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# Programming with Contracts in C++26

C++26 Contracts replace risky assert macros with explicit, compiler-enforced function boundaries. Learn how to trap errors at the source and achieve zero-overhead performance.

- **Speakers:** [Peter Bindels](https://www.wearedevelopers.com/@peter-bindels)
- **Event:** World Congress 2026 Europe
- **Published:** July 9, 2026
- **Duration:** 24:25
- **URL:** https://www.wearedevelopers.com/videos/100056-programming-with-contracts-in-c-26

## Summary

C++26 introduces Contracts, transforming how developers define and enforce function boundaries previously relegated to uncompilable code comments. By declaring explicit pre-conditions and post-conditions directly in the function signature, compilers can now universally understand valid inputs and expected return ranges. Much like guardrails on a mountain road, contracts remain invisible during normal operation but provide crucial safety nets the moment execution veers off track, trapping errors close to the source rather than deep within the call stack.<br><br>Contracts solve severe historical flaws associated with traditional C-style assert macros, such as comma parsing errors in templates, accidental state mutations, and code drift between debug and release builds. The new implementation introduces a contract_assert that is continually parsed even when disabled and enforces const evaluation to prevent unintended side effects. Developers have granular control over violation handling through four distinct semantic modes: ignore (for broken or expensive checks), observe (for validating new constraints in production safely), enforce (the recommended default for robust termination), and quick enforce (optimized for security and minimal binary footprint by bypassing logging).<br><br>Beyond mere safety, contracts enable profound global optimization by doing local analysis. Because data flow properties are strictly guaranteed by contract declarations, compilers can eliminate redundant bounds checks downstream, ensuring that well-written checks introduce zero systemic performance overhead. As the specification finalizes for C++26, with implementations already available in modern compilers like GCC 16, future proposals are targeting rich custom messages and contract tagging to allow modular execution controls across large codebases.

**Keywords:** c++26 contracts, design-by-contract boundaries, function preconditions and postconditions, contract violation handling, semantic evaluation modes, replacing c-style asserts, contract_assert semantics, compiler data flow analysis, global compilation optimization, quick enforce binary reduction, observe mode production testing, debug configuration code drift, const assertion evaluation, gcc 16 contract support

## Chapters

1. **The limitations of code comments for function boundaries** (00:04) — Exploring a mathematical function to demonstrate why standard code comments hide critical bounds from compilers.
1. **Syntax and boundary checks of C++26 contracts** (02:19) — Applying preconditions and postconditions on function declarations allows compilers to assume exact bounds.
1. **Contracts as guardrails for intended application execution** (04:08) — Design bounds do not alter normal execution but catch unintended code paths before they can be exploited.
1. **Handling contract violations through traps and termination** (05:57) — The compiler standard provides handler functions passing violation objects to trigger debuggers, logging, or process termination.
1. **Delaying process termination in mission-critical applications** (07:23) — Terminating failing processes automatically is not appropriate in strict safety contexts like algorithmic trading or self-driving cars.
1. **Semantic modes for managing variable contract checks** (08:50) — The C++ standard allows switching contract enforcement paths between ignore, observe, enforce, and quick enforce.
1. **Deciding between contract semantic evaluation modes** (10:29) — Standard code should apply enforce mode, utilizing observe for deployments and quick enforce for extreme security.
1. **Recognizing the functional limitations of legacy C asserts** (11:56) — The standard assert macro breaks on unparenthesized commas and often degrades in release builds by hiding functional drift.
1. **Fixing ad-hoc validation problems with contract asserts** (14:57) — Deploying contract assert establishes safe checks that parse accurately without actively hindering system performance thresholds.
1. **Enabling global optimization through local contract analysis** (16:16) — Structured declarations assist compilers in pruning unneeded runtime checks by successfully tracking local data flow.
1. **Adding labels and custom messages to contract checks** (20:30) — Future standard updates will introduce labeled error string configurations to trigger conditional violation routing handlers.
1. **Checking early compiler support for C++26 contracts** (22:55) — Access standard implementations today using early versions of GCC 16, Clang forks, and Ubuntu package managers.

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- [The original purpose of strict programming language contracts](https://www.wearedevelopers.com/videos/100529-when-humans-stop-writing-code-rethinking-languages-compilers-and-responsibility) (from "When Humans Stop Writing Code: Rethinking Languages, Compilers, and Responsibility")
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