[libcu++] Specialize `cuda::std::formatter` for `tuple` by davebayer · Pull Request #9593 · NVIDIA/cccl

davebayer · 2026-06-25T12:29:32Z

This PR implements:

tuple_formatter

coderabbitai · 2026-06-25T12:40:26Z

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Added tuple/pair formatting support to cuda::std::formatter, including the formatter plumbing needed for cppreference’s “tuple_formatter” feature and (when available) corresponding host-stdlib std::tuple/std::pair specializations.

Key changes:

Implemented width-aware tuple/pair formatting by introducing __fmt_retarget_buffer and a basic_format_context partial specialization that retargets output for tuple/pair formatting.
Added tuple-specific formatter infrastructure in cuda/std/__format/formatters/tuple.h, supporting configurable opening/closing brackets, separators, and the m format specifier for 2-element (pair-like) types; parse validation was extended accordingly.
Updated formatter declarations/forward declarations to support the new tuple formatter path (including removing the default _CharT = char for cuda::std::formatter and adjusting __fwd/format.h declarations).
Extended output formatting helpers to efficiently handle retarget-buffer iterators (__fmt_copy, __fmt_transform, __fmt_fill).
Added tuple-related formatter spec helpers (__fmt_spec_fields_tuple()) and supporting string-literal utilities/macros (notably _CCCL_STRLIT) with associated literal test coverage.
Expanded/added tuple-focused tests for tuple formatter behavior and format_to/vformat_to, including additional coverage gated by TEST_FORMAT_LIMITED_ITERATOR_TESTING, plus tests for parse, set_brackets, set_separator, formatting output, and relevant exception/validation paths.

Walkthrough

Adds tuple and pair formatting support in libcudacxx, including formatter plumbing, a retarget buffer/context path, tuple formatter specializations, and tests for format, parse, bracket, separator, format_to, and vformat_to behavior.

Changes

Tuple formatting support

Layer / File(s)	Summary
Formatter plumbing `libcudacxx/include/cuda/std/__fwd/format.h`, `libcudacxx/include/cuda/std/__format/formatter.h`, `libcudacxx/include/cuda/std/__format_`, `libcudacxx/include/cuda/std/__format/format_spec_parser.h`	Forward declares `formatter`, removes the `_CharT` default, includes tuple formatter support, and adds tuple-specific format-spec defaults.
Retarget buffer and context `libcudacxx/include/cuda/std/__format/buffer.h`, `libcudacxx/include/cuda/std/__format/format_context.h`, `libcudacxx/include/cuda/std/__format/output_utils.h`	Adds `__fmt_retarget_buffer<_CharT>`, a matching `basic_format_context` specialization, and retarget-aware output fast paths.
Tuple formatter implementation `libcudacxx/include/cuda/std/__format/formatters/tuple.h`	Implements `__fmt_formatter_tuple` and `formatter` specializations for `cuda::std::pair`, `cuda::std::tuple`, and host `std::pair`/`std::tuple`.
Direct formatter tests `libcudacxx/test/support/format_functions_common.h`, `libcudacxx/test/libcudacxx/std/text/format/format.tuple/*`	Adds parse-call validator support and direct tuple formatter tests for format, parse, bracket, and separator behavior.
Iterator tuple tests `libcudacxx/include/cuda/std/__string/literal.h`, `libcudacxx/test/support/literal.h`, `libcudacxx/test/libcudacxx/libcxx/literals/str.pass.cpp`, `libcudacxx/test/libcudacxx/std/text/format/format.functions/*`, `libcudacxx/test/libcudacxx/std/text/format/format.functions/tests/tuple.h`, `libcudacxx/test/libcudacxx/std/text/format/format.functions/format_to.tuple.pass.cpp`, `libcudacxx/test/libcudacxx/std/text/format/format.functions/vformat_to.tuple.pass.cpp`	Adds string-literal helpers, shared tuple test helpers, checker guards, and the `format_to`/`vformat_to` tuple pass files for iterator-based coverage.

Suggested labels

libcu++

Suggested reviewers

ericniebler
miscco

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coderabbitai

Actionable comments posted: 2

🧹 Nitpick comments (1)

libcudacxx/include/cuda/std/__format/formatters/tuple.h (1)
10-11: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

suggestion: Fix the include guard spelling to match the full path.

FORMATERS is missing a T; this violates the project’s full-path include-guard rule.
-#ifndef _CUDA_STD___FORMAT_FORMATERS_TUPLE_H
-#define _CUDA_STD___FORMAT_FORMATERS_TUPLE_H
+#ifndef _CUDA_STD___FORMAT_FORMATTERS_TUPLE_H
+#define _CUDA_STD___FORMAT_FORMATTERS_TUPLE_H
...
-#endif // _CUDA_STD___FORMAT_FORMATERS_TUPLE_H
+#endif // _CUDA_STD___FORMAT_FORMATTERS_TUPLE_H
As per coding guidelines, “Headers must use include guards named from the uppercase full path.”

Also applies to: 207-207

Source: Coding guidelines

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📥 Commits

Reviewing files that changed from the base of the PR and between 161beb9 and 856f4d6.

📒 Files selected for processing (20)

libcudacxx/include/cuda/std/__format/buffer.h
libcudacxx/include/cuda/std/__format/format_context.h
libcudacxx/include/cuda/std/__format/format_spec_parser.h
libcudacxx/include/cuda/std/__format/formatter.h
libcudacxx/include/cuda/std/__format/formatters/tuple.h
libcudacxx/include/cuda/std/__format_
libcudacxx/include/cuda/std/__fwd/format.h
libcudacxx/include/cuda/std/__string/literal.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/checkers/format_to.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/checkers/format_to_n.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/checkers/vformat_to.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/format_to.tuple.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.functions/tests/tuple.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/vformat_to.tuple.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.tuple/format.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.tuple/parse.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.tuple/set_brackets.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.tuple/set_separator.pass.cpp
libcudacxx/test/support/format_functions_common.h
libcudacxx/test/support/literal.h

miscco · 2026-06-25T12:40:24Z

+      }
+      else
+      {
+        ::cuda::std::__throw_format_error("Type m requires a pair or a tuple with two elements");


Should this reference "tuple presentation types"?

Yes, exactly!

coderabbitai

Actionable comments posted: 1

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📥 Commits

Reviewing files that changed from the base of the PR and between e335053 and 1d22f09.

📒 Files selected for processing (22)

libcudacxx/include/cuda/std/__format/buffer.h
libcudacxx/include/cuda/std/__format/format_context.h
libcudacxx/include/cuda/std/__format/format_spec_parser.h
libcudacxx/include/cuda/std/__format/formatter.h
libcudacxx/include/cuda/std/__format/formatters/tuple.h
libcudacxx/include/cuda/std/__format/output_utils.h
libcudacxx/include/cuda/std/__format_
libcudacxx/include/cuda/std/__fwd/format.h
libcudacxx/include/cuda/std/__string/literal.h
libcudacxx/test/libcudacxx/libcxx/literals/str.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.functions/checkers/format_to.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/checkers/format_to_n.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/checkers/vformat_to.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/format_to.tuple.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.functions/tests/tuple.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/vformat_to.tuple.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.tuple/format.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.tuple/parse.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.tuple/set_brackets.pass.cpp
libcudacxx/test/libcudacxx/std/text/format/format.tuple/set_separator.pass.cpp
libcudacxx/test/support/format_functions_common.h
libcudacxx/test/support/literal.h

✅ Files skipped from review due to trivial changes (4)

libcudacxx/test/libcudacxx/std/text/format/format.functions/checkers/format_to_n.h
libcudacxx/include/cuda/std/_format
libcudacxx/test/libcudacxx/std/text/format/format.functions/format_to.tuple.pass.cpp
libcudacxx/include/cuda/std/__fwd/format.h

🚧 Files skipped from review as they are similar to previous changes (12)

libcudacxx/test/libcudacxx/std/text/format/format.functions/vformat_to.tuple.pass.cpp
libcudacxx/include/cuda/std/__string/literal.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/checkers/vformat_to.h
libcudacxx/test/support/format_functions_common.h
libcudacxx/include/cuda/std/__format/format_spec_parser.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/checkers/format_to.h
libcudacxx/test/support/literal.h
libcudacxx/include/cuda/std/__format/formatter.h
libcudacxx/include/cuda/std/__format/buffer.h
libcudacxx/include/cuda/std/__format/formatters/tuple.h
libcudacxx/include/cuda/std/__format/format_context.h
libcudacxx/test/libcudacxx/std/text/format/format.functions/tests/tuple.h

github-actions · 2026-06-27T09:12:22Z

😬 CI Workflow Results

🟥 Finished in 1h 49m: Pass: 80%/120 | Total: 3d 07h | Max: 1h 48m | Hits: 54%/607498

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davebayer requested a review from a team as a code owner June 25, 2026 12:29

davebayer requested a review from ericniebler June 25, 2026 12:29

github-project-automation Bot added this to CCCL Jun 25, 2026

github-project-automation Bot moved this to Todo in CCCL Jun 25, 2026

cccl-authenticator-app Bot moved this from Todo to In Review in CCCL Jun 25, 2026

coderabbitai Bot reviewed Jun 25, 2026

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Comment thread libcudacxx/include/cuda/std/__format/format_context.h

Comment thread libcudacxx/test/libcudacxx/std/text/format/format.functions/tests/tuple.h

davebayer force-pushed the libcuxx_format_tuple branch from 856f4d6 to f378167 Compare June 25, 2026 12:43

miscco reviewed Jun 25, 2026

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[libcu++] Optimize integral formatters

1d22f09

davebayer force-pushed the libcuxx_format_tuple branch from e335053 to 1d22f09 Compare June 27, 2026 07:21

coderabbitai Bot reviewed Jun 27, 2026

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Comment thread libcudacxx/test/libcudacxx/std/text/format/format.tuple/format.pass.cpp

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[libcu++] Specialize `cuda::std::formatter` for `tuple`#9593

[libcu++] Specialize `cuda::std::formatter` for `tuple`#9593
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