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# nlohmann::basic_json::update
```
// (1)
void update(const_reference j, bool merge_objects = false);
// (2)
void update(const_iterator first, const_iterator last, bool merge_objects = false);
```
1. Inserts all values from JSON object `j`.
1. Inserts all values from range `[first, last)`
When `merge_objects` is `false` (default), existing keys are overwritten. When `merge_objects` is `true`, recursively merges objects with common keys.
If the JSON value is `null`, it is implicitly converted to an empty object before the values are inserted.
The function is motivated by Python's [dict.update](https://docs.python.org/3.6/library/stdtypes.html#dict.update) function.
## Iterator invalidation
For [`ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md), adding a value to an object can yield a reallocation, in which case all iterators (including the `end()` iterator) and all references to the elements are invalidated.
## Parameters
`j` (in) : JSON object to read values from
`merge_objects` (in) : when `true`, keys that exist in both objects and whose value in the source is itself an object are merged recursively; all other values are overwritten as usual (default: `false`)
`first` (in) : the beginning of the range of elements to insert
`last` (in) : the end of the range of elements to insert
## Exception safety
Basic guarantee: if an exception is thrown during the operation, the JSON value may be partially modified.
## Exceptions
1. The function can throw the following exceptions:
- Throws [`type_error.312`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error312) if called on JSON values other than objects; example: `"cannot use update() with string"`
1. The function can throw the following exceptions:
- Throws [`type_error.312`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error312) if called on JSON values other than objects; example: `"cannot use update() with string"`
- Throws [`invalid_iterator.210`](https://json.nlohmann.me/home/exceptions/#jsonexceptioninvalid_iterator210) if `first` and `last` do not belong to the same JSON value; example: `"iterators do not fit"`
## Complexity
1. O(N\*log(size() + N)), where N is the number of elements to insert.
1. O(N\*log(size() + N)), where N is the number of elements to insert.
## Examples
Example
The example shows how `update()` is used.
```
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
using namespace nlohmann::literals;
int main()
{
// create two JSON objects
json o1 = R"( {"color": "red", "price": 17.99, "names": {"de": "Flugzeug"}} )"_json;
json o2 = R"( {"color": "blue", "speed": 100, "names": {"en": "plane"}} )"_json;
json o3 = o1;
// add all keys from o2 to o1 (updating "color", replacing "names")
o1.update(o2);
// add all keys from o2 to o1 (updating "color", merging "names")
o3.update(o2, true);
// output updated object o1 and o3
std::cout << std::setw(2) << o1 << '\n';
std::cout << std::setw(2) << o3 << '\n';
}
```
Output:
```
{
"color": "blue",
"names": {
"en": "plane"
},
"price": 17.99,
"speed": 100
}
{
"color": "blue",
"names": {
"de": "Flugzeug",
"en": "plane"
},
"price": 17.99,
"speed": 100
}
```
Example
The example shows how `update()` is used.
```
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
using namespace nlohmann::literals;
int main()
{
// create two JSON objects
json o1 = R"( {"color": "red", "price": 17.99, "names": {"de": "Flugzeug"}} )"_json;
json o2 = R"( {"color": "blue", "speed": 100, "names": {"en": "plane"}} )"_json;
json o3 = o1;
// add all keys from o2 to o1 (updating "color", replacing "names")
o1.update(o2.begin(), o2.end());
// add all keys from o2 to o1 (updating "color", merging "names")
o3.update(o2.begin(), o2.end(), true);
// output updated object o1 and o3
std::cout << std::setw(2) << o1 << '\n';
std::cout << std::setw(2) << o3 << '\n';
}
```
Output:
```
{
"color": "blue",
"names": {
"en": "plane"
},
"price": 17.99,
"speed": 100
}
{
"color": "blue",
"names": {
"de": "Flugzeug",
"en": "plane"
},
"price": 17.99,
"speed": 100
}
```
Example
One common use case for this function is the handling of user settings. Assume your application can be configured in some aspects:
```
{
"color": "red",
"active": true,
"name": {"de": "Maus", "en": "mouse"}
}
```
The user may override the default settings selectively:
```
{
"color": "blue",
"name": {"es": "ratón"},
}
```
Then `update` manages the merging of default settings and user settings:
```
auto user_settings = json::parse("config.json");
auto effective_settings = get_default_settings();
effective_settings.update(user_settings);
```
Now `effective_settings` contains the default settings, but those keys set by the user are overwritten:
```
{
"color": "blue",
"active": true,
"name": {"es": "ratón"}
}
```
Note existing keys were just overwritten. To merge objects, `merge_objects` setting should be set to `true`:
```
auto user_settings = json::parse("config.json");
auto effective_settings = get_default_settings();
effective_settings.update(user_settings, true);
```
```
{
"color": "blue",
"active": true,
"name": {"de": "Maus", "en": "mouse", "es": "ratón"}
}
```
## See also
- [insert](https://json.nlohmann.me/api/basic_json/insert/index.md) add values to an array/object
- [merge_patch](https://json.nlohmann.me/api/basic_json/merge_patch/index.md) applies a JSON Merge Patch
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - the article on modifying values
## Version history
- Added in version 3.0.0.
- Added `merge_objects` parameter in 3.10.5.