More C++ Idioms/Capability Query

Capability Query

Intent

To check at runtime whether or not an object supports an interface.

Motivation

Separating interface from implementation is a good object oriented software design practice. In C++, the Interface Class idiom is used to separate interface from implementation and invoke the public methods of any abstraction using runtime polymorphism. Extending the example in the interface class idiom, a concrete class may implement multiple interfaces as shown below.

class Shape {  // An interface class.
  public:
    virtual ~Shape();
    virtual void draw() const = 0;
    //...
};

class Rollable {  // One more interface class.
  public:
    virtual ~Rollable();
    virtual void roll() = 0;
};

class Circle : public Shape, public Rollable {  // Circles roll - concrete class.
    //...
    void draw() const override;
    void roll() override;
    //...
};

class Square : public Shape {  // Squares don't roll - concrete class.
    //...
    void draw() const override;
    //...
};

Now, if we are given a container of pointers to abstract class Rollable, we can simply invoke the roll function on every pointer, as described in the interface class idiom.

std::vector<Rollable *> rollables;
//  Fill up rollables vector somehow.
for ( Rollable * rPtr : rollables )
  rPtr->roll();

Sometimes it is not possible to know in advance whether or not an object implements a particular interface. Such a situation commonly arises if an object inherits from multiple interface classes. To discover the presence or absence of the interface at runtime, a capability query is used.

Solution and Sample Code

In C++, a capability query is typically expressed as a dynamic_cast between unrelated types.

Shape *s = getSomeShape();
if (Rollable *roller = dynamic_cast<Rollable *>(s))
  roller->roll();

This use of dynamic_cast is often called a cross-cast, because it attempts a conversion across a hierarchy, rather than up or down a hierarchy. In our example hierarchy of shapes and rollables, dynamic_cast to Rollable will succeed only for Circle and not for Square, as the later one does not inherit from the Rollable interface class.

Excessive use of capability queries is often an indication of bad object-oriented design.

Known Uses

• Martin, Robert C. "Acyclic Visitor Pattern" (PDF). {{cite web}}: |archive-url= requires |archive-date= (help)

Related Idioms

• Interface Class
• Inner Class

References

• Dewhurst, Stephen C. "Capability Queries". C++ Common Knowledge. ISBN 0321321928.