Proxy 4 focuses on a composable skills API, refined core semantics for the basic building blocks, and leaner code generation. It remains header-only and keeps 3.x coexistence paths introduced in 3.4.0.

New: a refreshed documentation site at https://microsoft.github.io/proxy/ and out-of-the-box availability on Compiler Explorer. Proxy now also supports the Intel oneAPI C++ compilers in CI.

What's inside

• Skills and builder

  • basic_facade_builder gains add_skill and moves feature toggles into pro::skills (format, wformat, rtti, as_view, as_weak, slim). [Refactor basic_facade_builder #285]
  • {fmt} integration via proxy_fmt.h with pro::skills::fmt_format and fmt_wformat (requires {fmt} >= 6.1; include <proxy/proxy.h> and {fmt} before proxy_fmt.h). [Add skills for {fmt} #287]

• Core semantics and ergonomics

  • proxy::facade_type alias and stricter polymorphic ctor/assignment constraints (pointer-like only). [Improve semantics of proxy #278]
  • proxy_invoke now passes the proxy reference to indirect function pointers, enabling leaner call sequences around indirect calls; removed deprecated PRO_DEF_WEAK_DISPATCH (since 3.2.0).
  • Improved design contracts: removed access_proxy and proxiable_ptr_constraints; refined ProAccessible, ProBasicFacade, ProFacade, ProDispatch, and ProOverload requirements; accessing a null proxy is erroneous behavior; updated spec and tests. [Improve design quality #320, Revise the semantics of *ProAccessible* #336]
  • proxy_indirect_accessor is no longer copyable/movable to prevent misuse; added tests. [Improve proxy_indirect_accessor #335]
  • Introduced is_bitwise_trivially_relocatable and updated dispatch/pointer internals to support bitwise trivially relocatable but non-movable types. We did not adopt the C++26 "trivially relocatable" model; Proxy follows the "bitwise trivially relocatable" direction to preserve memcpy-based relocation semantics (see P3780R0). [Support bitwise trivial relocatability #330]
  • Fixed prefix ++/-- to return proxy& (like assignment operators). [Fix return type for prefix operator++ and operator-- #332]
  • weak_proxy supports upward conversion. [weak_proxy support upward conversion #286]

• C++20 modules

  • Introduced C++20 module interface files (.ixx) and separated convenience macros to support modules. A huge thanks to @SidneyCogdill for leading this effort. See the modules support docs for usage caveats and current CMake module export limitations. [Separate macro definitions and add cppm files #293]

• Code generation

  • Leaner indirect call paths via proxy_invoke adjustments reduce surrounding instructions. [Improve proxy_invoke #280]
  • Builds where <format> is unavailable are supported; features that need formatting are conditional. [Support build environments where <format> is not available #289]

• Toolchain and CI

  • Clang upgraded to 20 in CI; fixed build with GCC trunk; added Intel oneAPI pipeline; updated NVHPC; improved libc++ compatibility. [Upgrade Clang version in the CI pipeline #282, Fix build with GCC trunk #327, Improve toolchain coverage #333]

• Docs and site

  • New docs site (MkDocs) with dark theme and improved navigation. See the FAQ for major upgrade guidance. Extracted example code from docs; updated sample links. [Revise documentation #299, Allow dark theme on the website #331, Add FAQ for major version upgrade #311, Extract example code from docs #291, Update sample code links #328]
  • clang-format applied and enforced in CI; code and macro cleanup for standard naming. [Apply clang-format #309, Apply clang-format to the whole repository #314, Code clean up #283, Code cleanup #316, Clean up macros #317]

Upgrading

Most users can update includes and CMake targets and rebuild. If you customized dispatches/skills or relied on old behaviors, see notes below.

• Headers and namespaces

  • Preferred include: <proxy/proxy.h>. Versioned headers remain under <proxy/v4/>. Code lives in pro::v4 inline namespace.

• CMake package/targets

  • New package/namespace: msft_proxy4. Example:
    • find_package(msft_proxy4 REQUIRED)
    • target_link_libraries(your_target PRIVATE msft_proxy4::proxy)
    • For the module interface, see docs; .ixx sources are shipped but export workflows vary by toolchain.

• Builder and skills

  • basic_facade_builder::support(...) -> add_skill<...>(). Feature toggles are in pro::skills (e.g., pro::skills::rtti, ::as_view, ::as_weak, ::fmt_format).
  • New header proxy_fmt.h for {fmt} skills; include <proxy/proxy.h> and {fmt} first.

• Removed and changed behaviors

  • PRO_DEF_WEAK_DISPATCH removed (deprecated since 3.2.0). Use standard dispatch patterns; weak fallbacks can call back into proxy.
  • Accessing a null proxy is erroneous behavior. Audit code paths that may dereference disengaged proxies.
  • proxy_indirect_accessor isn't copyable/movable; avoid storing *p; consume it immediately.
  • Relocatability: prefer pointer-like indirections if a type fails proxiable checks; see relocatability docs and is_bitwise_trivially_relocatable guidance.

• Formatting availability

  • If <format> is not available on your toolchain, Proxy will still build; only format-dependent skills/features are disabled. Use {fmt} skills if desired.

Refer to the updated docs and FAQ for side-by-side 3.x/4.x migration in large codebases, module usage notes, and relocatability guidance.

Acknowledgments

• What's Changed (selected PRs)

  • Fix typo in the spec of weak_facade by @mingxwa in Fix typo in the spec of weak_facade #275
  • Remove offsetof to eliminate undefined behavior by @mingxwa in Remove offsetof to eliminate undefined behavior #276
  • Improve semantics of proxy by @mingxwa in Improve semantics of proxy #278
  • Improve proxy_invoke by @mingxwa in Improve proxy_invoke #280
  • Upgrade Clang version in the CI pipeline by @mingxwa in Upgrade Clang version in the CI pipeline #282
  • Code clean up by @mingxwa in Code clean up #283
  • Refactor basic_facade_builder by @mingxwa in Refactor basic_facade_builder #285
  • weak_proxy support upward conversion by @mingxwa in weak_proxy support upward conversion #286
  • Support build environments where <format> is not available by @mingxwa in Support build environments where <format> is not available #289
  • Add skills for {fmt} by @mingxwa in Add skills for {fmt} #287
  • Extract example code from docs by @mingxwa in Extract example code from docs #291
  • Separate macro definitions and add cppm files by @SidneyCogdill in Separate macro definitions and add cppm files #293
  • Project rename: Proxy -> Proxy 4 by @SidneyCogdill in Project rename: Proxy -> Proxy 4 #302
  • Make package name matches namespace name by @SidneyCogdill in Make package name matches namespace name #307
  • Apply clang-format by @mingxwa in Apply clang-format #309
  • Revise documentation by @mingxwa in Revise documentation #299
  • Add FAQ for major version upgrade by @mingxwa in Add FAQ for major version upgrade #311
  • Fix release pipeline by @mingxwa in Fix release pipeline #313
  • Apply clang-format to the whole repository by @mingxwa in Apply clang-format to the whole repository #314
  • Code cleanup by @mingxwa in Code cleanup #316
  • Clean up macros by @mingxwa in Clean up macros #317
  • Improve design quality by @mingxwa in Improve design quality #320
  • Support trivial relocatability by @mingxwa in Support trivial relocatability #321
  • Revise skills by @mingxwa in Revise skills #324
  • Fix MSVC build in debug mode by @mingxwa in Fix MSVC build in debug mode #329
  • Update sample code links by @mingxwa in Update sample code links #328
  • Fix build with GCC trunk by @mingxwa in Fix build with GCC trunk #327
  • Support bitwise trivial relocatability by @mingxwa in Support bitwise trivial relocatability #330
  • Allow dark theme on the website by @mingxwa in Allow dark theme on the website #331
  • Improve proxy_indirect_accessor by @mingxwa in Improve proxy_indirect_accessor #335
  • Fix return type for prefix operator++ and operator-- by @mingxwa in Fix return type for prefix operator++ and operator-- #332
  • Code cleanup by @mingxwa in Code cleanup #334
  • Improve toolchain coverage by @mingxwa in Improve toolchain coverage #333
  • Revise the semantics of ProAccessible by @mingxwa in Revise the semantics of *ProAccessible* #336
  • Fix release pipeline by @mingxwa in Fix release pipeline #337

• New Contributors

  • @SidneyCogdill made their first contribution in Separate macro definitions and add cppm files #293

Full changelog: 3.4.0...4.0.0