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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.
  2. 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.

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

Parameters

j (in)
JSON object to read values from
merge_objects (in)
when true, existing keys are not overwritten, but contents of objects are merged recursively (default: false)
first (in)
begin of the range of elements to insert
last (in)
end of the range of elements to insert

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.202 if called on an iterator which does not belong to the current JSON value; example: "iterator does not fit current value"
    • 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.

Examples

Example

The example shows how update() is used.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

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;

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 usecase for this function is the handling of user settings. Assume your application can 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"}
}

Version history

  • Added in version 3.0.0.
  • Added merge_objects parameter in 3.10.4.

Last update: May 1, 2022
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