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

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.
2. Inserts all values from range [first, last)

When merge_objects is #!c false (default), existing keys are overwritten. When merge_objects is #!c true, recursively merges objects with common keys.

If the JSON value is #!json null, it is implicitly converted to an empty object before the values are inserted.

The function is motivated by Python's dict.update function.

Iterator invalidation

For ordered_json, 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 #!c 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: #!c 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 if called on JSON values other than objects; example: "cannot use update() with string"
2. The function can throw the following exceptions:
   • Throws type_error.312 if called on JSON values other than objects; example: "cannot use update() with string"
   • Throws invalid_iterator.210 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.
2. O(N*log(size() + N)), where N is the number of elements to insert.

Notes

The argument j (or, for overload (2), the range [first, last)) may be #!cpp *this itself or refer to a value contained in #!cpp *this (for example, a subobject returned by #!cpp (*this)[key]); it is read as it was when update() was called, before any modification of #!cpp *this.

Examples

??? example "Example: (1) update with another object"

The example shows how `update()` is used.

```cpp
--8<-- "examples/update.cpp"
```

Output:

```json
--8<-- "examples/update.output"
```

??? example "Example: (2) update with an iterator range"

The example shows how `update()` is used.

```cpp
--8<-- "examples/update__range.cpp"
```

Output:

```json
--8<-- "examples/update__range.output"
```

??? example "Example: (1) merge user settings into default settings"

One common use case for this function is the handling of user settings. Assume your application can be configured in
some aspects:

```json
{
    "color": "red",
    "active": true,
    "name": {"de": "Maus", "en": "mouse"}
}
```

The user may override the default settings selectively:

```json
{
    "color": "blue",
    "name": {"es": "ratón"},
}
```

Then `update` manages the merging of default settings and user settings:

```cpp
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:

```json
{
    "color": "blue",
    "active": true,
    "name": {"es": "ratón"}
}
```

Note existing keys were just overwritten. To merge objects, `merge_objects` setting should be set to `#!c true`:

```cpp
auto user_settings = json::parse("config.json");
auto effective_settings = get_default_settings();
effective_settings.update(user_settings, true);
```

```json
{
    "color": "blue",
    "active": true,
    "name": {"de": "Maus", "en": "mouse", "es": "ratón"}
}
```

See also

• insert add values to an array/object
• merge_patch applies a JSON Merge Patch
• Modifying values - the article on modifying values

Version history

• Added in version 3.0.0.
• Added merge_objects parameter in 3.10.5.
• Fixed use of freed or relocated memory when the argument is #!cpp *this or refers to a value contained in #!cpp *this, in version 3.13.0.