_Alloc>' being compiled
1> with
1> [
1> _Alloc=std::allocator<Creature>
1> ]
1> visual studio 2013\projects\cpp_demo\cpp_demo\main.cpp(7) : see reference to class template instantiation 'std::vector<Creature,std::allocator<_Ty>>' being compiled
1> with
1> [
1> _Ty=Creature
1> ]
My code:
int main()
{
unique_ptr<vector<Creature>> pCreatures(new vector<Creature>);
unique_ptr<Creature> pHuman(new Human());
pCreatures->push_back(*pHuman);
}
#include "stdafx.h"
#include "Creature.h"
class Human : public Creature
{
public:
virtual string Move(vector<string> &log);
};
#include "stdafx.h"
#include "IMove.h"
class Creature : public IMove
{
public:
virtual string Move(vector<string> &log) = 0;
virtual string GetState(vector<string> &log);
};
Please help.

Answer 1

You CAN use abstract class in vector or unique_ptr, e.g.
#include <vector>
#include <memory>
using namespace std;
class Interface {
public:
virtual ~Interface() = 0;
};
Interface::~Interface() {}
class Implementation : public Interface {
};
int main(int argc, char** argv) {
unique_ptr<Interface> p(new Implementation);
vector<unique_ptr<Interface>> v;
v.emplace_back(new Implementation);
vector<Interface> vi;
// This leads to compile error: vi.emplace_back();
}
Moreover, you CAN use vector<Interface> as long as you don't call any methods that potentially calls new Interface. For example, if you just declare a variable vector<Interface> v; it compiles, but if you push_back or emplace_back or resize, then it will have compile error because they will call new Interface.
The above code is tested under gcc-4.6.3.

Answer 2

You can use, but instead of using:
unique_ptr<vector<Creature>> pCreatures(new vector<Creature>);
use
vector<unique_ptr<Creature>> pCreatures;
so you will have a vectors of Creatures pointers, managed by unique_ptr.
There are, at least, two ways to use this vector:
Creating the objects directly into the vector:
pCreatures.emplace_back(new Human());
Moving an unique_ptr to it:
unique_ptr pHuman(new Human());
pCreatures.push_back(move(pHuman));
Below is a compact usage:
int main()
{
vector<unique_ptr<Creature>> pCreatures;
pCreatures.emplace_back(new Human());
unique_ptr<Creature> pHuman(new Human());
pCreatures.push_back(move(pHuman));
// example of usage
pCreatures[0]->Move();
}