Test MDX File V5

This is a Test MDX File for the Pointer Lab interactive.

Pointers in C++

A pointer is a variable whose value is the address of another variable. Declaring a pointer with int* p = &a; stores the address of a inside p. The expression *p (“the thing p points at”) reads the value back from that address, interpreting the bytes through the pointer’s declared type.

The pointer’s declared type matters. int* p = &a; tells the compiler to treat whatever is at address &a as a four-byte int. If p actually points at a single-byte char, the dereference reaches past the character into whatever bytes happen to live next to it. This is a classic undefined behaviour.

Interactive Pointer Lab

The lab below is pre-loaded with an int a = 545;, a char b = 'X';, and a float c = 3.14;, plus a pointer p declared as int* pointing at a. Try these in order:

Keep the defaults and read the live cout panel — *p prints 545.
Change the pointer’s type to char* (still pointing at a). The same four bytes of a are reinterpreted as a single char — and *p prints whatever ASCII character sits in the lowest byte.
Enable Pick via click, then click b (the char). Now *p reads a byte from 'X' cleanly.
Switch to void* — the compiler refuses to dereference.
Choose nullptr from the dropdown to see the classic null-deref.

C++ Pointer Lab

Point p at a variable and see what *p produces — matching types and mismatched ones.

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Memorydata 0x1000–0x1009p at 0x100A

9 / 10 data bytes used1 free

This example assumes a 64-bit platform — pointer p stores an 8-byte address.

p →

1009

545

3.14

0x1000

byte 0

A pointer is a variable that stores an address

Select the dereference type of the pointer p and the variable at which it points:

Pointer p

Typep = &

int* p = &a; // p stores 0x1000

Type Palette — drag onto memory

char8-bit

int32-bit

float32-bit

main.cpp — live

int a = 545;

char b = 'X';

float c = 3.14;

int* p = &a;

cout << p; // output: 0x1000

cout << *p; // output: 545

Try this:Start with int* p = &a and watch *p print 545. Now change the pointer type to char* without changing the target — the same four bytes of memory are reinterpreted as a single char, and *p suddenly prints a different value. Finally, set p = nullptr to see the classic null-deref crash.

Here is a Class Hierarchy exercise, expanded by default …

Class Hierarchy

Build the Animal Hierarchy

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Drag class names from the pool into the slots so each class sits below its parent in the inheritance tree, then click Submit.

Inheritance Tree (base class on top)

·····

Available Classes

Dog

Animal

Plant

Reptile

Vehicle

Bird

Insect

Fish

Sparrow

Cat

Mammal

Class Hierarchy

Build the Vehicle Hierarchy

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Drag class names from the pool into the slots so each class sits below its parent in the inheritance tree, then click Submit.

Inheritance Tree (base class on top)

·····

Available Classes

Car

Aircraft

Submarine

Motorcycle

Watercraft

Engine

Vehicle

Bicycle

Sailboat

Wheel

LandVehicle

Here is a second Class Hierarchy exercise, collapsed by default … This one models a small slice of a C++ I/O class hierarchy.

Class Hierarchy

Reconstruct a C++ Stream Hierarchy

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The Abstract version

Abstract Class Hierarchy

Build the Shape Hierarchy

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Drag class names from the pool into the slots so each class sits below its parent in the inheritance tree. Abstract classes are shown in italics with an amber accent — slots expecting an abstract class are pre-marked with «abstract». Click Submit when you're done.

Inheritance Tree (base class on top)

«abstract»

·····

«abstract»

·····

Available Classes

Ellipse

Circle

Rectangle

Shape

Polygon

Pixel

Square

Curve

Triangle