JSON for Modern C++ 3.10.4

◆ basic_json() [3/10]

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, class BinaryType = std::vector<std::uint8_t>>
template<typename CompatibleType , typename U = detail::uncvref_t<CompatibleType>, detail::enable_if_t< !detail::is_basic_json< U >::value &&detail::is_compatible_type< basic_json_t, U >::value, int > = 0>
nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::basic_json ( CompatibleType &&  val)
inlinenoexcept

This is a "catch all" constructor for all compatible JSON types; that is, types for which a to_json() method exists. The constructor forwards the parameter val to that method (to json_serializer<U>to_json method with U = uncvref_t<CompatibleType>, to be exact).

Template type CompatibleType includes, but is not limited to, the following types:

  • arrays: array_t and all kinds of compatible containers such as std::vector, std::deque, std::list, std::forward_list, std::array, std::valarray, std::set, std::unordered_set, std::multiset, and std::unordered_multiset with a value_type from which a basic_json value can be constructed.
  • objects: object_t and all kinds of compatible associative containers such as std::map, std::unordered_map, std::multimap, and std::unordered_multimap with a key_type compatible to string_t and a value_type from which a basic_json value can be constructed.
  • strings: string_t, string literals, and all compatible string containers can be used.
  • numbers: number_integer_t, number_unsigned_t, number_float_t, and all convertible number types such as int, size_t, int64_t, float or double can be used.
  • boolean: boolean_t / bool can be used.
  • binary: binary_t / std::vector<std::uint8_t> may be used, unfortunately because string literals cannot be distinguished from binary character arrays by the C++ type system, all types compatible with const char* will be directed to the string constructor instead. This is both for backwards compatibility, and due to the fact that a binary type is not a standard JSON type.

See the examples below.

Template Parameters
CompatibleTypea type such that:
  • CompatibleType is not derived from std::istream,
  • CompatibleType is not basic_json (to avoid hijacking copy/move constructors),
  • CompatibleType is not a different basic_json type (i.e. with different template arguments)
  • CompatibleType is not a basic_json nested type (e.g., json_pointer, iterator, etc ...)
  • json_serializer<U> has a to_json(basic_json_t&, CompatibleType&&) method
U= uncvref_t<CompatibleType>
Parameters
[in]valthe value to be forwarded to the respective constructor
Complexity
Usually linear in the size of the passed val, also depending on the implementation of the called to_json() method.
Exception safety
Depends on the called constructor. For types directly supported by the library (i.e., all types for which no to_json() function was provided), strong guarantee holds: if an exception is thrown, there are no changes to any JSON value.
Example
The following code shows the constructor with several compatible types.
1#include <iostream>
2#include <deque>
3#include <list>
4#include <forward_list>
5#include <set>
6#include <unordered_map>
7#include <unordered_set>
8#include <valarray>
9#include <nlohmann/json.hpp>
10
11using json = nlohmann::json;
12
13int main()
14{
15 // ============
16 // object types
17 // ============
18
19 // create an object from an object_t value
20 json::object_t object_value = { {"one", 1}, {"two", 2} };
21 json j_object_t(object_value);
22
23 // create an object from std::map
24 std::map<std::string, int> c_map
25 {
26 {"one", 1}, {"two", 2}, {"three", 3}
27 };
28 json j_map(c_map);
29
30 // create an object from std::unordered_map
31 std::unordered_map<const char*, double> c_umap
32 {
33 {"one", 1.2}, {"two", 2.3}, {"three", 3.4}
34 };
35 json j_umap(c_umap);
36
37 // create an object from std::multimap
38 std::multimap<std::string, bool> c_mmap
39 {
40 {"one", true}, {"two", true}, {"three", false}, {"three", true}
41 };
42 json j_mmap(c_mmap); // only one entry for key "three" is used
43
44 // create an object from std::unordered_multimap
45 std::unordered_multimap<std::string, bool> c_ummap
46 {
47 {"one", true}, {"two", true}, {"three", false}, {"three", true}
48 };
49 json j_ummap(c_ummap); // only one entry for key "three" is used
50
51 // serialize the JSON objects
52 std::cout << j_object_t << '\n';
53 std::cout << j_map << '\n';
54 std::cout << j_umap << '\n';
55 std::cout << j_mmap << '\n';
56 std::cout << j_ummap << "\n\n";
57
58
59 // ===========
60 // array types
61 // ===========
62
63 // create an array from an array_t value
64 json::array_t array_value = {"one", "two", 3, 4.5, false};
65 json j_array_t(array_value);
66
67 // create an array from std::vector
68 std::vector<int> c_vector {1, 2, 3, 4};
69 json j_vec(c_vector);
70
71 // create an array from std::valarray
72 std::valarray<short> c_valarray {10, 9, 8, 7};
73 json j_valarray(c_valarray);
74
75 // create an array from std::deque
76 std::deque<double> c_deque {1.2, 2.3, 3.4, 5.6};
77 json j_deque(c_deque);
78
79 // create an array from std::list
80 std::list<bool> c_list {true, true, false, true};
81 json j_list(c_list);
82
83 // create an array from std::forward_list
84 std::forward_list<int64_t> c_flist {12345678909876, 23456789098765, 34567890987654, 45678909876543};
85 json j_flist(c_flist);
86
87 // create an array from std::array
88 std::array<unsigned long, 4> c_array {{1, 2, 3, 4}};
89 json j_array(c_array);
90
91 // create an array from std::set
92 std::set<std::string> c_set {"one", "two", "three", "four", "one"};
93 json j_set(c_set); // only one entry for "one" is used
94
95 // create an array from std::unordered_set
96 std::unordered_set<std::string> c_uset {"one", "two", "three", "four", "one"};
97 json j_uset(c_uset); // only one entry for "one" is used
98
99 // create an array from std::multiset
100 std::multiset<std::string> c_mset {"one", "two", "one", "four"};
101 json j_mset(c_mset); // both entries for "one" are used
102
103 // create an array from std::unordered_multiset
104 std::unordered_multiset<std::string> c_umset {"one", "two", "one", "four"};
105 json j_umset(c_umset); // both entries for "one" are used
106
107 // serialize the JSON arrays
108 std::cout << j_array_t << '\n';
109 std::cout << j_vec << '\n';
110 std::cout << j_valarray << '\n';
111 std::cout << j_deque << '\n';
112 std::cout << j_list << '\n';
113 std::cout << j_flist << '\n';
114 std::cout << j_array << '\n';
115 std::cout << j_set << '\n';
116 std::cout << j_uset << '\n';
117 std::cout << j_mset << '\n';
118 std::cout << j_umset << "\n\n";
119
120
121 // ============
122 // string types
123 // ============
124
125 // create string from a string_t value
126 json::string_t string_value = "The quick brown fox jumps over the lazy dog.";
127 json j_string_t(string_value);
128
129 // create a JSON string directly from a string literal
130 json j_string_literal("The quick brown fox jumps over the lazy dog.");
131
132 // create string from std::string
133 std::string s_stdstring = "The quick brown fox jumps over the lazy dog.";
134 json j_stdstring(s_stdstring);
135
136 // serialize the JSON strings
137 std::cout << j_string_t << '\n';
138 std::cout << j_string_literal << '\n';
139 std::cout << j_stdstring << "\n\n";
140
141
142 // ============
143 // number types
144 // ============
145
146 // create a JSON number from number_integer_t
147 json::number_integer_t value_integer_t = -42;
148 json j_integer_t(value_integer_t);
149
150 // create a JSON number from number_unsigned_t
151 json::number_integer_t value_unsigned_t = 17;
152 json j_unsigned_t(value_unsigned_t);
153
154 // create a JSON number from an anonymous enum
155 enum { enum_value = 17 };
156 json j_enum(enum_value);
157
158 // create values of different integer types
159 short n_short = 42;
160 int n_int = -23;
161 long n_long = 1024;
162 int_least32_t n_int_least32_t = -17;
163 uint8_t n_uint8_t = 8;
164
165 // create (integer) JSON numbers
166 json j_short(n_short);
167 json j_int(n_int);
168 json j_long(n_long);
169 json j_int_least32_t(n_int_least32_t);
170 json j_uint8_t(n_uint8_t);
171
172 // create values of different floating-point types
173 json::number_float_t v_ok = 3.141592653589793;
174 json::number_float_t v_nan = NAN;
175 json::number_float_t v_infinity = INFINITY;
176
177 // create values of different floating-point types
178 float n_float = 42.23;
179 float n_float_nan = 1.0f / 0.0f;
180 double n_double = 23.42;
181
182 // create (floating point) JSON numbers
183 json j_ok(v_ok);
184 json j_nan(v_nan);
185 json j_infinity(v_infinity);
186 json j_float(n_float);
187 json j_float_nan(n_float_nan);
188 json j_double(n_double);
189
190 // serialize the JSON numbers
191 std::cout << j_integer_t << '\n';
192 std::cout << j_unsigned_t << '\n';
193 std::cout << j_enum << '\n';
194 std::cout << j_short << '\n';
195 std::cout << j_int << '\n';
196 std::cout << j_long << '\n';
197 std::cout << j_int_least32_t << '\n';
198 std::cout << j_uint8_t << '\n';
199 std::cout << j_ok << '\n';
200 std::cout << j_nan << '\n';
201 std::cout << j_infinity << '\n';
202 std::cout << j_float << '\n';
203 std::cout << j_float_nan << '\n';
204 std::cout << j_double << "\n\n";
205
206
207 // =============
208 // boolean types
209 // =============
210
211 // create boolean values
212 json j_truth = true;
213 json j_falsity = false;
214
215 // serialize the JSON booleans
216 std::cout << j_truth << '\n';
217 std::cout << j_falsity << '\n';
218}
NumberIntegerType number_integer_t
a type for a number (integer)
Definition: json.hpp:18212
StringType string_t
a type for a string
Definition: json.hpp:18114
NumberFloatType number_float_t
a type for a number (floating-point)
Definition: json.hpp:18351
ArrayType< basic_json, AllocatorType< basic_json > > array_t
a type for an array
Definition: json.hpp:18061
ObjectType< StringType, basic_json, object_comparator_t, AllocatorType< std::pair< const StringType, basic_json > > > object_t
a type for an object
Definition: json.hpp:18015
basic_json<> json
default JSON class
Definition: json.hpp:3472

Output (play with this example online):
{"one":1,"two":2}
{"one":1,"three":3,"two":2}
{"one":1.2,"three":3.4,"two":2.3}
{"one":true,"three":false,"two":true}
{"one":true,"three":false,"two":true}

["one","two",3,4.5,false]
[1,2,3,4]
[10,9,8,7]
[1.2,2.3,3.4,5.6]
[true,true,false,true]
[12345678909876,23456789098765,34567890987654,45678909876543]
[1,2,3,4]
["four","one","three","two"]
["four","three","two","one"]
["four","one","one","two"]
["four","two","one","one"]

"The quick brown fox jumps over the lazy dog."
"The quick brown fox jumps over the lazy dog."
"The quick brown fox jumps over the lazy dog."

-42
17
17
42
-23
1024
-17
8
3.141592653589793
null
null
42.22999954223633
null
23.42

true
false
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/basic_json__CompatibleType.cpp -o basic_json__CompatibleType 
Since
version 2.1.0

Definition at line 19071 of file json.hpp.