JSON for Modern C++  3.7.0

◆ get() [3/6]

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 ValueTypeCV , typename ValueType = detail::uncvref_t<ValueTypeCV>, detail::enable_if_t< not detail::is_basic_json< ValueType >::value and detail::has_from_json< basic_json_t, ValueType >::value and not detail::has_non_default_from_json< basic_json_t, ValueType >::value, int > = 0>
ValueType nlohmann::basic_json::get ( ) const
inlinenoexcept

Explicit type conversion between the JSON value and a compatible value which is CopyConstructible and DefaultConstructible. The value is converted by calling the json_serializer<ValueType> from_json() method.

The function is equivalent to executing

ValueType ret;
JSONSerializer<ValueType>::from_json(*this, ret);
return ret;

This overloads is chosen if:

Template Parameters
ValueTypeCVthe provided value type
ValueTypethe returned value type
Returns
copy of the JSON value, converted to ValueType
Exceptions
whatjson_serializer<ValueType> from_json() method throws
Example
The example below shows several conversions from JSON values to other types. There a few things to note: (1) Floating-point numbers can be converted to integers, (2) A JSON array can be converted to a standard std::vector<short>, (3) A JSON object can be converted to C++ associative containers such as std::unordered_map<std::string, json>.
1 #include <iostream>
2 #include <unordered_map>
3 #include <nlohmann/json.hpp>
4 
5 using json = nlohmann::json;
6 
7 int main()
8 {
9  // create a JSON value with different types
10  json json_types =
11  {
12  {"boolean", true},
13  {
14  "number", {
15  {"integer", 42},
16  {"floating-point", 17.23}
17  }
18  },
19  {"string", "Hello, world!"},
20  {"array", {1, 2, 3, 4, 5}},
21  {"null", nullptr}
22  };
23 
24  // use explicit conversions
25  auto v1 = json_types["boolean"].get<bool>();
26  auto v2 = json_types["number"]["integer"].get<int>();
27  auto v3 = json_types["number"]["integer"].get<short>();
28  auto v4 = json_types["number"]["floating-point"].get<float>();
29  auto v5 = json_types["number"]["floating-point"].get<int>();
30  auto v6 = json_types["string"].get<std::string>();
31  auto v7 = json_types["array"].get<std::vector<short>>();
32  auto v8 = json_types.get<std::unordered_map<std::string, json>>();
33 
34  // print the conversion results
35  std::cout << v1 << '\n';
36  std::cout << v2 << ' ' << v3 << '\n';
37  std::cout << v4 << ' ' << v5 << '\n';
38  std::cout << v6 << '\n';
39 
40  for (auto i : v7)
41  {
42  std::cout << i << ' ';
43  }
44  std::cout << "\n\n";
45 
46  for (auto i : v8)
47  {
48  std::cout << i.first << ": " << i.second << '\n';
49  }
50 }
basic_json<> json
default JSON class
Definition: json.hpp:2355

Output (play with this example online):
1
42 42
17.23 17
Hello, world!
1 2 3 4 5 

string: "Hello, world!"
number: {"floating-point":17.23,"integer":42}
null: null
boolean: true
array: [1,2,3,4,5]
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/get__ValueType_const.cpp -o get__ValueType_const 
Since
version 2.1.0

Definition at line 16990 of file json.hpp.