What completing C++26 means for existing C++ programs

The final C++26 meeting did not add or remove a headline feature. It completed the work of resolving national-body comments on a draft whose scope was already established. In his March 29 trip report, Herb Sutter reports that technical work finished in Croydon on March 28. The committee is preparing the document for its Draft International Standard approval ballot and final editorial work.

That distinction matters to anyone deciding what the announcement changes today. Technical completion settles the intended language and library specification at this stage of the process. It does not mean every compiler and standard library has shipped a complete implementation. The post is news about the standardization milestone, with practical consequences that arrive through particular toolchains.

My strongest reason to value C++ remains RAII. Scope-bound ownership gives cleanup a form that ordinary code can express and reviewers can follow. The language also carries several generations of programming practice into the same codebase. C++26 interests me where it helps that existing code become safer, or lets new library machinery replace techniques that are difficult to read and maintain.

Completion includes disagreement

Sutter reports that the committee finished addressing the 411 national-body comments received in the Committee Draft ballot. He also reports a final plenary vote of 114 in favor, 12 opposed and three abstaining. Sustained concerns about contracts were the main source of opposition. Completion therefore records a decision after review, without implying unanimity about every technical choice.

That is a useful corrective to treating a standard as the discovery of one obviously correct design. A language with existing implementations and deployed programs has to choose among competing costs. The result can be coherent enough to ship while remaining contentious. Library authors and application authors still need to understand the chosen behavior rather than infer it from the strength of the announcement.

The substantive question is how the completed features change a program's obligations. Safety improvements in existing operations have a different adoption path from a new way to write a library. Contracts add another distinction because the written assertion and the selected checking behavior are separate parts of their design.

Erroneous behavior preserves the fact that a program is wrong

P2795R5 explains the motivation for erroneous behavior using an uninitialized automatic variable. The old problem is that a common programming mistake can lead to undefined behavior. Simply giving the variable a useful default value would also make it easier for incorrect code to depend on that default intentionally.

The paper instead distinguishes erroneous values from a valid program's intended values. An implementation can diagnose an erroneous operation while giving the underlying storage a defined initial representation. The purpose is to constrain the consequences of a mistake without turning the mistake into a recommended programming technique.

This design addresses a real tension in language evolution. Defaults affect both accidental omissions and deliberate code. If the language silently gives an omitted initializer a meaning that programmers can rely on, it has changed the expressive distinction between initializing an object and failing to initialize it. Erroneous behavior tries to retain that distinction while reducing the danger of the failure.

The paper also spells out limits. Its change concerns initialization, and it does not remove every later source of undefined behavior. A pointer with an erroneous initial representation does not become safe to dereference. That distinction is essential when discussing what recompilation can accomplish. Constraining one operation does not prove that the rest of a computation is valid.

Library hardening puts checks at existing preconditions

P3471R4 describes standard library hardening through selected preconditions. A hardened implementation checks those preconditions at runtime and treats their violation as a contract violation. The proposal focuses on checks useful enough and cheap enough for production, particularly bounds-related failures.

Its scope is deliberately narrower than checking all library requirements. Many semantic requirements cannot be checked cheaply, and some cannot be checked by the callee at all. The proposal also aims to avoid requiring an ABI change. That helps explain why hardening can be relevant to existing programs without requiring a new representation for every container.

The operational value is the location of the check. A bounded operation already knows something about the range it is allowed to access. Checking that requirement at the operation can prevent the failure from proceeding through an unchecked library access. Application code still owns the validity of the index it computes, but the library can provide an enforced boundary for selected operations.

This is the part of the release that most directly fits my preference for keeping useful abstractions inexpensive. The interesting engineering question is which checks can run in ordinary builds. A comprehensive mode that nobody enables in deployed software has a different practical effect from a limited set of checks that an implementation can keep enabled there.

Contracts describe requirements without choosing one checking policy

The contracts proposal P2900R14 defines precondition assertions, postcondition assertions and assertion statements. It also defines evaluation semantics called ignore, observe, enforce and quick-enforce, with a mechanism for a user-defined violation handler. The assertion's presence in source is therefore not, by itself, a promise that a particular build will stop on its violation.

That separation makes sense as a mechanism for varied build and deployment requirements, but it adds something a reader must learn. The source states the requirement. The implementation's selected semantics determine how the assertion participates in execution. Documentation for a library using contracts needs to explain both, especially where users might assume a check is always active.

An API requirement also differs from validation of untrusted input. A caller may need ordinary control flow to reject an invalid request and return an error. A contract expresses an obligation within the program. Treating those two tasks as interchangeable would leave behavior dependent on checking policy where the application may need a stable response.

Sutter's account of the continued debate is relevant here. The committee retained contracts after repeated review. That tells readers the feature survived the standardization process, but it does not discharge the work of understanding its policies. The paper's vocabulary is necessary to discuss what an individual program actually gains.

Reflection moves structural knowledge into constant evaluation

P2996R13 describes static reflection using an opaque std::meta::info value. The ^^ operator obtains a reflection, constant-evaluation functions inspect it, and splicers turn reflections back into grammatical elements. Its examples include converting enumeration values to strings and deriving operations from members.

The useful change is where structural knowledge can come from. A library can inspect a program entity during compilation instead of requiring the author to reproduce every relevant detail in an unrelated description. That reduces one source of disagreement between a declaration and the machinery built around it. It still leaves policy decisions to the library author.

For example, learning which members exist does not decide which members should belong to a stable external format. A declaration can contain an implementation detail that should never become a serialized field. Reflection supplies a way to examine structure. Compatibility policy remains a separate design responsibility, and a generated operation must encode it deliberately.

The paper presents this as an initial set of reflection facilities with room for further work. That is a more useful expectation than assuming every metaprogramming task becomes straightforward. Some repetitive structural work gains a direct mechanism. The maintainability of the resulting library will depend on how clearly it exposes the choices it makes from that information.

Execution introduces a shared model for asynchronous composition

P2300R10 supplies the sender and receiver execution framework that Sutter identifies as another major C++26 addition. The framework describes asynchronous work and its completion channels so operations can compose within a common model. This is a different kind of adoption from enabling checks on an existing container operation.

Existing asynchronous APIs need adapters to participate in that model. Sutter explicitly warns that documentation and supporting libraries are still incomplete. A team can value the direction while recognizing that integration requires learning and additional code. The standard provides common concepts, but the surrounding application does not acquire those concepts automatically.

For existing programs, the release consequently offers several distinct decisions. Toolchain adoption can bring changed core behavior. Library hardening depends on the implementation and its mode. New contracts and reflection-based libraries require intentional source-level design. Completing technical work makes those decisions concrete enough to study against a settled feature set while the document proceeds through its remaining approval and editorial stages.