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# <small>nlohmann::basic_json::</small>basic_json
```cpp
// (1)
basic_json(const value_t v);
// (2)
basic_json(std::nullptr_t = nullptr) noexcept;
// (3)
template<typename CompatibleType>
basic_json(CompatibleType&& val) noexcept(noexcept(
JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),
std::forward<CompatibleType>(val))));
// (4)
template<typename BasicJsonType>
basic_json(const BasicJsonType& val);
// (5)
basic_json(initializer_list_t init,
bool type_deduction = true,
value_t manual_type = value_t::array);
// (6)
basic_json(size_type cnt, const basic_json& val);
// (7)
basic_json(iterator first, iterator last);
basic_json(const_iterator first, const_iterator last);
// (8)
basic_json(const basic_json& other);
// (9)
basic_json(basic_json&& other) noexcept;
```
1. Create an empty JSON value with a given type. The value will be default initialized with an empty value which depends
on the type:
| Value type | initial value |
|------------|----------------|
| null | `#!json null` |
| boolean | `#!json false` |
| string | `#!json ""` |
| number | `#!json 0` |
| object | `#!json {}` |
| array | `#!json []` |
| binary | empty array |
The postcondition of this constructor can be restored by calling [`clear()`](clear.md).
2. Create a `#!json null` JSON value. It either takes a null pointer as parameter (explicitly creating `#!json null`)
or no parameter (implicitly creating `#!json null`). The passed null pointer itself is not read -- it is only used to
choose the right constructor.
3. 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`](array_t.md) 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`](object_t.md) 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_integer_t.md), [`number_unsigned_t`](number_unsigned_t.md),
[`number_float_t`](number_float_t.md), 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<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.
4. This is a constructor for existing `basic_json` types. It does not hijack copy/move constructors, since the parameter
has different template arguments than the current ones.
The constructor tries to convert the internal `m_value` of the parameter. Each member value (object, array, string,
etc.) is serialized via the corresponding `to_json()` overload. For objects and strings, the conversion requires
that the *target* `basic_json` type's `object_t::key_type` (or `string_t`) be directly constructible from the
*source* type's corresponding member type via `is_constructible`. If this requirement is not met, the conversion
does not fail to compile; instead, it silently falls back to the array-conversion path, which represents objects
as arrays of `[key, value]` pairs and strings as arrays of character codes. This is a known limitation tracked in
[issue #3425](https://github.com/nlohmann/json/issues/3425).
5. Creates a JSON value of type array or object from the passed initializer list `init`. In case `type_deduction` is
`#!cpp true` (default), the type of the JSON value to be created is deducted from the initializer list `init`
according to the following rules:
1. If the list is empty, an empty JSON object value `{}` is created.
2. If the list consists of pairs whose first element is a string, a JSON object value is created where the first
elements of the pairs are treated as keys and the second elements are as values.
3. In all other cases, an array is created.
The following flowchart also takes into account what happens when `type_deduction` is `#!cpp false`, in which case
`manual_type` decides between object and array, and an object can only be forced if `init` actually matches rule 2
(or is empty):
```mermaid
flowchart TD
A(["initializer_list init"]) --> B{"empty, or every element is a 2-element<br/>array whose first element is a string?"}
B -->|"yes"| C{"type_deduction"}
B -->|"no"| D{"type_deduction"}
C -->|"true"| OBJ["create object"]
C -->|"false"| E{"manual_type"}
E -->|"object"| OBJ
E -->|"array"| ARR["create array"]
D -->|"true"| ARR
D -->|"false"| F{"manual_type"}
F -->|"array"| ARR
F -->|"object"| ERR["throw type_error.301"]
```
The rules aim to create the best fit between a C++ initializer list and JSON values. The rationale is as follows:
1. The empty initializer list is written as `#!cpp {}` which is exactly an empty JSON object.
2. C++ has no way of describing mapped types other than to list a list of pairs. As JSON requires that keys must be
of type string, rule 2 is the weakest constraint one can pose on initializer lists to interpret them as an
object.
3. In all other cases, the initializer list could not be interpreted as a JSON object type, so interpreting it as a
JSON array type is safe.
With the rules described above, the following JSON values cannot be expressed by an initializer list:
- the empty array (`#!json []`): use `array(initializer_list_t)` with an empty initializer list in this case
- arrays whose elements satisfy rule 2: use `array(initializer_list_t)` with the same initializer list in this case
Function [`array()`](array.md) and [`object()`](object.md) force array and object creation from initializer lists,
respectively.
!!! warning "Brace initialization yields arrays"
Because this constructor takes an `initializer_list_t`, brace-initializing a `json`/`ordered_json` from
another `json` value wraps it in a single-element array rather than copying it:
```cpp
json j1 = "hello";
json j2{j1}; // [!] j2 is ["hello"], NOT a copy of j1
json j3(j1); // j3 is "hello" -- parentheses copy as expected
```
See the FAQ entry on [brace initialization](../../home/faq.md#brace-initialization-yields-arrays) for the
full explanation, an opt-in macro to change this behavior, and how to explicitly create a single-element
array (`json::array({value})`) if that is what you want.
6. Constructs a JSON array value by creating `cnt` copies of a passed value. In case `cnt` is `0`, an empty array is
created.
7. Constructs the JSON value with the contents of the range `[first, last)`. The semantics depend on the different
types a JSON value can have:
- In case of a `#!json null` type, [invalid_iterator.206](../../home/exceptions.md#jsonexceptioninvalid_iterator206)
is thrown.
- In case of other primitive types (number, boolean, string, or binary), `first` must be `begin()` and `last`
must be `end()`. In this case, the value is copied. Otherwise,
[`invalid_iterator.204`](../../home/exceptions.md#jsonexceptioninvalid_iterator204) is thrown.
- In case of structured types (array, object), the constructor behaves as similar versions for `std::vector` or
`std::map`; that is, a JSON array or object is constructed from the values in the range.
8. Creates a copy of a given JSON value.
9. Move constructor. Constructs a JSON value with the contents of the given value `other` using move semantics. It
"steals" the resources from `other` and leaves it as JSON `#!json null` value.
## Template parameters
`CompatibleType`
: a 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.)
- if [`JSON_DISABLE_TUPLE_REFERENCE_CONVERSION`](../macros/json_disable_tuple_reference_conversion.md) is defined
to `1`: `CompatibleType` is not a one-element `std::tuple` holding a reference to `basic_json`
- `json_serializer<U>` (with `U = uncvref_t<CompatibleType>`) has a `to_json(basic_json_t&, CompatibleType&&)`
method
`BasicJsonType`:
: a type such that:
- `BasicJsonType` is a `basic_json` type.
- `BasicJsonType` has different template arguments than `basic_json_t`.
**Note:** For cross-`basic_json` conversions to produce correct results, the target `basic_json`'s
[`object_t`](object_t.md)`::key_type` and [`string_t`](string_t.md) must be directly constructible from the source
`basic_json`'s corresponding types. See the description of overload (4) above for details on what happens when
this requirement is not met.
`U`:
: `uncvref_t<CompatibleType>`
## Parameters
`v` (in)
: the type of the value to create
`val` (in)
: the value to be forwarded to the respective constructor
`init` (in)
: initializer list with JSON values
`type_deduction` (in)
: internal parameter; when set to `#!cpp true`, the type of the JSON value is deducted from the initializer list
`init`; when set to `#!cpp false`, the type provided via `manual_type` is forced. This mode is used by the functions
`array(initializer_list_t)` and `object(initializer_list_t)`.
`manual_type` (in)
: internal parameter; when `type_deduction` is set to `#!cpp false`, the created JSON value will use the provided type
(only `value_t::array` and `value_t::object` are valid); when `type_deduction` is set to `#!cpp true`, this
parameter has no effect
`cnt` (in)
: the number of JSON copies of `val` to create
`first` (in)
: the beginning of the range to copy from (included)
`last` (in)
: the end of the range to copy from (excluded)
`other` (in)
: the JSON value to copy/move
## Exception safety
1. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
2. No-throw guarantee: this constructor never throws exceptions.
3. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no
`to_json()` function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any
JSON value.
4. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no
`to_json()` function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any
JSON value.
5. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
6. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
7. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
8. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
9. No-throw guarantee: this constructor never throws exceptions.
## Exceptions
1. (none)
2. The function does not throw exceptions.
3. (none)
4. (none)
5. The function can throw the following exceptions:
- Throws [`type_error.301`](../../home/exceptions.md#jsonexceptiontype_error301) if `type_deduction` is
`#!cpp false`, `manual_type` is `value_t::object`, but `init` contains an element which is not a pair whose first
element is a string. In this case, the constructor could not create an object. If `type_deduction` would have been
`#!cpp true`, an array would have been created. See `object(initializer_list_t)` for an example.
6. (none)
7. The function can throw the following exceptions:
- Throws [`invalid_iterator.201`](../../home/exceptions.md#jsonexceptioninvalid_iterator201) if iterators `first`
and `last` are not compatible (i.e., do not belong to the same JSON value). In this case, the range
`[first, last)` is undefined.
- Throws [`invalid_iterator.204`](../../home/exceptions.md#jsonexceptioninvalid_iterator204) if iterators `first`
and `last` belong to a primitive type (number, boolean, string, or binary), but `first` does not point to the
first element anymore. In this case, the range `[first, last)` is undefined. See the example code below.
- Throws [`invalid_iterator.206`](../../home/exceptions.md#jsonexceptioninvalid_iterator206) if iterators `first`
and `last` belong to a `#!json null` value. In this case, the range `[first, last)` is undefined.
8. (none)
9. The function does not throw exceptions.
## Complexity
1. Constant.
2. Constant.
3. Usually linear in the size of the passed `val`, also depending on the implementation of the called `to_json()`
method.
4. Usually linear in the size of the passed `val`, also depending on the implementation of the called `to_json()`
method.
5. Linear in the size of the initializer list `init`.
6. Linear in `cnt`.
7. Linear in distance between `first` and `last`.
8. Linear in the size of `other`.
9. Constant.
## Notes
- Overload 5:
!!! note "Empty initializer list"
When used without parentheses around an empty initializer list, `basic_json()` is called instead of this
function, yielding the JSON `#!json null` value.
- Overload 4:
!!! info "Implicit conversion"
The conversion is implicit unless [`JSON_USE_IMPLICIT_CONVERSIONS`](../macros/json_use_implicit_conversions.md)
is defined to `0` and `BasicJsonType::string_t` differs from `string_t`. In that case, the constructor is
`explicit`, so a JSON value with a different string type is no longer silently converted, for example when it is
passed to a function taking `#!cpp const json&`. Write `#!cpp json(other)` or `#!cpp other.get<json>()` instead.
- Overload 7:
!!! info "Preconditions"
- Iterators `first` and `last` must be initialized. **This precondition is enforced with a
[runtime assertion](../../features/assertions.md).
- Range `[first, last)` is valid. Usually, this precondition cannot be checked efficiently. Only certain edge
cases are detected; see the description of the exceptions above. A violation of this precondition yields
undefined behavior.
!!! danger "Runtime assertion"
A precondition is enforced with a [runtime assertion](../../features/assertions.md).
- Overload 8:
!!! info "Postcondition"
`#!cpp *this == other`
- Overload 9:
!!! info "Postconditions"
- `#!cpp `*this` has the same value as `other` before the call.
- `other` is a JSON `#!json null` value
## Examples
??? example "Example: (1) create an empty value with a given type"
The following code shows the constructor for different `value_t` values.
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create the different JSON values with default values
json j_null(json::value_t::null);
json j_boolean(json::value_t::boolean);
json j_number_integer(json::value_t::number_integer);
json j_number_float(json::value_t::number_float);
json j_object(json::value_t::object);
json j_array(json::value_t::array);
json j_string(json::value_t::string);
// serialize the JSON values
std::cout << j_null << '\n';
std::cout << j_boolean << '\n';
std::cout << j_number_integer << '\n';
std::cout << j_number_float << '\n';
std::cout << j_object << '\n';
std::cout << j_array << '\n';
std::cout << j_string << '\n';
}
```
Output:
```json
null
false
0
0.0
{}
[]
""
```
??? example "Example: (2) create a `#!json null` object"
The following code shows the constructor with and without a null pointer parameter.
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// implicitly create a JSON null value
json j1;
// explicitly create a JSON null value
json j2(nullptr);
// serialize the JSON null value
std::cout << j1 << '\n' << j2 << '\n';
}
```
Output:
```json
null
null
```
??? example "Example: (3) create a JSON value from compatible types"
The following code shows the constructor with several compatible types.
```cpp
#include <iostream>
#include <deque>
#include <list>
#include <forward_list>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <valarray>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// ============
// object types
// ============
// create an object from an object_t value
json::object_t object_value = { {"one", 1}, {"two", 2} };
json j_object_t(object_value);
// create an object from std::map
std::map<std::string, int> c_map
{
{"one", 1}, {"two", 2}, {"three", 3}
};
json j_map(c_map);
// create an object from std::unordered_map
std::unordered_map<const char*, double> c_umap
{
{"one", 1.2}, {"two", 2.3}, {"three", 3.4}
};
json j_umap(c_umap);
// create an object from std::multimap
std::multimap<std::string, bool> c_mmap
{
{"one", true}, {"two", true}, {"three", false}, {"three", true}
};
json j_mmap(c_mmap); // only one entry for key "three" is used
// create an object from std::unordered_multimap
std::unordered_multimap<std::string, bool> c_ummap
{
{"one", true}, {"two", true}, {"three", false}, {"three", false}
};
json j_ummap(c_ummap); // only one entry for key "three" is used
// serialize the JSON objects
std::cout << j_object_t << '\n';
std::cout << j_map << '\n';
std::cout << j_umap << '\n';
std::cout << j_mmap << '\n';
std::cout << j_ummap << "\n\n";
// ===========
// array types
// ===========
// create an array from an array_t value
json::array_t array_value = {"one", "two", 3, 4.5, false};
json j_array_t(array_value);
// create an array from std::vector
std::vector<int> c_vector {1, 2, 3, 4};
json j_vec(c_vector);
// create an array from std::valarray
std::valarray<short> c_valarray {10, 9, 8, 7};
json j_valarray(c_valarray);
// create an array from std::deque
std::deque<double> c_deque {1.2, 2.3, 3.4, 5.6};
json j_deque(c_deque);
// create an array from std::list
std::list<bool> c_list {true, true, false, true};
json j_list(c_list);
// create an array from std::forward_list
std::forward_list<std::int64_t> c_flist {12345678909876, 23456789098765, 34567890987654, 45678909876543};
json j_flist(c_flist);
// create an array from std::array
std::array<unsigned long, 4> c_array {{1, 2, 3, 4}};
json j_array(c_array);
// create an array from std::set
std::set<std::string> c_set {"one", "two", "three", "four", "one"};
json j_set(c_set); // only one entry for "one" is used
// create an array from std::unordered_set
std::unordered_set<std::string> c_uset {"one", "one"};
json j_uset(c_uset); // only one entry for "one" is used
// create an array from std::multiset
std::multiset<std::string> c_mset {"one", "two", "one", "four"};
json j_mset(c_mset); // both entries for "one" are used
// create an array from std::unordered_multiset
std::unordered_multiset<std::string> c_umset {"one", "one"};
json j_umset(c_umset); // both entries for "one" are used
// serialize the JSON arrays
std::cout << j_array_t << '\n';
std::cout << j_vec << '\n';
std::cout << j_valarray << '\n';
std::cout << j_deque << '\n';
std::cout << j_list << '\n';
std::cout << j_flist << '\n';
std::cout << j_array << '\n';
std::cout << j_set << '\n';
std::cout << j_uset << '\n';
std::cout << j_mset << '\n';
std::cout << j_umset << "\n\n";
// ============
// string types
// ============
// create string from a string_t value
json::string_t string_value = "The quick brown fox jumps over the lazy dog.";
json j_string_t(string_value);
// create a JSON string directly from a string literal
json j_string_literal("The quick brown fox jumps over the lazy dog.");
// create string from std::string
std::string s_stdstring = "The quick brown fox jumps over the lazy dog.";
json j_stdstring(s_stdstring);
// serialize the JSON strings
std::cout << j_string_t << '\n';
std::cout << j_string_literal << '\n';
std::cout << j_stdstring << "\n\n";
// ============
// number types
// ============
// create a JSON number from number_integer_t
json::number_integer_t value_integer_t = -42;
json j_integer_t(value_integer_t);
// create a JSON number from number_unsigned_t
json::number_integer_t value_unsigned_t = 17;
json j_unsigned_t(value_unsigned_t);
// create a JSON number from an anonymous enum
enum { enum_value = 17 };
json j_enum(enum_value);
// create values of different integer types
short n_short = 42;
int n_int = -23;
long n_long = 1024;
int_least32_t n_int_least32_t = -17;
uint8_t n_uint8_t = 8;
// create (integer) JSON numbers
json j_short(n_short);
json j_int(n_int);
json j_long(n_long);
json j_int_least32_t(n_int_least32_t);
json j_uint8_t(n_uint8_t);
// create values of different floating-point types
json::number_float_t v_ok = 3.141592653589793;
json::number_float_t v_nan = NAN;
json::number_float_t v_infinity = INFINITY;
// create values of different floating-point types
float n_float = 42.23;
float n_float_nan = 1.0f / 0.0f;
double n_double = 23.42;
// create (floating point) JSON numbers
json j_ok(v_ok);
json j_nan(v_nan);
json j_infinity(v_infinity);
json j_float(n_float);
json j_float_nan(n_float_nan);
json j_double(n_double);
// serialize the JSON numbers
std::cout << j_integer_t << '\n';
std::cout << j_unsigned_t << '\n';
std::cout << j_enum << '\n';
std::cout << j_short << '\n';
std::cout << j_int << '\n';
std::cout << j_long << '\n';
std::cout << j_int_least32_t << '\n';
std::cout << j_uint8_t << '\n';
std::cout << j_ok << '\n';
std::cout << j_nan << '\n';
std::cout << j_infinity << '\n';
std::cout << j_float << '\n';
std::cout << j_float_nan << '\n';
std::cout << j_double << "\n\n";
// =============
// boolean types
// =============
// create boolean values
json j_truth = true;
json j_falsity = false;
// serialize the JSON booleans
std::cout << j_truth << '\n';
std::cout << j_falsity << '\n';
}
```
Output:
```json
{"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"]
["one"]
["four","one","one","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
```
Note the output is platform-dependent.
??? example "Example: (4) create a JSON value from another `basic_json` specialization"
The example below shows how a `json` value is converted to an `ordered_json` value and back using the converting
constructor. Note how the original insertion order of `oj` is not restored, because it was already given up when
converting to `json`, whose `object_t` sorts by key.
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
using ordered_json = nlohmann::ordered_json;
int main()
{
// create an ordered_json value; insertion order is preserved
ordered_json oj = {{"c", 3}, {"a", 1}, {"b", 2}};
// convert to json -- overload (4) is used; keys end up sorted
json j(oj);
// convert back to ordered_json -- the original insertion order is lost,
// because it was already given up when converting to json
ordered_json oj2(j);
std::cout << oj << '\n';
std::cout << j << '\n';
std::cout << oj2 << '\n';
}
```
Output:
```json
{"c":3,"a":1,"b":2}
{"a":1,"b":2,"c":3}
{"a":1,"b":2,"c":3}
```
??? example "Example: (5) create a container (array or object) from an initializer list"
The example below shows how JSON values are created from initializer lists.
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create JSON values
json j_empty_init_list = json({});
json j_object = { {"one", 1}, {"two", 2} };
json j_array = {1, 2, 3, 4};
json j_nested_object = { {"one", {1}}, {"two", {1, 2}} };
json j_nested_array = { {{1}, "one"}, {{1, 2}, "two"} };
// serialize the JSON value
std::cout << j_empty_init_list << '\n';
std::cout << j_object << '\n';
std::cout << j_array << '\n';
std::cout << j_nested_object << '\n';
std::cout << j_nested_array << '\n';
}
```
Output:
```json
{}
{"one":1,"two":2}
[1,2,3,4]
{"one":[1],"two":[1,2]}
[[[1],"one"],[[1,2],"two"]]
```
??? example "Example: (6) construct an array with count copies of a given value"
The following code shows examples for creating arrays with several copies of a given value.
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create an array by creating copies of a JSON value
json value = "Hello";
json array_0 = json(0, value);
json array_1 = json(1, value);
json array_5 = json(5, value);
// serialize the JSON arrays
std::cout << array_0 << '\n';
std::cout << array_1 << '\n';
std::cout << array_5 << '\n';
}
```
Output:
```json
[]
["Hello"]
["Hello","Hello","Hello","Hello","Hello"]
```
??? example "Example: (7) construct a JSON container given an iterator range"
The example below shows several ways to create JSON values by specifying a subrange with iterators.
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create JSON values
json j_array = {"alpha", "bravo", "charly", "delta", "easy"};
json j_number = 42;
json j_object = {{"one", "eins"}, {"two", "zwei"}};
// create copies using iterators
json j_array_range(j_array.begin() + 1, j_array.end() - 2);
json j_number_range(j_number.begin(), j_number.end());
json j_object_range(j_object.begin(), j_object.find("two"));
// serialize the values
std::cout << j_array_range << '\n';
std::cout << j_number_range << '\n';
std::cout << j_object_range << '\n';
// example for an exception
try
{
json j_invalid(j_number.begin() + 1, j_number.end());
}
catch (const json::invalid_iterator& e)
{
std::cout << e.what() << '\n';
}
}
```
Output:
```json
["bravo","charly"]
42
{"one":"eins"}
[json.exception.invalid_iterator.204] iterators out of range
```
??? example "Example: (8) copy constructor"
The following code shows an example for the copy constructor.
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create a JSON array
json j1 = {"one", "two", 3, 4.5, false};
// create a copy
json j2(j1);
// serialize the JSON array
std::cout << j1 << " = " << j2 << '\n';
std::cout << std::boolalpha << (j1 == j2) << '\n';
}
```
Output:
```json
["one","two",3,4.5,false] = ["one","two",3,4.5,false]
true
```
??? example "Example: (9) move constructor"
The code below shows the move constructor explicitly called via `std::move`.
```cpp
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create a JSON value
json a = 23;
// move contents of a to b
json b(std::move(a));
// serialize the JSON arrays
std::cout << a << '\n';
std::cout << b << '\n';
}
```
Output:
```json
null
23
```
## See also
- [array](array.md) create a JSON array value, forcing array creation from an initializer list even when it looks like
an object
- [object](object.md) create a JSON object value, forcing object creation from an initializer list
- [binary](binary.md) create a JSON binary array value
- [operator=](operator=.md) copy assignment operator
- [Creating JSON values](../../features/creating_values.md) - the article on creating JSON values
## Version history
1. Since version 1.0.0.
2. Since version 1.0.0.
3. Since version 2.1.0.
4. Since version 3.2.0. Explicit for different string types if `JSON_USE_IMPLICIT_CONVERSIONS` is `0` since
version 3.13.0.
5. Since version 1.0.0.
6. Since version 1.0.0.
7. Since version 1.0.0. Fixed in version 3.13.0 to also check the iterator range for binary values; before, a range
that did not cover the whole value (such as `(end(), end())`) was accepted and the whole binary value was copied,
unlike the other primitive types.
8. Since version 1.0.0.
9. Since version 1.0.0.
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