JSON for Modern C++  3.7.0

◆ operator[]() [6/8]

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 >
const_reference nlohmann::basic_json::operator[] ( T *  key) const
inline

Returns a const reference to the element at with specified key key. No bounds checking is performed.

Warning
If the element with key key does not exist, the behavior is undefined.
Parameters
[in]keykey of the element to access
Returns
const reference to the element at key key
Precondition
The element with key key must exist. This precondition is enforced with an assertion.
Exceptions
type_error.305if the JSON value is not an object; in that case, 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 using the [] operator.
1 #include <iostream>
2 #include <nlohmann/json.hpp>
3 
4 using json = nlohmann::json;
5 
6 int main()
7 {
8  // create a JSON object
9  const json object =
10  {
11  {"one", 1}, {"two", 2}, {"three", 2.9}
12  };
13 
14  // output element with key "two"
15  std::cout << object["two"] << '\n';
16 }
basic_json<> json
default JSON class
Definition: json.hpp:2355

Output (play with this example online):
2
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/operatorarray__key_type_const.cpp -o operatorarray__key_type_const 
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 17765 of file json.hpp.