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C++ abstract classes and virtual dispatch

Invoke derived behavior through a base reference.

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Before you start

Classes, inheritance, and references.

Invoke derived behavior through a base reference.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A pure virtual method defines a required operation and makes the base class abstract. A derived class supplies the implementation with override.

A base reference preserves the dynamic type of the object. Virtual dispatch calls Rectangle::area. A virtual destructor supports correct destruction if a derived object is later owned through a base pointer.

Code example

C++

#include <iostream>
class Shape {
public:
    virtual int area() const = 0;
    virtual ~Shape() = default;
};
class Rectangle : public Shape {
    int width_, height_;
public:
    Rectangle(int w, int h) : width_(w), height_(h) {}
    int area() const override { return width_ * height_; }
};
int main() {
    Rectangle rectangle(3, 4);
    const Shape& shape = rectangle;
    std::cout << shape.area() << '\n';
}

Output

Example details

Follow the code

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1#include <iostream>

2class Shape {

3public:

4 virtual int area() const = 0;

5 virtual ~Shape() = default;

6};

7class Rectangle : public Shape {

8 int width_, height_;

9public:

10 Rectangle(int w, int h) : width_(w), height_(h) {}

11 int area() const override { return width_ * height_; }

12};

13int main() {

14 Rectangle rectangle(3, 4);

15 const Shape& shape = rectangle;

16 std::cout << shape.area() << '\n';

17}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Construct the concrete shape.	`width_ = 3, height_ = 4`	—
Bind a base reference to the same object.	`dynamic type = Rectangle`	—
Dispatch to the derived override.	`return = 12`	—
Print the result.	`return = 12`	12

Behind the scenes

How C++ processes this code

The C++ compiler produces a native executable. Execution enters main, where objects and function calls perform the work.

Cost and common mistakes

O(1) arithmetic work and fixed object storage.

Copying derived objects into non-abstract base values can slice derived state.
This sample assumes small nonnegative dimensions; it does not validate dimensions or multiplication overflow.

Try it yourself

Implement a Square derived from Shape.

Reveal answer and reasoning

Store one side and override area to return side * side. A side of 5 gives 25 through a Shape reference.