json/docs/mkdocs/docs/api/basic_json/parse.md

nlohmann::basic_json::parse

// (1)
template<typename InputType>
static basic_json parse(InputType&& i,
                        const parser_callback_t cb = nullptr,
                        const bool allow_exceptions = true,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false);

// (2)
template<typename IteratorType, typename SentinelType = IteratorType>
static basic_json parse(IteratorType first, SentinelType last,
                        const parser_callback_t cb = nullptr,
                        const bool allow_exceptions = true,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false);

1. Deserialize from a compatible input.

2. Deserialize from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)

   The value_type of the iterator must be an integral type with size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If SentinelType differs from IteratorType, it must be comparable to the iterator type with operator!=.

   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.

Template parameters

InputType

A compatible input, for instance:

• an std::istream object
• a FILE pointer (throws if null)
• a C-style array of characters
• a pointer to a null-terminated string of single byte characters (throws if null)
• a std::string or a std::string_view (C++17)
• a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
• wide characters (wchar_t, char16_t, char32_t), which are converted to UTF-8: a wide string object (std::wstring, std::u16string, std::u32string) or an array (e.g. a wide string literal); a pointer to a null-terminated wide string (e.g. std::u16string::c_str()) is not supported

IteratorType

a compatible iterator type, for instance.

• a pair of std::string::iterator or std::vector<std::uint8_t>::iterator
• a pair of pointers such as ptr and ptr + len

SentinelType

defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

• a custom sentinel type for C++20 ranges
• std::default_sentinel_t, when IteratorType is std::counted_iterator

Parameters

i (in)

Input to parse from.

cb (in)

a parser callback function of type parser_callback_t which is used to control the deserialization by filtering unwanted values (optional)

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, or a sentinel value that compares equal to the end iterator with operator!=

Return value

Deserialized JSON value; in case of a parse error and allow_exceptions set to #!cpp false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

• Throws parse_error.101 in case of an unexpected token, or empty input like a null FILE* or char* pointer, or an std::istream without a stream buffer (#!cpp i.rdbuf() == nullptr, for instance #!cpp std::istream(nullptr)).
• If reading from an std::istream reaches the end of the input and eofbit is part of the stream's exceptions() mask, the std::ios_base::failure thrown by the stream itself propagates instead of a parse_error, the same as it would for the standard library's own extraction operators.

Complexity

Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the parser callback function cb or reading from (1) the input i or (2) the iterator range [first, last] has a super-linear complexity.

Notes

A UTF-8 byte order mark is silently ignored.

Invalid Unicode escapes and unpaired surrogates in the input are reported as parse_error.101 with a detailed message.

By default, a '\0' (NUL) byte anywhere in the input is treated as end of input, rather than as an ordinary (and, outside of a string, invalid) byte; see the FAQ entry for details and the JSON_STRICT_NUL_HANDLING macro to opt into rejecting it instead.

!!! tip "Parsing into a document instead of a value"

[`basic_json_document::parse`](../basic_json_document/parse.md) accepts the same inputs and the same options, but
keeps the result as a flat index of the parsed values instead of a tree of `basic_json` values. It is faster and uses
less memory when only parts of the document are read, and it also accepts a pointer to a null-terminated wide string,
which `parse()` does not. See [Zero-copy JSON views](../../features/json_view.md).

!!! tip "Performance"

Contiguous inputs (strings, string views, vectors, arrays, pointer ranges) are the fastest to parse. A
`#!cpp std::istream` comes close: the parser scans the characters its stream buffer holds in bulk, without reading
ahead, so the stream is left exactly where it would be after reading one character at a time. A `#!cpp FILE*` is
read one character at a time, but the file is locked only once, so other threads that use the same `FILE` object
wait until parsing is done. See [input sources and performance](../../features/parsing/index.md#input-sources-and-performance),
[`JSON_DISABLE_STREAM_BULK_SCAN`](../macros/json_disable_stream_bulk_scan.md), and
[`JSON_DISABLE_UNLOCKED_FILE_IO`](../macros/json_disable_unlocked_file_io.md).

Examples

??? example "Example: (1) parse from a character array"

The example below demonstrates the `parse()` function reading from an array.

```cpp
--8<-- "examples/parse__array__parser_callback_t.cpp"
```

Output:

```json
--8<-- "examples/parse__array__parser_callback_t.output"
```

??? example "Example: (1) parse from a string"

The example below demonstrates the `parse()` function with and without callback function.

```cpp
--8<-- "examples/parse__string__parser_callback_t.cpp"
```

Output:

```json
--8<-- "examples/parse__string__parser_callback_t.output"
```

??? example "Example: (1) parse from an input stream"

The example below demonstrates the `parse()` function with and without callback function.

```cpp
--8<-- "examples/parse__istream__parser_callback_t.cpp"
```

Output:

```json
--8<-- "examples/parse__istream__parser_callback_t.output"
```

??? example "Example: (1) parse from a contiguous container"

The example below demonstrates the `parse()` function reading from a contiguous container.

```cpp
--8<-- "examples/parse__contiguouscontainer__parser_callback_t.cpp"
```

Output:

```json
--8<-- "examples/parse__contiguouscontainer__parser_callback_t.output"
```

??? example "Example: (2) parse from a non-null-terminated string"

The example below demonstrates the `parse()` function reading from a string that is not null-terminated.

```cpp
--8<-- "examples/parse__pointers.cpp"
```

Output:

```json
--8<-- "examples/parse__pointers.output"
```

??? example "Example: (2) parse from an iterator pair"

The example below demonstrates the `parse()` function reading from an iterator pair.

```cpp
--8<-- "examples/parse__iterator_pair.cpp"
```

Output:

```json
--8<-- "examples/parse__iterator_pair.output"
```

??? example "Example: effect of allow_exceptions parameter"

The example below demonstrates the effect of the `allow_exceptions` parameter in the `parse()` function.

```cpp
--8<-- "examples/parse__allow_exceptions.cpp"
```

Output:

```json
--8<-- "examples/parse__allow_exceptions.output"
```

??? example "Example: effect of ignore_comments parameter"

The example below demonstrates the effect of the `ignore_comments` parameter in the `parse()` function.

```cpp
--8<-- "examples/comments.cpp"
```

Output:

```
--8<-- "examples/comments.output"
```

??? example "Example: effect of ignore_trailing_commas parameter"

The example below demonstrates the effect of the `ignore_trailing_commas` parameter in the `parse()` function.

```cpp
--8<-- "examples/trailing_commas.cpp"
```

Output:

```
--8<-- "examples/trailing_commas.output"
```

??? example "Example: parse from UTF-16, UTF-32, and wide strings"

The encoding is deduced from the character type of the input; `std::u16string` and `std::u32string` are interpreted
as UTF-16 and UTF-32, and `std::wstring` uses the platform's `wchar_t` width. String literals like `u"..."`,
`U"..."`, and `L"..."` work as well, as do iterators over wide characters.

```cpp
--8<-- "examples/parse__wide_string.cpp"
```

Output:

```json
--8<-- "examples/parse__wide_string.output"
```

See also

• accept - check if the input is valid JSON
• sax_parse - parse input using the SAX interface
• operator>> - deserialize from stream
• JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input
• JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time
• JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with the locking stdio functions

Version history

• Added in version 1.0.0.
• Overload for contiguous containers (1) added in version 2.0.3.
• Ignoring comments via ignore_comments added in version 3.9.0.
• Changed runtime assertion in case of FILE* null pointers to exception in version 3.12.0.
• Added ignore_trailing_commas in version 3.13.0.
• Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0.
• Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
• JSON_STRICT_NUL_HANDLING added in version 3.13.0 to optionally reject a NUL byte in the input instead of treating it as end of input; planned to become the default in version 4.0.0.
• Extended empty-input detection to also cover an std::istream without a stream buffer, and fixed a crash (std::terminate) when parsing from an std::istream with eofbit in its exception mask, in version 3.13.0.
• Parsing from an std::istream scans the characters in its stream buffer in bulk in version 3.13.0 (JSON_DISABLE_STREAM_BULK_SCAN restores reading one character at a time).
• Parsing from a FILE* locks the file once instead of once per character in version 3.13.0 (JSON_DISABLE_UNLOCKED_FILE_IO restores std::fgetc).
• 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 in version 3.13.0; before that, some such sequences were accepted or ended the input silently.

!!! warning "Deprecation"

Overload (2) replaces calls to `parse` with a pair of iterators as their first parameter which has been
deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like
`#!cpp parse({ptr, ptr+len}, ...);` with `#!cpp parse(ptr, ptr+len, ...);`.

You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated
function.

See the [migration guide](../../integration/migration_guide.md#parsing) for how to update existing code.