C++26 Contracts: Introduction to Preconditions and Postconditions

Original Article ↗ Hacker News Discussion ↗

C++26’s new Contracts feature, which adds standardized preconditions and postconditions to function declarations, is drawing both enthusiasm and skepticism. Supporters see it as a way to turn “code as documentation” into machine-checkable invariants that can aid debugging, static analysis, optimization and even AI tooling, while remaining optional at runtime. Critics argue that the semantics are muddled—especially around input validation and when checks may be compiled out—making Contracts feel like a heavier, more confusing form of `assert` that may not deliver on formal verification hopes.

Role of Contracts and Formal Verification

  • Many see C++26 contracts as a way to encode Hoare triples (pre/post/invariants) and generate verification conditions that tools/solvers can check.
  • Examples from other ecosystems (Racket, SPARK/Ada, specialized C/C++ verifiers) are cited as evidence that a restricted subset can be statically proved, even if full C++ never will be.
  • Contracts can help isolate “messy” functions: prove everything around them assuming pre/post hold, and localize where proof fails.

Runtime Behavior vs Input Validation

  • Broad agreement: contracts are for “this must never happen; if it does, it’s a bug,” not for normal user input validation.
  • Several people note the boundary between “input validation” and “assumptions” is blurry and will likely be misused in practice.
  • Contracts are compared to asserts: similar role, but standardized, visible at declaration, multiple evaluation modes (ignore, observe, checking).
  • Critiques focus on the “ghost code” model: you must assume checks might be omitted, so they cannot be relied on for safety-critical behavior.

Optimization and Performance

  • Some hope compilers will use contracts like assume for optimization or to eliminate redundant checks; others say that is not a primary design goal.
  • There’s disappointment that in current implementations, “ignore” mode (no runtime checks) doesn’t appear to still guide optimizations.
  • Embedded and high‑performance developers like being able to enable checks on workstations but disable them on target builds.

Placement, Headers, and Modules

  • Debate over where to put contracts: headers expose them as documentation but may force heavy includes; .cpp files avoid that but lose visibility.
  • Standard requires the first declaration to carry the contracts, pushing usage toward headers; some argue modules will mitigate compile-time blowups.
  • Suggested pattern: keep header contracts simple or delegate to helper functions defined elsewhere.

Language Complexity and Comparisons

  • Some view C++ as already too large and see contracts as more complexity for modest gain; others say this just formalizes a long‑used pattern.
  • Comparisons made with Eiffel, D, Clojure, Racket, Rust, Go: many other languages have contracts or pre/post facilities, with mixed experiences on real‑world usefulness.

Usefulness, Safety, and Tooling

  • Fans emphasize benefits for IDEs, static analyzers, AI agents, and as precise “executable documentation.”
  • Skeptics argue contracts are “just fancier asserts,” can’t guarantee execution, and may even be less safe than plain assert in some patterns.
  • Overall: cautious optimism about tooling and verification potential, substantial skepticism about practical everyday payoff and design trade‑offs.