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3815: MINOR: [C++] fix missing format_context when compiled with C++23 #39

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3815: MINOR: [C++] fix missing format_context when compiled with C++23 #39

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MINOR: [C++] fix missing format_context when compiled with C++23
wgtmac Jun 9, 2026
1c88b5b
Add AI config files
martin-g Jun 10, 2026
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5 changes: 5 additions & 0 deletions .cursor/rules.md
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The AI agents should never link to any issue or a pull request
in any GitHub repository in the code reviews!

The AI agents should not review AI agents' config files like CLAUDE.md or AGENTS.md!

5 changes: 5 additions & 0 deletions .gemini/rules.md
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The AI agents should never link to any issue or a pull request
in any GitHub repository in the code reviews!

The AI agents should not review AI agents' config files like CLAUDE.md or AGENTS.md!

5 changes: 5 additions & 0 deletions AGENTS.md
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The AI agents should never link to any issue or a pull request
in any GitHub repository in the code reviews!

The AI agents should not review AI agents' config files like CLAUDE.md or AGENTS.md!

5 changes: 5 additions & 0 deletions CLAUDE.md
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The AI agents should never link to any issue or a pull request
in any GitHub repository in the code reviews!

The AI agents should not review AI agents' config files like CLAUDE.md or AGENTS.md!

3 changes: 2 additions & 1 deletion lang/c++/include/avro/Node.hh
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Expand Up @@ -225,7 +225,8 @@ inline std::ostream &operator<<(std::ostream &os, const avro::Node &n) {

template<>
struct std::formatter<avro::Name, char> : std::formatter<std::string, char> {
auto format(const avro::Name &n, std::format_context &ctx) const {
template<typename FormatContext>
auto format(const avro::Name &n, FormatContext &ctx) const {
return std::formatter<std::string, char>::format(n.fullname(), ctx);
}
Comment on lines +228 to 231

@coderabbitai coderabbitai Bot Jun 10, 2026

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⚠️ Potential issue | 🟡 Minor

🧩 Analysis chain

🌐 Web query:

Does C++23 require std::formatter specializations to use a template FormatContext parameter instead of std::format_context in the format method signature?

💡 Result:

No, C++23 does not require std::formatter specializations to use a template FormatContext parameter in the format method signature [1]. The standard requirements for a formatter specialization [2][1] specify that the format method must accept a context object of type std::basic_format_context<OutputIt, CharT> [3][2][1]. In practice, you may either explicitly use the concrete type (such as std::format_context) [4][5] or use a template parameter to make the formatter more generic [6][1]. Specifically, the Formatter requirements [1] state that given a formatter object f, an lvalue arg of type Arg, and an lvalue fc of type basic_format_context<OutputIt, CharT>, the expression f.format(arg, fc) must be valid and return an object of type basic_format_context<OutputIt, CharT>::iterator [2][1]. Because std::format_context is a type alias for std::basic_format_context<std::back_insert_iteratorstd::string, char> (or the wide character equivalent) [3][7], using it in your signature restricts your formatter to that specific context [4][5]. Using a template parameter for the context allows your formatter to work with different output iterators and contexts [6][1], but it is not a requirement imposed by the standard [1]. You are free to write the signature as: auto format(const T& val, std::format_context& ctx) const; or template auto format(const T& val, FormatContext& ctx) const; Both are valid, provided they meet the semantic requirements of the Formatter interface [2][1].

Citations:

Clarify C++23 std::formatter::format signature requirements (optional change)
C++23 doesn’t require the format method to use a templated FormatContext&; the requirement is that it accepts a std::basic_format_context<OutputIt, CharT>&—std::format_context is an allowed concrete alias. For lang/c++/include/avro/Node.hh:228-231, ensure the C++23 build failure you’re targeting is actually caused by the format signature mismatch (otherwise the template generalization is just an optional flexibility improvement).

🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@lang/c`++/include/avro/Node.hh around lines 228 - 231, The current templated
FormatContext signature on the member formatter for avro::Name (the format
method in Node.hh) may not match the C++23 requirement; replace the template
with a concrete allowed context type—either accept std::format_context& ctx or
define the signature as template<typename CharT, typename OutputIt> auto
format(const avro::Name &n, std::basic_format_context<OutputIt, CharT> &ctx)
const—then call std::formatter<std::string, CharT>::format(n.fullname(), ctx);
also confirm the failing C++23 build error is actually triggered by this
signature mismatch before changing anything else.

};
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3 changes: 2 additions & 1 deletion lang/c++/include/avro/Types.hh
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Expand Up @@ -112,7 +112,8 @@ std::ostream &operator<<(std::ostream &os, const Null &null);

template<>
struct std::formatter<avro::Type, char> : std::formatter<std::string, char> {
auto format(avro::Type t, std::format_context &ctx) const {
template<typename FormatContext>
auto format(avro::Type t, FormatContext &ctx) const {
return std::formatter<std::string, char>::format(avro::toString(t), ctx);
}
};
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