nlohmann::basic_json::unflatten¶

basic_jsonunflatten()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.109 if an array index in a key is not a number; example: "array index 'one' is not a number"

Complexity¶

Linear in the size of the JSON value.

Notes¶

Empty objects and arrays are flattened by flatten() to null values and cannot unflattened to their original type. Apart from this example, for a JSON value j, the following is always true: j==j.flatten().unflatten().

Examples¶

#include<iostream>
#include<iomanip>
#include<nlohmann/json.hpp>
usingjson=nlohmann::json;
intmain()
{
// create JSON value
jsonj_flattened=
{
{"/answer/everything",42},
{"/happy",true},
{"/list/0",1},
{"/list/1",0},
{"/list/2",2},
{"/name","Niels"},
{"/nothing",nullptr},
{"/object/currency","USD"},
{"/object/value",42.99},
{"/pi",3.141}
};
// call unflatten()
std::cout<<std::setw(4)<<j_flattened.unflatten()<<'\n';
}

{
"answer":{
"everything":42
},
"happy":true,
"list":[
1,
0,
2
],
"name":"Niels",
"nothing":null,
"object":{
"currency":"USD",
"value":42.99
},
"pi":3.141
}

See also¶

• flatten the reverse function

Version history¶

• Added in version 2.0.0.