mirror of
https://github.com/nlohmann/json.git
synced 2026-07-10 12:35:09 +00:00
nlohmann
62 lines
3.0 KiB
Markdown
62 lines
3.0 KiB
Markdown
# Parsing
|
|
|
|
This library can create a JSON value from a wide range of inputs. This page gives an overview of the available parsing
|
|
functions and how they behave; the linked pages go into more detail.
|
|
|
|
## Input
|
|
|
|
The [`parse`](../../api/basic_json/parse.md) function reads a JSON value from an input. The input can be
|
|
|
|
- a string (`#!cpp std::string`, C string, or string literal),
|
|
- a `#!cpp std::istream` (e.g., an `#!cpp std::ifstream` reading from a file),
|
|
- a `#!cpp FILE*` pointer,
|
|
- a pair of iterators over a contiguous range (e.g., a `#!cpp std::vector<std::uint8_t>`), or
|
|
- a contiguous container.
|
|
|
|
```cpp
|
|
// parse from a string
|
|
json j = json::parse(R"({"happy": true, "pi": 3.141})");
|
|
|
|
// parse from a file
|
|
std::ifstream f("example.json");
|
|
json data = json::parse(f);
|
|
```
|
|
|
|
The input must be encoded in UTF-8; other encodings are not supported. A single input may contain only one JSON value.
|
|
Inputs consisting of multiple values separated by newlines are handled by the [JSON Lines](json_lines.md) format.
|
|
|
|
By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the `parse`
|
|
function — see [comments](../comments.md) and [trailing commas](../trailing_commas.md).
|
|
|
|
## SAX vs. DOM parsing
|
|
|
|
The library offers two parsing models:
|
|
|
|
- **DOM parsing** (the default): the complete input is read and stored as an in-memory `basic_json` value that can be
|
|
traversed and modified freely. This is what [`parse`](../../api/basic_json/parse.md) does, and it is the right choice
|
|
for most use cases.
|
|
- **SAX parsing**: instead of building a value, the parser reports events (such as "a string was read" or "an object
|
|
started") to a handler that you implement. This avoids building the full value in memory and is useful for very large
|
|
inputs or when you only need to extract parts of the input. See the [SAX interface](sax_interface.md) for details and
|
|
[`sax_parse`](../../api/basic_json/sax_parse.md) for the API.
|
|
|
|
You can influence a DOM parse without switching to the SAX interface by passing a
|
|
[parser callback](parser_callbacks.md), which is called during parsing and can, for example, discard parts of the input.
|
|
|
|
## Exceptions
|
|
|
|
When the input is not valid JSON, the `parse` function throws an exception by default. If exceptions are undesired or
|
|
unavailable, the parser can instead return a discarded value, or [`accept`](../../api/basic_json/accept.md) can be used
|
|
to only check whether an input is valid JSON. See [parsing and exceptions](parse_exceptions.md) for the available
|
|
options.
|
|
|
|
## See also
|
|
|
|
- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
|
|
- [`accept`](../../api/basic_json/accept.md) - check if the input is valid JSON
|
|
- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
|
|
- [JSON Lines](json_lines.md) - parse newline-delimited JSON
|
|
- [parser callbacks](parser_callbacks.md) - influence the parsing by a callback function
|
|
- [SAX interface](sax_interface.md) - implement a custom SAX handler
|
|
- [parsing and exceptions](parse_exceptions.md) - control error handling
|