C++ Contracts and Input Validation: Keep Both Boundaries

C++ contract assertions describe expectations about program state. Required validation decides what the program does with untrusted or otherwise invalid input. Keep those responsibilities explicit when reviewing new contract support or changing a codebase's assertion policy.

The current C++ contract-assertion draft section describes several evaluation semantics, including an ignore semantic. Compiler support and build configuration matter. Do not assume that an annotation visible in source guarantees a particular production response to bad input. Verify the toolchain and configuration you actually deploy.

Draw the input boundary

Identify where external bytes become application values: a request parser, file reader, database import, command-line interface, or plugin boundary. Decide which invalid states must receive a defined error rather than enter an internal function unchecked.

For example, a public division endpoint should handle a zero denominator deliberately. Whether an internal helper also documents a nonzero expectation is a separate design choice. The endpoint still needs the required behavior in every supported release configuration.

Do not classify every failure as a programmer bug. A malformed customer request may be expected operational input. The interface should reject it predictably without relying on a debug-only assumption or an assertion behavior that the release build might change.

Keep the required check ordinary and visible

Here is a small C++ example using unsigned integers and an explicit result:

#include <optional>

std::optional<unsigned> divide(unsigned a, unsigned b) {
    if (b == 0) return std::nullopt;
    return a / b;
}

The caller must handle the empty result. This example illustrates a defined check; it is not a complete request parser or universal error-handling pattern. Parsing, authorization, size limits, and other interface rules remain outside this helper.

The unsigned type also avoids introducing a signed-overflow corner case into this particular illustration. In real code, review all relevant arithmetic states rather than copying a denominator check and declaring the entire operation safe.

Understand contract purpose

A precondition can express what must hold when a function is entered, and a postcondition can express an expected state after successful execution. These declarations can help communicate and test internal assumptions. Their use should fit the codebase's toolchain and operating policy.

Keep predicates focused on observation. Do not put essential work, state changes, logging relied on for compliance, or security decisions inside a predicate whose evaluation behavior can vary. Required effects belong in ordinary controlled execution paths.

Avoid presenting a failed contract as automatically recoverable in the same way as a normal validation error. Review the selected evaluation semantic and handler behavior. A program that terminates on an internal violation has a different interface from one that returns a documented client error.

Verify the build, not just the syntax

Record compiler version, language mode, relevant flags, and how dependencies are built. A code sample compiling in one experimental environment does not establish support across your release matrix. Keep unsupported configurations from silently skipping the intended test.

Build a small controlled program with an intentionally violated expectation and observe the documented outcome for each supported configuration. Run this in a disposable test process, not inside a production service. Capture exit status and diagnostics without assuming that every compiler uses the same switches.

Check libraries and translation units as well as the main executable. A consistent policy needs evidence about the code that actually contains the assertions. A release label alone does not prove that every component was compiled under the intended settings.

Test invalid input separately

Create interface tests for missing fields, malformed numbers, out-of-range values, zero where forbidden, and oversized input. Specify expected responses and resource limits. These tests should pass under every supported release configuration, including configurations where optional assertions do not run.

Then create internal correctness tests that exercise the documented assumptions. Keep them separate so an interface-validation failure is not hidden by a general assertion test. The test names should tell reviewers which boundary each case covers.

Include negative cases in continuous integration and run them against the actual release artifact where feasible. Our release tag and artifact guide explains why testing a source revision and delivering a different binary breaks the evidence chain.

Plan diagnostics and failure handling

Decide what information is safe to log when a check fails. A diagnostic should identify the component and condition without dumping credentials, personal data, or an entire untrusted payload. Keep failure paths bounded so malformed input cannot create unlimited logs.

If a contract violation terminates a worker, test the supervisor's recovery and the request's observable outcome. If ordinary validation rejects the input, confirm that the service remains healthy and returns the intended error. Do not mix these outcomes in one generic “validation worked” status.

Text Diff can compare redacted build settings between configurations. It does not compile C++ or prove an assertion executes. For generated code, our AI code acceptance tests provide a broader review workflow.

Conclusion

Use contracts to make internal expectations clearer, and use required validation to define behavior at trust boundaries. Verify evaluation semantics in the actual build and test bad inputs independently. Source annotations, compiler acceptance, and production safety are related but distinct pieces of evidence.