JSON for Modern C++  3.7.0

◆ operator[]() [5/8]

template<template< typename U, typename V, typename... Args > class ObjectType = std::map, template< typename U, typename... Args > class ArrayType = std::vector, class StringType = std::string, class BooleanType = bool, class NumberIntegerType = std::int64_t, class NumberUnsignedType = std::uint64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator, template< typename T, typename SFINAE=void > class JSONSerializer = adl_serializer>
template<typename T >
reference nlohmann::basic_json::operator[] ( T *  key)
inline

Returns a reference to the element at with specified key key.

Note
If key is not found in the object, then it is silently added to the object and filled with a null value to make key a valid reference. In case the value was null before, it is converted to an object.
Parameters
[in]keykey of the element to access
Returns
reference to the element at key key
Exceptions
type_error.305if the JSON value is not an object or null; in that cases, using the [] operator with a key makes no sense.
Complexity
Logarithmic in the size of the container.
Example
The example below shows how object elements can be read and written using the [] operator.
1 #include <iostream>
2 #include <iomanip>
3 #include <nlohmann/json.hpp>
4 
5 using json = nlohmann::json;
6 
7 int main()
8 {
9  // create a JSON object
10  json object =
11  {
12  {"one", 1}, {"two", 2}, {"three", 2.9}
13  };
14 
15  // output element with key "two"
16  std::cout << object["two"] << "\n\n";
17 
18  // change element with key "three"
19  object["three"] = 3;
20 
21  // output changed array
22  std::cout << std::setw(4) << object << "\n\n";
23 
24  // mention nonexisting key
25  object["four"];
26 
27  // write to nonexisting key
28  object["five"]["really"]["nested"] = true;
29 
30  // output changed object
31  std::cout << std::setw(4) << object << '\n';
32 }
basic_json<> json
default JSON class
Definition: json.hpp:2355

Output (play with this example online):
2

{
    "one": 1,
    "three": 3,
    "two": 2
}

{
    "five": {
        "really": {
            "nested": true
        }
    },
    "four": null,
    "one": 1,
    "three": 3,
    "two": 2
}
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/operatorarray__key_type.cpp -o operatorarray__key_type 
See also
at(const typename object_t::key_type&) for access by reference with range checking
value() for access by value with a default value
Since
version 1.1.0

Definition at line 17714 of file json.hpp.