Niels LohmannandGitHub
e9c3985f0a
* 📝 Fix documentation gaps found in a full GitHub Discussions review Reviewed all 1008 GitHub Discussions (2020-2026) for recurring questions that better or more visible documentation would have avoided. Adds/expands documentation for ~26 distinct gaps, including: - New "Debugging" page collecting natvis, GDB pretty printer, LLDB status, and JSON_DIAGNOSTICS pointers (previously scattered/undiscoverable) - Thread-safety and schema-validation FAQ entries - StringType's char-based requirement (no wstring/u16string/u32string) - Brace-initialization-yields-arrays warning directly on the constructor reference page (previously only in the FAQ, missed by users reading the constructor docs) - std::any exclusion from get<T>(), with a manual-dispatch example - Non-string-keyed std::map serializing as an array of pairs - ordered_json compatibility with NLOHMANN_DEFINE_TYPE_* macros (already worked, was undocumented) - std::array truncation on size-mismatched conversion (no exception) - static_cast vs. get<std::optional<T>>() divergence - Recipe for omitting a std::optional field instead of emitting null - No built-in nesting-depth limit during parsing + a callback-based workaround recipe - Recipe for streaming a large homogeneous array via parser callbacks - operator>> stream-position semantics for concatenated JSON values - JSON Pointer array-vs-object creation rule for non-existing paths - CMake target name (nlohmann_json_modules) needed to link C++20 modules - ESP-IDF/PlatformIO: no official package, link to a community fork - get(key, default) as the Python dict.get() equivalent - reserve() recipe for pre-allocating array capacity - JSONC as an alias for the existing ignore_comments/ignore_trailing_commas combination (distinct from the unsupported JSON5) - items() dereferenced-element type: decltype() idiom + detail-namespace stability caveat - Various macro/type-conversion limitations (MSGPACK_DEFINE_ARRAY equivalent, char-array round-tripping, ADL serializer macro gap) Signed-off-by: Niels Lohmann <mail@nlohmann.me> * 🎨 fix format Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Comments
This library does not support comments by default. It does so for three reasons:
-
Comments are not part of the JSON specification. You may argue that
//or/* */are allowed in JavaScript, but JSON is not JavaScript. -
This was not an oversight: Douglas Crockford wrote on this in May 2012:
I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.
Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.
-
It is dangerous for interoperability if some libraries add comment support while others do not. Please check The Harmful Consequences of the Robustness Principle on this.
However, you can set parameter ignore_comments to #!cpp true in the parse function to ignore // or /* */ comments. Comments will then be treated as whitespace. Combined with ignore_trailing_commas (also a parse parameter), this covers what is commonly referred to as JSONC (JSON with Comments, as used e.g. by Visual Studio Code's .jsonc files) -- comments and trailing commas, nothing more. This is a different, smaller extension than JSON5, which additionally allows unquoted keys, single-quoted strings, and other syntax changes that this library does not support.
For more information, see JSON With Commas and Comments (JWCC).
!!! example
Consider the following JSON with comments.
```json
{
// update in 2006: removed Pluto
"planets": ["Mercury", "Venus", "Earth", "Mars",
"Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
}
```
When calling `parse` without additional argument, a parse error exception is thrown. If `ignore_comments` is set to `#!cpp true`, the comments are ignored during parsing:
```cpp
--8<-- "examples/comments.cpp"
```
Output:
```
--8<-- "examples/comments.output"
```