Pointers to Objects in a Program

A drawing program rarely works with a single shape. It reads shapes from some input, keeps them while the program works, draws them, transforms them, and finally cleans up. Each of those stages touches the same collection of shapes, so a small mistake in one stage can show up as a wrong result somewhere else. This article follows one short program through its stages so you can practice tracing a result back to the stage that produced it.

The program you will study is a single user function. It reads shapes until input runs out, draws every shape, rotates every shape by 45, and then deletes every shape. Read it as a sequence of stages. The question to keep in mind is: at which stage would the output stop matching what you expected?

The program uses a shape reader to get shapes from input. The source shows the reader being called as read_shape(cin), and its result being passed directly to push_back on the vector. Each call to read_shape produces one shape, and push_back adds that shape to the collection. The reader is therefore the only place in this program where new shapes come into existence from outside.

Read this diagram once per pass through the loop. Each pass moves exactly one shape from the input into the end of the vector, so the vector grows by one element each time. When the input runs out, the loop stops and the vector holds every shape that was read, in the order they were read.

This choice matters for every later stage. Drawing, rotating, and deleting all work through those pointers. When draw_all reaches a slot, it does not find a shape sitting in that slot; it finds a pointer, and the shape it needs is the object that pointer refers to. The pointer is the link between the slot and the shape.

Once the vector holds the shapes, the program calls draw_all(v) and then rotate_all(v,45). The source's comments describe what each call does to the vector: draw_all calls draw() for each element, and rotate_all calls rotate(45) for each element. Both functions work on the whole vector, so the caller does not write a loop over the shapes for these two stages.

Each function steps through the vector one element at a time. On each iteration, the element is a pointer, and the operation is applied to the shape that pointer refers to. The draw operation therefore runs once for every shape, and the rotation operation runs once for every shape with the argument 45.

The program runs in a fixed order: reading, storing, drawing, rotating, and then deleting. Because the order is fixed, you can rule stages out one at a time when a result is wrong. The diagram below lays out those stages in sequence.

The order matters. Drawing happens before rotating in this program, so the drawing stage reflects the shapes as they were read, and the rotation stage then changes the shapes that remain in the vector. If the result of drawing looks wrong, the rotation stage has not run yet and cannot be the cause. This ordering is a useful way to narrow down a failure.

The final stage is a loop that deletes each element: for (auto p : v) delete p. The source marks this with the comment that you should remember to delete the elements. Each iteration copies one pointer out of the vector into p and then deletes the shape that p points to. This is the only place in the program where shapes are released.

Deleting the pointed-to shape does not change the vector. The vector still contains the same pointer values, but after this loop those pointers no longer refer to live objects. The vector is not cleared by this loop, so v still reports the same number of elements afterward. Keeping that distinction in mind is essential when you trace the final state of the program.

When the output of a shape program is wrong, the most useful first question is which stage produced the wrong value. Because the program runs in a fixed order of reading, drawing, rotating, and deleting, you can rule stages out one at a time. If the shapes drawn are already wrong, the problem lies in reading or in the shapes before drawing. If the drawn shapes are right but the rotated result is wrong, the problem lies in the rotation stage or in how it was called.