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Modifying values

Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page gives an overview of the available operations. For read access, see element access.

Adding to arrays

New elements are appended to an array with push_back or constructed in place with emplace_back. If the value is null, it is converted to an array first, so these functions can also be used to build an array from scratch.

json j;                 // null
j.push_back(1);         // [1]
j.push_back(2);         // [1,2]
j.emplace_back(3);      // [1,2,3]

// operator+= is a shorthand for push_back
j += 4;                 // [1,2,3,4]

Adding to objects

The most common way to add or replace a member is operator[], which inserts the key if it does not exist yet:

json j;
j["name"] = "Mary";     // {"name":"Mary"}
j["name"] = "John";     // {"name":"John"}  (replaced)

emplace inserts a member only if the key is not already present, and reports whether the insertion happened — useful for "add if absent" semantics.

Merging objects

To merge one object into another, update copies all members from another object, overwriting existing keys (similar to Python's dict.update). This is the idiomatic way to combine two objects.

Example 
#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
}

For a recursive merge that follows RFC 7386, see JSON Merge Patch. To apply a sequence of well-defined edit operations, see JSON Patch.

Removing elements

Elements are removed with erase, which accepts an object key, an array index, or an iterator. clear empties a value while keeping its type, and operator[] combined with assignment can overwrite a value entirely.

json j = {{"a", 1}, {"b", 2}, {"c", 3}};
j.erase("b");           // {"a":1,"c":3}

json a = {1, 2, 3, 4};
a.erase(1);             // [1,3,4]  (erase by index)

See also