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📡 Document cross-basic_json conversion limitation (#3425) (#5249)

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When converting objects or strings between different basic_json specializations,
the target's object_t::key_type or string_t must be directly constructible from
the source's corresponding type. If this requirement is not met, the conversion
silently falls back to the array-conversion path, producing incorrect results.

This documents the limitation and provides references to issue #3425, which tracks
this behavior. The comment in unit-alt-string.cpp is clarified to reference the
known limitation with a link to the issue, and suggests the parse() workaround.

Fixes #3425 (documentation; full fix deferred pending type-trait redesign)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-07-09 06:44:51 +02:00
committed by GitHub
parent 366f3d26e5
commit fe0299545a
4 changed files with 36 additions and 9 deletions
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docs/mkdocs/docs/api/basic_json/basic_json.md
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@@ -82,7 +82,13 @@ basic_json(basic_json&& other) noexcept;
4. This is a constructor for existing `basic_json` types. It does not hijack copy/move constructors, since the parameter
has different template arguments than the current ones.
The constructor tries to convert the internal `m_value` of the parameter.
The constructor tries to convert the internal `m_value` of the parameter. Each member value (object, array, string,
etc.) is serialized via the corresponding `to_json()` overload. For objects and strings, the conversion requires
that the *target* `basic_json` type's `object_t::key_type` (or `string_t`) be directly constructible from the
*source* type's corresponding member type via `is_constructible`. If this requirement is not met, the conversion
does not fail to compile; instead, it silently falls back to the array-conversion path, which represents objects
as arrays of `[key, value]` pairs and strings as arrays of character codes. This is a known limitation tracked in
[issue #3425](https://github.com/nlohmann/json/issues/3425).
5. Creates a JSON value of type array or object from the passed initializer list `init`. In case `type_deduction` is
`#!cpp true` (default), the type of the JSON value to be created is deducted from the initializer list `init`
@@ -146,6 +152,11 @@ basic_json(basic_json&& other) noexcept;
- `BasicJsonType` is a `basic_json` type.
- `BasicJsonType` has different template arguments than `basic_json_t`.
**Note:** For cross-`basic_json` conversions to produce correct results, the target `basic_json`'s
`object_t::key_type` and `string_t` must be directly constructible from the source `basic_json`'s
corresponding types. See the description of overload (4) above for details on what happens when
this requirement is not met.
`U`:
: `uncvref_t<CompatibleType>`