From ddab4935516c3edd98e0daeabf123b8af540bc23 Mon Sep 17 00:00:00 2001
From: Mark Probst <mark.probst@gmail.com>
Date: Mon, 20 Jul 2026 16:58:48 -0400
Subject: [PATCH] fix(cpp): support std::optional in Utf16_Utf8 string
 conversion (#2383)

Co-Authored-By: gpt-5.6-sol via pi <noreply@openai.com>
---
 .../language/CPlusPlus/CPlusPlusRenderer.ts   | 22 +++++++++++++++++++
 test/languages.ts                             |  4 +++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts b/packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts
index ec99e8029b..55e371d735 100644
--- a/packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts
+++ b/packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts
@@ -3403,6 +3403,28 @@ export class CPlusPlusRenderer extends ConvenienceRenderer {
                 );
                 this.superThis.ensureBlankLine();
 
+                if (this.superThis.haveOptionalProperties) {
+                    this.superThis.emitLine(
+                        "template<typename TF, typename TT>",
+                    );
+                    this.superThis.emitBlock(
+                        [
+                            "static toType convert(tag<",
+                            this.superThis._optionalType,
+                            "<TF> >, tag<",
+                            this.superThis._optionalType,
+                            "<TT> >, fromType opt)",
+                        ],
+                        false,
+                        () => {
+                            this.superThis.emitLine(
+                                "if (!opt) return toType(); else return toType(Utf16_Utf8<TF,TT>::convert(*opt));",
+                            );
+                        },
+                    );
+                    this.superThis.ensureBlankLine();
+                }
+
                 this.superThis.emitLine("template<typename TF, typename TT>");
                 this.superThis.emitBlock(
                     [
diff --git a/test/languages.ts b/test/languages.ts
index 6bcfd43611..95f7f17d3e 100644
--- a/test/languages.ts
+++ b/test/languages.ts
@@ -739,7 +739,9 @@ export const CPlusPlusLanguage: Language = {
     quickTestRendererOptions: [
         { "source-style": "multi-source" },
         { "code-format": "with-struct" },
-        { wstring: "use-wstring" },
+        // bug2521.json has an optional string, exercising UTF conversion
+        // through std::optional.
+        ["bug2521.json", { wstring: "use-wstring" }],
         { "const-style": "east-const" },
         // The default is boost=false (C++17); this keeps the boost code
         // path covered.  Pinned to specific inputs because the default