mirror of
https://github.com/nlohmann/json.git
synced 2026-07-10 12:35:09 +00:00
nlohmann
108 lines
4.1 KiB
Markdown
108 lines
4.1 KiB
Markdown
# 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](https://json.nlohmann.me/features/parsing/index.md).
|
|
|
|
## 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](https://json.nlohmann.me/features/conversions/index.md) 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.
|
|
|
|
Ambiguous cases: `{}` vs. `[]`
|
|
|
|
Because the same `{}` syntax is used for both arrays and objects, some cases are ambiguous. To force a particular type, use the explicit factory functions [`json::array`](https://json.nlohmann.me/api/basic_json/array/index.md) and [`json::object`](https://json.nlohmann.me/api/basic_json/object/index.md):
|
|
|
|
```
|
|
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 `json j{value};` wraps the element in a single-element **array** by default, and its behavior even differs between compilers. See the [FAQ](https://json.nlohmann.me/home/faq/#brace-initialization-yields-arrays) for details and the opt-in [`JSON_BRACE_INIT_COPY_SEMANTICS`](https://json.nlohmann.me/api/macros/json_brace_init_copy_semantics/index.md) macro.
|
|
|
|
## Building incrementally
|
|
|
|
A value can also be built up piece by piece. Accessing a non-existing object key or array index with [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) 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](https://json.nlohmann.me/features/modifying_values/index.md) for [`push_back`](https://json.nlohmann.me/api/basic_json/push_back/index.md), [`emplace`](https://json.nlohmann.me/api/basic_json/emplace/index.md), and related functions.
|
|
|
|
## With the `_json` literal
|
|
|
|
The `_json` [user-defined literal](https://json.nlohmann.me/api/operator_literal_json/index.md) parses a string at the call site and is a convenient way to write a JSON value inline:
|
|
|
|
Example
|
|
|
|
```
|
|
#include <iostream>
|
|
#include <iomanip>
|
|
#include <nlohmann/json.hpp>
|
|
|
|
using json = nlohmann::json;
|
|
using namespace nlohmann::literals;
|
|
|
|
int main()
|
|
{
|
|
json j = R"( {"hello": "world", "answer": 42} )"_json;
|
|
|
|
std::cout << std::setw(2) << j << '\n';
|
|
}
|
|
```
|
|
|
|
Output:
|
|
|
|
```
|
|
{
|
|
"answer": 42,
|
|
"hello": "world"
|
|
}
|
|
```
|
|
|
|
Note this **parses** the string, so `"42"_json` is the number `42`, whereas `json("42")` is the JSON string `"42"`.
|
|
|
|
## See also
|
|
|
|
- [`basic_json` constructors](https://json.nlohmann.me/api/basic_json/basic_json/index.md) - all ways to construct a value
|
|
- [`array`](https://json.nlohmann.me/api/basic_json/array/index.md) / [`object`](https://json.nlohmann.me/api/basic_json/object/index.md) - force array or object type
|
|
- [`operator""_json`](https://json.nlohmann.me/api/operator_literal_json/index.md) - the `_json` literal
|
|
- [Converting values](https://json.nlohmann.me/features/conversions/index.md) - which C++ types can be used
|
|
- [Parsing](https://json.nlohmann.me/features/parsing/index.md) - create a value from JSON text
|