Question

Lets say I have an abstract base class and a derived class with some unique functionality:

class Shape
{
    public:
    Shape();
};

class Circle : public Shape
{
    public:
    Circle(int radius) : radius(radius);
    int getRadius() { return this->radius; };
    private:
    int radius;
};

Now I can put Circle*:s in std::vector and check afterwards if vector has Shape*

std::vector< Circle* > v;

Circle* circle = new Circle(1);

v.push_back(circle);

// ...

Shape* someShape = ...; // I know it's a Circle but the pointer comes from class that only knows about Shapes

for(std::vector< Circle* >::iterator it = v.begin(); it != v.end(); ++it)
{
    Circle* c = *it;
    if((Shape)c == someShape)
    {
        // found
    }
}

// ...

delete c;

But I want to use std::unordered_set (applies also to std::unordered_map, std::set, std::map) because it has faster-than-linear find() and count()

std::unordered_set< Circle* > u;

Circle* circle = new Circle(1);

u.insert(circle);

// ...

Shape* someShape = ...; // I know it's a Circle but the pointer comes from class that only knows about Shapes

if(u.count(someShape) == 1)
{
    // found
}

Instead of the obivious I get “undefined function” as there only exists std::unordered_set< Key >::count(Key& k);

Is there any way I can find() or count() from std::unordered_set using base class?

Answer 1

If you know it’s a circle then you can simply cast someShape to Circle * and pass that to find().

Shape* someShape = ...; // I know it's a Circle but the pointer comes from class that only knows about Shapes
Circle* someCircle = dynamic_cast<Circle*>(someShape);

if (someCircle != NULL && u.count(someCircle) == 1)
{
    // found 
}