Creating JSON values

There are several ways to create a JSON value in memory. This page gives an overview; to read a value from JSON text instead, see parsing. A JSON text can also be parsed into a document and read through a view instead of into a value; see Zero-copy JSON views.

From C++ values

Any value of a supported C++ type can be assigned to or used to construct a json:

json j_number  = 42;
json j_float   = 3.141;
json j_string  = "Hello";
json j_boolean = true;
json j_null    = nullptr;
json j_vector  = std::vector<int>{1, 2, 3};   // array

See converting values for the full set of supported types.

With initializer lists

Objects and arrays can be written concisely with brace-enclosed initializer lists:

// an array
json array = {1, 2, 3, 4};

// an object (a list of key/value pairs)
json object = {
    {"pi", 3.141},
    {"happy", true},
    {"name", "Niels"},
    {"nothing", nullptr},
    {"list", {1, 0, 2}},
    {"object", {{"currency", "USD"}, {"value", 42.99}}}
};

The library decides between an array and an object based on the content: a list whose elements are all two-element lists with a string as the first element is treated as an object, everything else as an array.

!!! warning "Ambiguous cases: #!cpp {} vs. #!cpp []"

Because the same `#!cpp {}` syntax is used for both arrays and objects, some cases are ambiguous. To force a
particular type, use the explicit factory functions [`json::array`](../api/basic_json/array.md) and
[`json::object`](../api/basic_json/object.md):

```cpp
json empty_array_explicit = json::array();               // []
json empty_object_explicit = json::object();             // {}

// a JSON array with one object, not an object with one member
json array_of_objects = json::array({{"key", "value"}}); // [{"key":"value"}]
```

Related to this, single-element brace initialization such as `#!cpp json j{value};` wraps the element in a
single-element **array** by default, and its behavior even differs between compilers. See the
[FAQ](../home/faq.md#brace-initialization-yields-arrays) for details and the opt-in
[`JSON_BRACE_INIT_COPY_SEMANTICS`](../api/macros/json_brace_init_copy_semantics.md) macro.

What does this produce?

The following table shows how some common spellings are interpreted. When in doubt, use #!cpp json::array(), #!cpp json::object(), or the #!cpp json(value) form with parentheses.

code	result	note
`#!cpp json j = {};` / `#!cpp json j{};`	`#!json null`	use `json::object()` or `json::array()`
`#!cpp json j = json::object();`	`#!json {}`
`#!cpp json j = json::array();`	`#!json []`
`#!cpp json j(true);`	`#!json true`	parentheses create the value itself
`#!cpp json j = {true};`	`#!json [true]`	braces create a one-element array (`#!cpp json j{true};` depends on the compiler, see the FAQ)
`#!cpp json j = {{"a", 1}};`	`#!json {"a":1}`	all elements are `[string, value]` pairs: object
`#!cpp json j = {{"a", 1}, {"b"}};`	`#!json [["a",1],["b"]]`	one element is not a pair: array
`#!cpp json j = {{"a", 1}, 2};`	`#!json [["a",1],2]`	a plain value among pairs: array
`#!cpp json j = {{"a", 1, 2}};`	`#!json [["a",1,2]]`	a three-element list is not a pair: array
`#!cpp json j = "{\"a\":1}";`	`#!json "{\"a\":1}"`	a string value; it is not parsed
`#!cpp json j = json::parse("{\"a\":1}");`	`#!json {"a":1}`	parse text with `parse` or `_json`

Building incrementally

A value can also be built up piece by piece. Accessing a non-existing object key or array index with operator[] creates the element on the fly:

json j;                    // null
j["answer"]["everything"] = 42;   // becomes an object
j["list"] = {1, 0, 2};
j["list"].push_back(3);           // [1,0,2,3]

See modifying values for push_back, emplace, and related functions.

With the `_json` literal

The _json user-defined literal parses a string at the call site and is a convenient way to write a JSON value inline:

??? example

```cpp
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    json j = R"( {"hello": "world", "answer": 42} )"_json;

    std::cout << std::setw(2) << j << '\n';
}

```

Output:

```json
{
  "answer": 42,
  "hello": "world"
}

```

Note this parses the string, so #!cpp "42"_json is the number #!cpp 42, whereas #!cpp json("42") is the JSON string #!json "42".

6.4 KiB Raw Blame History

Creating JSON values

From C++ values

With initializer lists

What does this produce?

Building incrementally

With the _json literal

See also

6.4 KiB

Raw Blame History

With the `_json` literal