Prompt and scenario
A large C++ codebase uses several compilers, standard-library versions, and build modes. The team wants C++26 Contracts for API preconditions and postconditions, but worries about uneven compiler support, online-process behavior after a violation, and an old build fleet that cannot upgrade. Design a staged rollout and explain validation and rollback.
What the interviewer is testing
- Whether you distinguish contract expressions from test assertions, exception handling, and undefined behavior.
- Whether you identify compatibility boundaries across the standard, compiler, linker, runtime mode, and ABI.
- Whether you include violation handling, performance cost, and production safety in the release design.
- Whether you can advance a language feature with small migrations and observable evidence instead of a one-shot rewrite.
Clarifying questions to ask first
- What compiler, standard-library, build-system, and target-platform matrix exists, and which targets must keep old versions?
- Are contracts meant for public APIs, internal module boundaries, or data invariants? Should a violation terminate, log, pause debugging, or continue?
- What are the online performance budget, incident rate, rollback method, and symbolication capability for core services?
- Do assertions, tests, static analysis, and release gates already provide a migration baseline?
A 30-second answer
I would treat Contracts as interface semantics and diagnostics, not as a replacement for all tests or exception handling. First I would map compiler support, then pilot one internal library and compare enabled, disabled, and violation-handling modes. Public headers must remain consumable by old compilers through a conditional or contract-disabled path. I would migrate from internal APIs to critical services, measuring build success, violations, latency, and binary compatibility. Every online violation needs an explicit pause and rollback path.
Deep dive
1. Define the problem a contract solves
Preconditions describe input obligations, postconditions describe what should hold at return, and invariants constrain object state. They put assumptions at an interface boundary, but do not replace business-error handling, graceful behavior for ambiguous input, or complete tests. Start with cross-module constraints that callers cannot infer easily instead of exposing every implementation detail.
2. Map the toolchain and ABI boundary
List compiler versions, standard libraries, language modes, linkers, platforms, and build configurations. Verify parsing, code generation, violation-handling support, and debug information for each. New syntax in a public header can make an old compiler fail immediately; even successful builds may diagnose differently in different runtime modes. Use capability detection and a minimal sample before choosing conditional-build boundaries.
3. Choose a violation-handling strategy
Violation handling must match service risk. Development builds can stop with a stack trace; test builds should fail the test; production builds may terminate safely, isolate a request, or record and continue depending on contract severity. Continuing cannot hide that state may be untrusted. Records should contain contract location, an input summary, and a version without exposing sensitive data.
4. Evaluate semantics and side effects
Contract expressions should be side-effect free, repeatable, and independent of undefined evaluation order. The team must know whether expressions are evaluated in enabled, disabled, or alternate checking modes and whether handling changes control flow. Do not put state mutation, network calls, or timing dependencies in a contract; keep those behaviors in implementation and tests.
5. Design staged migration and compatibility layers
Start with non-public libraries, pure functions, and high-value invariants on one compiler and CI target. Public APIs can use versioned headers, conditional builds, or macros to keep the old fleet compiling, but macros should handle capability differences rather than hide different business semantics. Record covered functions, support matrix, and unmigrated callers for each expansion.
6. Use evidence to expand or roll back
Establish a pre-migration baseline for build time, binary size, critical-path latency, contract violations, crashes, and test defects. During a canary, compare the same traffic and input distribution before and after, separating real input problems, code defects, and toolchain false positives. If violations concentrate on critical paths, performance exceeds budget, or an old target cannot build reliably, pause migration, disable the new syntax path, and restore the previous build artifact.
A complete strong answer
I would inventory compilers, standard libraries, language modes, platforms, and runtime modes and verify C++26 Contracts parsing and violation handling for every target. Start with internal pure functions and explicit invariants; use contracts as interface documentation and diagnostics, not as a replacement for exceptions, graceful degradation, or tests. Let development and test builds fail fast, while production chooses safe termination, isolation, or logging based on risk and keeps secrets out of records. Preserve old-fleet compilation with capability detection and conditional builds. Gate the canary on build success, latency, binary changes, violation rate, and crashes, pausing and rolling back on any high-risk breach.
Common failure modes
- Treating Contracts as a complete replacement for assertions, exceptions, or tests.
- Saying only “upgrade to C++26” without a compiler, standard-library, linker, and ABI matrix.
- Ignoring how checking modes and violation handling affect control flow, performance, and online safety.
- Putting side-effecting operations in a contract so diagnostics change program behavior.
- Expanding after a successful compile without a baseline, canary, or rollback.
Follow-ups and extensions
Follow-up 1: Should a contract violation throw an exception?
There is no universal answer. A violation means program state or a calling convention was broken. Choose based on safe recoverability, whether exceptions cross an ABI boundary, and the team’s error policy. For unrecoverable state, continuing or using an ordinary exception can hide a larger failure.
Follow-up 2: How do you support old compilers?
First confirm whether public headers must be parsed by them. Capability detection, conditional builds, or compatibility macros can send old targets through a contract-disabled path, but preserve equivalent tests and documentation so business semantics do not diverge.
Follow-up 3: Will contracts slow production?
Measure rather than guess. Compare CPU, latency, and binary changes across checking modes, optimization levels, and input distributions. Restrict expensive checks to development, tests, or a small canary while keeping low-cost monitoring for critical invariants.
Follow-up 4: How do you prevent contract abuse?
Set rules: express only verifiable invariants and boundary assumptions, forbid side effects, document violation and sensitive-data handling, and review them in code review. Every contract needs tests, an owner, and a condition for removal or revision.