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Niels Lohmann bfea6f36d3 Fix to_msgpack() reading the inactive number union member (#5694 )

* Fix to_msgpack() reading the inactive number union member

basic_json stores number_integer and number_unsigned in a union, and
number_unsigned_t only has to be at least as wide as number_integer_t
(with the default types, both are 64-bit and have the same
representation). When number_integer_t is narrower, write_msgpack()
read the wrong union member in two places:

- The number_unsigned case wrote number_integer's bits instead of
  number_unsigned's, silently writing the wrong value whenever it
  did not fit in number_integer_t.
- The number_integer case (non-negative branch) picked the encoded
  width by comparing number_unsigned's bits, which is undefined
  behavior, though the value written was still number_integer's, so
  at worst a too-wide encoding was chosen.

Read the active member in both cases, like the other binary writers
(CBOR, UBJSON, BJData, BSON, BON8) already do.

Fixes #5644.

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

* Cast number_integer to number_unsigned_t only once in to_msgpack()

Addresses review comment by @gregmarr.

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

---------

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

2026-09-30 20:07:19 +02:00

2.7 KiB

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nlohmann::basic_json::to_msgpack

// (1)
static std::vector<std::uint8_t> to_msgpack(const basic_json& j);

// (2)
static void to_msgpack(const basic_json& j, detail::output_adapter<std::uint8_t> o);
static void to_msgpack(const basic_json& j, detail::output_adapter<char> o);

Serializes a given JSON value j to a byte vector using the MessagePack serialization format. MessagePack is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

Returns a byte vector containing the MessagePack serialization.
Writes the MessagePack serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in): JSON value to serialize
o (in): output adapter to write serialization to

Return value

MessagePack serialization as a byte vector
(none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

Throws out_of_range.412 if the length of a string, binary value, array, or object exceeds 4294967295, the maximum MessagePack can store; example: "MessagePack length 4294967296 exceeds maximum of 4294967295"
Throws out_of_range.415 if the subtype of a binary value exceeds 255, the maximum of the MessagePack ext type; example: "subtype 70000 is too large for the MessagePack ext type (max 255)"

Complexity

Linear in the size of the JSON value j.

Examples

??? example

The example shows the serialization of a JSON value to a byte vector in MessagePack format.
 
```cpp
--8<-- "examples/to_msgpack.cpp"
```

Output:

```json
--8<-- "examples/to_msgpack.output"
```

Version history

Added in version 2.0.9.
Throws out_of_range.412 and out_of_range.415 since version 3.13.0.
Fixed in version 3.13.0 to serialize number_integer_t/number_unsigned_t pairs of different width correctly; before, integers could be serialized with the wrong value if number_integer_t was narrower than number_unsigned_t.

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