#reflection
21 posts tagged #reflection.
Defaulting a destructor costs you both move operations
Howard Hinnant's table tells you which special member functions survive once you declare one yourself. C++26 reflection can check it rather than trusting it, and the result for a defaulted destructor is the one worth internalising: a line that looks like a no-op removes move construction and move assignment.
reflect_tracing: zero-overhead spans on Maciek Gajewski's ring-buffer engine
Tracing and metrics split into sibling libraries: reflect_telemetry for aggregates over time, reflect_tracing for spans at microsecond resolution. The engine is Maciek Gajewski's 2021 Wro.cpp technique — thread-local ring buffers, ~200 ns per span, function address as payload, DWARF resolution at dump time. Reflection contributes the instrumentation layer, which today is one explicit scope-guard line per function and tomorrow (C++29, P3294/P3157) becomes a pure annotation.
reflect_telemetry: compile-time Prometheus metrics from annotated fields
Every microservice has hand-registered counter() lines that drift from the variable they count. Annotate a field [[=metric(counter)]] and let reflection emit the Prometheus exposition + OpenTelemetry OTLP exporter, keeping names in lockstep with code. Genuinely novel — nobody is doing this yet.
reflect_dx: auto-generated debugger pretty-printers and docs
Every C++ shop writes .natvis files by hand and they go stale. Every C++ shop wishes their headers were documented but can't face Doxygen XML. reflect_dx ships as a build-step tool: walk your headers with reflection, emit LLDB / GDB / Visual Studio pretty-printers + markdown docs. Your struct is your documentation and its debugger visualisation.
Define your own function colors: compile-time caller checks with C++26 reflection
C++ already enforces function colors -- consteval, CUDA's __device__, Clang's [[clang::nonblocking]] -- but every one of them needed a compiler change. C++26's std::meta::current_function() (P3795R1) lets a callee reflect on its caller at compile time, so you can paint your own colors as a 20-line library: audio-thread safety, capability tokens, or architectural layers that refuse to compile when called from the wrong place. Here is the pattern, three professional uses, and exactly where it breaks.
reflect_optics: Haskell-style lenses for C++26
Haskell's lens library has been the gold standard for nested data access for fifteen years. Ports to C++ needed macro hell and never felt right. Reflection makes the pattern genuinely tractable. field<"address.city">(person) is now a first-class, zero-overhead, type-safe, composable lens.
Reflection + annotations for hashing: opt out of fields, not whole structs
Krystian Piekos's May 29 post on infotraining.pl shows the cleanest demo so far of P3394 annotations combined with P2996 reflection: opt structs into hashing with [[=hashable]], and opt individual fields out with [[=skipped_for_hash]]. The Hashable concept and the calculate_hash walker fit in 40 lines. No macros.
reflect_arbitrary: property-based test generators, inferred from your types
QuickCheck in Haskell, mockall in Rust, jqwik in Java — every major language auto-derives Arbitrary<T> from a struct declaration. C++ was last. Reflection fixes that: reflect the struct, recurse into field types, emit a generator. RapidCheck and FuzzTest adapters included.
reflect_llmschema: C++ functions to LLM tool-use schemas, at compile time
In the AI agent era every tool you expose to Claude/GPT needs a JSON-Schema description. Today you write it by hand next to the C++ function. With C++26 reflection, a single consteval call emits the tool-use JSON at compile time — parameter names, types, docstrings, dispatch, done.
The fastest JVM is the C++26 compiler
Koen Samyn's BeCPP talk used std::meta::substitute to transform Java bytecode into executable C++ at compile time. The compiler constant-folds the entire program. The loop doesn't run faster. It doesn't exist. This is reflection used not for serialization or enum-to-string, but as a compile-time metaprogramming substrate for building language interpreters.
Revzin at C++Now: 'Reflection Is Only Half the Story' -- what generation looks like next
Barry Revzin's C++Now 2026 keynote (Mon 4 May, Aspen) landed the line the C++ reflection community has been circling for two years: C++26 reflection lets you OBSERVE -- the next question is what it looks like to GENERATE. The 90-minute talk is a tour of source-code-generation design space (macros, templates, Rust proc-macros, Swift macros, D mixins) and reads as the natural sequel to wro.cpp's whole 'Where this is heading' triptych framing.
The five most useful things C++26 reflection unlocks (in effort order)
C++26 reflection looks like it requires a PhD in template metaprogramming. It does not. Five concrete projects you can build with it, ranked from five minutes to weeks. Use this as a triage when picking your first reflection-powered project.
Is std::vector consteval or constexpr? A reader's question, answered
A colleague asked: 'Is std::vector consteval and not constexpr?' Short answer: no, but the intuition is right. Long answer: the constexpr/consteval/constinit trio, the transient-allocation rule, and why std::define_static_array exists.
Auto-generating std::formatter<T> for any aggregate
Rust's #[derive(Debug)] in C++: make any struct printable via std::format and std::println by dropping in one partial specialisation of std::formatter. Nested types, containers, enum names — all handled.
Goodbye magic_enum: enum reflection done right
A reflection-driven enum↔string library in 30 lines. No __PRETTY_FUNCTION__ tricks, no compile-time range knob, no compiler-specific behaviour — and unbounded, including enum values beyond 128.
GCC 16.1 ships C++26 reflection -- your 30-line hands-on
GCC 16.1 dropped on 30 April 2026 with -freflection. C++26 was finalized at Croydon five weeks earlier. Here is a 25-line program you can compile and run today on your laptop.
template for: iterating reflections at compile time
Expansion statements (proposal P1306) unroll a loop at compile time, instantiating the body once per element. They are the natural partner of reflection — one loop, N specialisations.
C++26 is done -- five weeks since Croydon, here's what shipped
Five weeks ago WG21 voted C++26 to publication in Croydon. The dust has settled enough to take stock of what landed, what compiles today, and what it means for the wro.cpp reflection series.
Splicing: [: r :] and putting reflections back into code
Splicing is the inverse of ^^: it takes a std::meta::info and drops the referred entity back into your source. Types, expressions, template arguments, member accesses — all round-trippable.
Your first ^^: reflecting types and walking members
Hands-on introduction to C++26 reflection: the ^^ operator, std::meta::info, and walking struct members. We take the post-1 teaser apart and rebuild it from primitives.
C++26 Reflection: What changes, and why it matters
Part 1 of the C++26 reflection series. A 40-line JSON serializer nobody could write in C++ before 2026 — plus the strategic story of why static reflection changes the ecosystem.