making a map in which the value type is an abstract class in C++

问题

I have an abstract class element and a child class elasticFrame :

class element
{
public:
    virtual Matrix getStiffness() = 0;
protected:
    Matrix K;
};


class elasticFrame3d:public element
{
public:
    elasticFrame3d(double E, double G);
    virtual Matrix getStiffness();
virtual Matrix getTransform();
private:
    double E, G;
};

what I want is to make a map like this:

map<int, element> elementMap;

but when I get this error:

error C2259: 'element' : cannot instantiate abstract class

is it even possible to do this? if yes how?

回答1:

You won't be able to create a value of type element as it has abstract function. If you want to store objects of a type derived from element, you'll need to store a suitable pointer or reference to these objects. You can, e.g., use std::unique_ptr<element> or std::shared_ptr<element> (you need to include #include <memory>) and allocate the concrete objects in a suitable memory area.

That is, you would use something like this:

std::map<int, std::unique_ptr<element>> elementMap;
elementMap[17] = std::unique_ptr<element>(new elasticFrame3D(3.14, 2.71));

BTW, you are using an unusal naming convention: when using CamelCase types are normally written with a capital letter and objects using a lowercase initial letter.

回答2:

Pointers!

Declaration:
map<int, element*> elementMap;

Use:

elasticFrame3d thing = elasticFrame3d(1,1);
elementMap[0] = &thing;

来源：https://stackoverflow.com/questions/25465224/making-a-map-in-which-the-value-type-is-an-abstract-class-in-c