json/docs/mkdocs/docs/api/basic_json/unflatten.md

nlohmann::basic_json::unflatten

basic_json unflatten() const;

The function restores the arbitrary nesting of a JSON value that has been flattened before using the flatten() 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)
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 if value is not an object
• Throws type_error.315 if object values are not primitive
• Throws type_error.313 if a key (JSON pointer) leads to a conflicting nesting; example: "invalid value to unflatten"
• Throws parse_error.106 if an array index in a key begins with '0'; example: "array index '01' must not begin with '0'"
• Throws parse_error.107 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 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 if an array index in a key is not a number; example: "array index 'one' is not a number"
• Throws out_of_range.404 if a level becomes an array (because one of its keys is 0) and another key at that level is empty (as in /a/), is -, or begins with a digit but is not a valid array index (such as 1a); examples: "unresolved reference token '-'", "unresolved reference token ''"

Complexity

Linear in the size of the JSON value.

Notes

Empty objects and arrays are flattened by flatten() to #!json null values and cannot be 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.

A third case is an object that has the key 0 next to a key that is not a valid array index: #!json {"0": 1, "b": 2} is flattened to #!json {"/0": 1, "/b": 2}, and unflatten treats the level as an array and throws parse_error.109 because b is not a number. (For the keys -, 1a, or the empty key instead of b, it throws out_of_range.404.)

Apart from these three 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 - the reverse function
• JSON Pointer - the article on JSON Pointer and flattening

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 instead of out_of_range.404 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.