api/basic_json_document/parse.md

# <small>nlohmann::basic_json_document::</small>parse

```cpp
// (1)
template<typename InputType>
static basic_json_document parse(InputType&& input,
                                 const bool allow_exceptions = true,
                                 const bool ignore_comments = false,
                                 const bool ignore_trailing_commas = false);

// (2)
template<typename IteratorType>
static basic_json_document parse(IteratorType first, IteratorType last,
                                 const bool allow_exceptions = true,
                                 const bool ignore_comments = false,
                                 const bool ignore_trailing_commas = false);
```

1. Deserialize from a compatible input, borrowing or owning it depending on its value category and type (see Notes).
2. Deserialize from a pair of input iterators.

Both overloads accept the same JSON text as [`BasicJsonType::parse()`](../basic_json/parse.md), with the same
`ignore_comments`/`ignore_trailing_commas` options, but build a [`basic_json_document`](index.md) (a flat index into
the input) instead of a tree of `BasicJsonType` values. They accept every input `BasicJsonType::parse()` takes, and in
addition a pointer to a null-terminated wide string (which `BasicJsonType::parse()` rejects). Contiguous inputs of
single-byte characters that stay alive can be borrowed; all others (streams, `#!cpp FILE*`, wide characters, which
must be converted to UTF-8, and non-contiguous iterators) are read into a copy that the document owns — see the
Notes. Use [`parse_copy()`](parse_copy.md) to get an owned copy of a borrowable input
as well.

## Template parameters

`InputType`
:   A compatible input, for instance:

    - a `#!cpp std::string`, a `#!cpp std::string_view` (C++17), or a C-style array of single-byte characters
    - a pointer to a null-terminated string of single-byte characters (`#!cpp char`, `#!cpp signed char`,
      `#!cpp unsigned char`, `#!cpp std::uint8_t`)
    - a container for which `#!cpp obj.data()` and `#!cpp obj.size()` give contiguous single-byte access, e.g.
      `#!cpp std::vector<char>` or `#!cpp std::vector<std::uint8_t>`
    - an `#!cpp std::istream` object
    - a `#!cpp FILE*` pointer
    - wide characters (`#!cpp wchar_t`, `#!cpp char16_t`, `#!cpp char32_t`), which are converted to UTF-8: a wide
      string object (`#!cpp std::wstring`, `#!cpp std::u16string`, `#!cpp std::u32string`), an array (e.g. a wide
      string literal), or a pointer to a null-terminated wide string

`IteratorType`
:   an input iterator type, for instance a pair of pointers such as `ptr` and `ptr + len`, or a pair of
    `#!cpp std::string::iterator`; the iterators of single-byte characters are borrowed or read like the byte inputs
    above, those of wide characters are converted to UTF-8

Ill-formed UTF-16 and UTF-32 sequences are rejected: a lone surrogate, a high surrogate that is not followed by its low
surrogate, and a code point above `U+10FFFF` all yield a
[`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101).

## Parameters

`input` (in)
:   Input to parse from.

`allow_exceptions` (in)
:   whether to throw exceptions in case of a parse error (optional, `#!cpp true` by default)

`ignore_comments` (in)
:   whether comments should be ignored and treated like whitespace (`#!cpp true`) or yield a parse error
    (`#!cpp false`); (optional, `#!cpp false` by default)

`ignore_trailing_commas` (in)
:   whether trailing commas in arrays or objects should be ignored and treated like whitespace (`#!cpp true`) or
    yield a parse error (`#!cpp false`); (optional, `#!cpp false` by default)

`first` (in)
:   iterator to the start of a character range

`last` (in)
:   iterator to the end of a character range

## Return value

The parsed document. If `allow_exceptions` is `#!cpp false` and the input is not valid JSON, the returned document is
discarded; see [`is_discarded`](is_discarded.md).

## Exceptions

Throws the same exception [`BasicJsonType::parse()`](../basic_json/parse.md) throws for the same input and options —
the same exception id, message, and position — because on a failing input the library's own parser is run on the
same bytes to produce the diagnostic. This replay runs only for a failing input, and costs about a lexing pass up to
the error (its SAX handler builds no values); with `allow_exceptions` set to `#!cpp false`, it is skipped and a
discarded document is returned instead. Additionally throws
[`out_of_range.416`](../../home/exceptions.md#jsonexceptionout_of_range416) if the input is 4294967280 bytes (4 GiB
minus 16 bytes) or larger, a size [`BasicJsonType::parse()`](../basic_json/parse.md) does not reject.

## Complexity

Linear in the length of the input.

## Notes

**Ownership.** Whether the document borrows `input` or owns a copy of it depends on its value category and type:

| `input`                                                                             | ownership                                                    |
|--------------------------------------------------------------------------------------|--------------------------------------------------------------|
| lvalue byte container (`std::string`, `std::vector<char>`, ...), `std::string_view`, C string, character array | **borrowed** — `input` must outlive the document |
| non-const rvalue `#!cpp std::string`                                                  | **owned**, moved in without a copy                            |
| other rvalue byte container (including a `#!cpp const` rvalue `#!cpp std::string`)  | **owned**, copied                                             |
| stream, `#!cpp FILE*`, wide characters, or anything else read through the general input adapter | **owned**, read into a buffer (a stream is read to its end) |

For overload (2), a pair of pointers to single-byte integers (e.g. `#!cpp const char*`, `#!cpp std::uint8_t*`) is
borrowed. From C++20 on, so is any other contiguous iterator over single bytes, such as
`#!cpp std::vector<char>::iterator` or `#!cpp std::string::const_iterator`. Before C++20 these iterators cannot be
told apart from other class-type iterators, so their range is read into an owned buffer, as is any non-contiguous
range (e.g. of a `#!cpp std::list<char>`).

See [`owns_source`](owns_source.md) to check which happened after a call, and the
[feature page](../../features/json_view.md) for the reasoning.

**Numbers.** As for [`BasicJsonType::parse()`](../basic_json/parse.md), an integer literal too large for the 64-bit
integer type becomes a floating-point value.

**No lengths.** An integer argument that is not a `#!cpp bool` where the flags are expected — for example
`#!cpp parse(ptr, len)` — does not compile (the overload is deleted). Such a call would convert `len` to
`allow_exceptions` and read `ptr` as a null-terminated string, past the end of a buffer that has none. To parse a
buffer of a given length, pass a pair of pointers: `#!cpp parse(ptr, ptr + len)`. The same holds for
[`parse_copy`](parse_copy.md), [`accept`](accept.md), and [`read`](read.md).

## Examples

??? example "Example: (1) borrowed vs. owned input, and errors identical to `BasicJsonType::parse()`"

    ```cpp
    #include <iostream>
    #include <string>
    #include <nlohmann/json.hpp>
    #include <nlohmann/json_view.hpp>
    
    using nlohmann::json_document;
    
    int main()
    {
        std::cout << std::boolalpha;
    
        // an lvalue std::string is BORROWED: the document only stores a pointer
        // into `text`, so `text` must outlive `borrowed`
        std::string text = R"({"count": 3})";
        json_document borrowed = json_document::parse(text);
        std::cout << borrowed.owns_source() << '\n'; // false
    
        // an rvalue std::string is MOVED into the document -- no copy of the text
        json_document owned = json_document::parse(std::string(R"({"count": 3})"));
        std::cout << owned.owns_source() << '\n'; // true
    
        // errors are identical to basic_json::parse: same exception id, message,
        // and position, because the library parser runs on the same bytes on a
        // failing input
        try
        {
            static_cast<void>(json_document::parse(R"({"count": )"));
        }
        catch (const nlohmann::json::parse_error& e)
        {
            std::cout << e.id << '\n';
        }
    }
    
    ```

    Output:

    ```json
    false
    true
    101
    
    ```

??? example "Example: (2) parse an iterator range (no NUL terminator required)"

    ```cpp
    #include <iostream>
    #include <vector>
    #include <nlohmann/json_view.hpp>
    
    using nlohmann::json_document;
    
    int main()
    {
        std::cout << std::boolalpha;
    
        // a JSON value embedded in a larger, non-null-terminated buffer (e.g. a
        // slice received over the network)
        std::vector<char> buffer = {'[', '1', ',', '2', ']', 'j', 'u', 'n', 'k'};
    
        // a pointer pair is BORROWED, exactly like a byte container lvalue: the
        // document points into the buffer. (From C++20 on, std::vector<char>
        // iterators are borrowed as well; before, they are copied.)
        const char* first = buffer.data();
        json_document doc = json_document::parse(first, first + 5);
        std::cout << doc.root().is_array() << ' ' << doc.root().size() << '\n';
        std::cout << doc.owns_source() << '\n'; // false: borrowed
    }
    
    ```

    Output:

    ```json
    true 2
    false
    
    ```

## See also

- [`parse_copy`](parse_copy.md) - deserialize a copy of a compatible input
- [`accept`](accept.md) - check whether the input is valid JSON
- [`read`](read.md) - (re-)parse into this document, reusing its memory
- [`owns_source`](owns_source.md) - return whether the document holds its own copy of the text
- [`load`](load.md) - read a document from an image instead of parsing JSON text
- [`BasicJsonType::parse`](../basic_json/parse.md) - the corresponding function of `basic_json`

## Version history

- Added in version 3.13.0.
- Ill-formed UTF-16 and UTF-32 input (a lone surrogate, a high surrogate that is not followed by its low surrogate, or
  a code point above `U+10FFFF`) is rejected with `parse_error.101`.