Niels Lohmann
2cca04ae6f
* Report one-character non-numeric array indices like longer ones
A JSON pointer reference token that is not a number but has only one
character (e.g. "/a/x") was reported as out_of_range.404 ("unresolved
reference token"), because the "is not a number" check only ran for
tokens longer than one character; "/a/xy" got parse_error.109. Both now
throw parse_error.109. "-" and the empty token are still reported as
out_of_range.404. As a consequence, value(json_pointer, default) on an
array now throws for "/x" as it already did for "/xy".
Also document why ordered_map::erase's destroy/placement-new loop on
pair<const Key, T> is kept despite [basic.life]/8 before C++20.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Document the one-character array index change
Add 3.13.0 version-history entries to at, operator[], value, patch,
patch_inplace, and unflatten, and describe in exceptions.md which array
indices throw parse_error.109 and which out_of_range.404.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---------
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
87 lines
3.6 KiB
Markdown
87 lines
3.6 KiB
Markdown
# <small>nlohmann::basic_json::</small>unflatten
|
|
|
|
```cpp
|
|
basic_json unflatten() const;
|
|
```
|
|
|
|
The function restores the arbitrary nesting of a JSON value that has been flattened before using the
|
|
[`flatten()`](flatten.md) function. The JSON value must meet certain constraints:
|
|
|
|
1. The value must be an object.
|
|
2. The keys must be JSON pointers (see [RFC 6901](https://tools.ietf.org/html/rfc6901))
|
|
3. The mapped values must be primitive JSON types.
|
|
|
|
## Return value
|
|
|
|
the original JSON from a flattened version
|
|
|
|
## Exception safety
|
|
|
|
Strong exception safety: if an exception occurs, the original value stays intact.
|
|
|
|
## Exceptions
|
|
|
|
The function can throw the following exceptions:
|
|
|
|
- Throws [`type_error.314`](../../home/exceptions.md#jsonexceptiontype_error314) if value is not an object
|
|
- Throws [`type_error.315`](../../home/exceptions.md#jsonexceptiontype_error315) if object values are not primitive
|
|
- Throws [`type_error.313`](../../home/exceptions.md#jsonexceptiontype_error313) if a key (JSON pointer) leads to a
|
|
conflicting nesting; example: `"invalid value to unflatten"`
|
|
- Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in a key begins
|
|
with '0'; example: `"array index '01' must not begin with '0'"`
|
|
- Throws [`parse_error.107`](../../home/exceptions.md#jsonexceptionparse_error107) if a key is not empty and does not
|
|
begin with a slash (`/`); example: `"JSON pointer must be empty or begin with '/' - was: 'a'"`
|
|
- Throws [`parse_error.108`](../../home/exceptions.md#jsonexceptionparse_error108) if a tilde (`~`) in a key is not
|
|
followed by `0` or `1`; example: `"escape character '~' must be followed with '0' or '1'"`
|
|
- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in a key is not a
|
|
number; example: `"array index 'one' is not a number"`
|
|
- Throws [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) if a level becomes an array
|
|
(because one of its keys is `0`) and another key at that level begins with a digit but is not a valid array index
|
|
(such as `1a`), or is `-`; example:
|
|
`"unresolved reference token '-'"`
|
|
|
|
## Complexity
|
|
|
|
Linear in the size of the JSON value.
|
|
|
|
## Notes
|
|
|
|
Empty objects and arrays are flattened by [`flatten()`](flatten.md) to `#!json null` values and cannot unflattened to
|
|
their original type.
|
|
|
|
A flattened array and a flattened object whose keys are array indices are indistinguishable, because both are
|
|
described by the same JSON pointers. A value is therefore restored as an array if and only if one of its keys is the
|
|
reference token `0`, and as an object otherwise: `#!json {"2": 1}` is restored unchanged, whereas `#!json {"0": 1}` is
|
|
restored as `#!json [1]`. This decision does not depend on the order in which the flattened object is iterated.
|
|
|
|
Apart from these two cases, for a JSON value `j`, the following is always true:
|
|
`#!cpp j == j.flatten().unflatten()`.
|
|
|
|
## Examples
|
|
|
|
??? example
|
|
|
|
The following code shows how a flattened JSON object is unflattened into the original nested JSON object.
|
|
|
|
```cpp
|
|
--8<-- "examples/unflatten.cpp"
|
|
```
|
|
|
|
Output:
|
|
|
|
```json
|
|
--8<-- "examples/unflatten.output"
|
|
```
|
|
|
|
## See also
|
|
|
|
- [flatten](flatten.md) the reverse function
|
|
|
|
## Version history
|
|
|
|
- Added in version 2.0.0.
|
|
- Made the array/object decision independent of the object's iteration order in version 3.13.0.
|
|
- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) instead of
|
|
[`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) for a one-character array index that is
|
|
not a digit (e.g., `/x`) in version 3.13.0, as it already did for longer ones.
|