The most appropriate group of people to review this topic would be Senior Software Engineers specializing in C++ Language Standardization, Metaprogramming, and Compiler Architecture.

Abstract:

This presentation provides a detailed technical overview of the C++ static reflection feature adopted in the P2996 proposal for the C++26 standard, emphasizing its design, core components, and semantic implications. Static reflection exposes the program's structure at compile time via the compiler, contrasting with prior template metaprogramming attempts due to performance and implementation complexity concerns. Key language features introduced include the reflection operator (<<...>>, nicknamed "unibrow"), the splicing operator (...), and the unified meta-object type, std::meta::info. A critical design choice is the stateful nature of meta-objects, meaning they dynamically update based on the compilation context (e.g., completion of an incomplete type). The talk reviews meta-functions, covering challenges like achieving consistent parameter names across function declarations and definitions, and introduces constexpr exceptions as a mechanism for reporting reflection errors at compile time. The speaker advocates for reflection as a groundbreaking feature that will enable a new generation of standard and third-party C++ libraries.

Summarization:

• 0:46 Introduction to Reflection: The session focuses on C++ reflection, defined as the ability of software to expose its structure, noting that the C++26 proposal centers strictly on static reflection performed by the compiler at compile time.
• 2:24 The C++ Language Evolution Context: The speaker, an ISO C++ Foundation and Boost Foundation board member, identifies the proposal as one of the largest ever, requiring incubation across both the Language (Core/Evolution) and Library Evolution (LEWG) working groups.
• 7:48 History and Design Divergence: Early attempts at reflection (e.g., Matuš Cholchick's 2006 "Mirror" library and Boost HANA) relied on template metaprogramming, which was deemed too problematic due to excessive compile times, leading the committee to pursue dedicated language syntax.
• 11:27 Meta-Object Monotype Design: The resulting design utilizes a monotype, std::meta::info, to represent all program elements (types, functions, templates, etc.) by value. This was chosen primarily because compiler implementers suggested that a hierarchical type system would be too heavy and complex to standardize and maintain.
• 18:10 Core Reflection Operators and Realm Shift: The proposal introduces:
  • The Reflection Operator (<<...>>, formerly a single caret), which lifts a program entity (like a type) into the reflection realm, resulting in a std::meta::info object.
  • The Splicing Operator (...), which extracts or interpolates the C++ expression back from the std::meta::info object, allowing the creation of new types or variables based on the reflection result.
• 22:42 Constraint: constexpr Requirement: Meta-type variables (those holding the result of a reflection operation) must be declared constexpr because the compiler must be able to hold and reason about them at compile time.
• 34:01 Stateful Reflection Semantics: A non-trivial semantic detail is that reflection is stateful. A std::meta::info object representing an incomplete type will automatically reflect the type's completion later in the compilation unit, without needing a re-reflection operation. This behavior was adopted to avoid the implementation burden of snapshotting and storing meta-object states.
• 42:01 Meta-Function Details: Meta-functions typically return a std::meta::info object, a vector of meta-info, a boolean, a size, or a string view. For instance, identifier_of returns the simple name in the closest scope, rather than a fully qualified name, a point relevant for logging applications.
• 42:58 Template Substitution/Instantiation: The substitute meta-function serves as the reflection-realm equivalent of template instantiation, allowing programmatic construction of types (e.g., std::array<int, 3>) using reflected parameters. Instantiation only occurs when the meta-object is spliced back into the program context.
• 49:40 Param Naming Consistency Rule: For the meta-function querying parameter names (function_param_names), the committee voted to enforce consistent naming between function declarations and definitions. If parameter names diverge, the code will fail to compile, imposing a new restriction on existing C++ codebases utilizing reflection-based libraries.
• 56:49 Comparison to AI and Rust: The speaker maintains that reflection is necessary despite the rise of AI code generation tools, citing the lack of consistent output from AI agents. When comparing C++ reflection to Rust's procedural macros and syn library, the speaker views the C++ approach as offering a more intuitive user interface for observation than token stream manipulation.
• 59:30 constexpr Exceptions for Error Handling: The proposal includes constexpr exceptions, allowing reflection logic to throw exceptions that are handled entirely at compile time (via try-catch within constexpr blocks). This provides users with better, more descriptive error messages compared to traditional compiler errors or standard template failure diagnostics.
• 1:01:04 Future Library Expectations: The introduction of reflection is expected to significantly increase the number of library proposals submitted to the Library Evolution Working Group, as developers can now write powerful, cross-compiler utilities without modifying the language itself.