D / APPLIED ENGINEERING
Unit tests, contracts, and memory checks
Learn how unit tests, contracts, and memory checks works in D, why the underlying model matters, and how to apply it in a small program without hiding important trade-offs.
What you will learn
- Explain unit tests in D using the correct mental model
- Trace a focused D example and predict its result before execution
- Recognize a boundary case involving contracts and handle it deliberately
Understanding Unit tests, contracts, and memory checks
Unit tests, contracts, and memory checks belongs to the practical core of D. Start by identifying the values or state involved and the rule that connects the input to the result.
Trace the example one operation at a time. Keep unit tests visible in the code rather than hiding it behind an abstraction before the behavior is understood.
Test a normal case and a boundary case. The difference between the prediction and the observed result is the most useful signal for deciding what to review next.
Unit tests, contracts, and memory checks is a defining part of practical D work. Start by identifying the data or state involved, then trace the operation that changes or interprets it. Pay attention to the rules D applies at this boundary, because those rules explain both the useful behavior and the common failure modes. This lesson keeps the example deliberately small, then connects it to capstone: a generic systems utility in d so the ideas form a coherent progression rather than a list of isolated syntax facts.
Worked examples
Unit tests, contracts, and memory checks example
A focused D example for unit tests.
import std.stdio;
void main() { writeln("Hello from D"); }
// Lesson 15: unit tests. Change one value and predict the result before running it.Example explained
Line 1Identify where unit tests appears in the D example and name the data it operates on.
Line 2Trace the relevant D rule one operation at a time, recording any state, type, or control-flow change.
Line 3Change one input or boundary condition, predict the result, and compare that prediction with the documented outcome.
Important notes
Keep the first unit tests example small enough to trace completely.
Use the normal D toolchain or browser workspace to compare the actual result with your prediction.
Common mistakes
Treating unit tests as punctuation to memorize instead of a D behavior to reason about.
Ignoring contracts until it appears in production data or a larger program.
Try it yourself
Change, predict, then run
Create a small D example that demonstrates unit tests. Add a normal case and a boundary case, write down the expected result for each, then explain which D rule produces that result. Lesson 15 should remain small enough to trace without guessing.
Open D workspaceCheck your understanding
What is the best first step when working with unit tests?
- Identify the data and predict the result
- Add more abstraction immediately
- Ignore boundary cases
- Memorize punctuation only
Show answer
A clear input, operation, and predicted result create a testable mental model.