Add an error_handler parameter for UTF-8 to the binary readers and writers (#5746)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/output/error_handler.hpp

@@ -0,0 +1,50 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <nlohmann/detail/abi_macros.hpp>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+
+/// how to treat decoding errors
+///
+/// @ref basic_json::dump uses this to decide what to do with ill-formed
+/// UTF-8 while escaping a string, and the binary writers (@ref
+/// basic_json::to_cbor, @ref basic_json::to_ubjson, @ref
+/// basic_json::to_bjdata, @ref basic_json::to_bson) use it the same way for
+/// string values and object keys. The binary readers (@ref
+/// basic_json::from_cbor, @ref basic_json::from_msgpack, @ref
+/// basic_json::from_ubjson, @ref basic_json::from_bjdata, @ref
+/// basic_json::from_bson) use it to decide whether to check text strings
+/// and object keys for well-formed UTF-8 at all, since none of those
+/// formats requires a decoder to do so.
+enum class error_handler_t
+{
+    strict,  ///< throw a type_error/parse_error exception in case of invalid UTF-8
+    replace, ///< replace invalid UTF-8 sequences with U+FFFD
+    ignore,  ///< ignore invalid UTF-8 sequences
+    keep     ///< keep invalid UTF-8 sequences unchanged
+};
+
+/// the default error handler of the CBOR, UBJSON, BJData, and BSON writers:
+/// error_handler_t::strict if JSON_STRICT_BINARY_UTF8 is enabled, otherwise
+/// error_handler_t::keep (the behavior before version 3.13.0)
+constexpr error_handler_t binary_writer_default_error_handler() noexcept
+{
+#if JSON_STRICT_BINARY_UTF8
+    return error_handler_t::strict;
+#else
+    return error_handler_t::keep;
+#endif
+}
+
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END