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!=.

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
• 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)

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::counted_iterator with a different sentinel type

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.

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.

Examples

??? example "Parsing 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 "Parsing 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 "Parsing 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 "Parsing 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 "Parsing 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 "Parsing 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 "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 "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 "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"
```

See also

• accept - check if the input is valid JSON
• sax_parse - parse input using the SAX interface
• operator>> - deserialize from stream

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.

!!! 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.