Use assertions to expose violated internal assumptions and broken invariants—not to handle conditions the software should expect and respond to. In embedded C and C++, Design by Contract (DbC) makes those assumptions explicit as preconditions, postconditions, and invariants; the project must also decide what the device does when one fails.
What assertions and Design by Contract mean
An assertion is an executable check that a condition the program expects to be true is, in fact, true. A failed assertion points to a defect or a broken assumption in the software, such as using an invalid index or accessing a peripheral before initialization. It is not automatically a recoverable error message for a user.
Design by Contract (DbC) frames a software component as having obligations between caller and callee. Preconditions describe what must be true when an operation is called; postconditions describe what it promises when it returns; invariants describe properties that must hold across relevant operations. Assertions can make some of these conditions executable as well as documented. The embedded guidance describes runtime validation as a way to find bugs and communicate component obligations (Embedded.com’s discussion of Design by Contract for embedded software). A WG21 paper likewise uses preconditions, postconditions, and assertions as contract concepts, though it is a historical proposal rather than final current standard wording (WG21 paper P0542R5).
Contracts clarify internal expectations; they do not replace requirements, system-level safety analysis, testing, or a complete safety case. A check can reveal that an assumption was violated, but it cannot prove the system is safe or that every relevant condition has been considered.
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When to assert—and when to handle an error
Choose the response based on whether the condition is a programming defect or a foreseeable part of operation. If a component’s internal logic assumes a valid index and receives an invalid one because of a bug, an assertion can expose that defect. If a peripheral is used before initialization when the code requires initialization first, an assertion can make the ordering assumption visible.
By contrast, an absent file or another expected external condition should ordinarily be handled through normal control flow. Return an error, select an appropriate fallback, or report the condition through the interface. If outside input can legitimately be invalid, validate it at the boundary and provide a defined response rather than using an assertion as the user-facing behavior. This distinction between defects and expected exceptional conditions is central to the embedded guidance (Embedded.com).
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| Situation | Typical response | Reason |
|---|---|---|
| An internal invariant is broken, such as an impossible state or invalid index caused by a defect | Assert and follow the project’s defined failure behavior | The condition indicates that the program’s assumptions no longer hold. |
| A normal external condition occurs, such as a file being absent | Handle it through explicit error or fallback logic | The program should expect the condition and respond without treating it as an internal programming failure. |
| Input at a component boundary is invalid but possible | Validate it and return or communicate a defined error | The interface must handle inputs that callers or the environment can legitimately provide. |
Decide what a failed assertion does on the target
Do not assume a desktop-oriented assertion response suits a microcontroller. Quantum Leaps’ DbC material notes that standard C assert() behavior on a false expression prints an error and exits, a response it says is rarely applicable to embedded systems (Quantum Leaps’ Design by Contract material). Embedded.com notes that an assertion handler can offer a last opportunity to transition to a fail-safe state (Embedded.com).
There is no universal recovery action: the right response depends on the device, its hazards, and its system design. Before deploying assertions, specify what the handler does after a contract violation.
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- Capture useful context. Decide which information, such as the failed condition or location, can be recorded and later retrieved.
- Prevent unsafe continuation. Stop the affected work, contain the fault, reset, or transition to the system’s defined safe behavior as appropriate to the design.
- Keep the handler independent. Do not rely on services that may be unavailable or unsafe after the failure, such as an uninitialized peripheral or an operating-system facility that is no longer functioning.
- Account for operating constraints. Diagnostic output, timing, memory use, and the safe response all need to fit the target and its hazard analysis.
A handler is not a promise that every failure can be recovered. Its purpose and limits should be explicit, and its behavior should be consistent with the system’s safety design.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Distinguish C and C++ assertion mechanisms
The traditional C/C++ assert macro is not the same mechanism as the language-level contract assertions described for C++26. The macro’s behavior depends on how the program is configured and built; confirm the project’s settings and toolchain rather than assuming a check is active or will terminate in a particular way.
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The consulted cppreference page labels C++ contract assertions as a C++26 feature and describes four evaluation semantics—ignore, observe, enforce, and quick-enforce. Which semantics apply depends on the language version and implementation, so verify the compiler and its support before relying on a contract check’s behavior (cppreference: Contracts). The WG21 paper cited above is useful for vocabulary and motivation, but should not be presented as the final standard wording.
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Put the contract into practice
- Define the boundary. Identify what callers must guarantee, what the operation promises on return, and which properties must remain true across operations.
- Classify each condition. Use an assertion for an internal assumption whose violation signals a defect. Use validation and ordinary error handling for invalid external input or expected environmental conditions.
- Choose the failure behavior. Specify diagnostic capture, containment, and the system-defined safe response; do not assume print-and-exit is appropriate.
- Verify build and language behavior. Check whether traditional assertions are enabled in the relevant build configuration. For C++26 contract assertions, verify the implementation and evaluation semantics used by the project.
- Review the whole safety argument. Treat assertions as one aid to visibility and bug detection, not as a substitute for requirements, testing, or system-level safety analysis.
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