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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);

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

  1. Deserialize from a compatible input.

  2. Deserialize from a pair of character iterators

    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.

Template parameters

InputType : A compatible input, for instance:

- an `std::istream` object
- a `FILE` pointer (must not be null)
- a C-style array of characters
- a pointer to a null-terminated string of single byte characters
- a `std::string`
- an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.

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`

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)

first (in) : iterator to start of character range

last (in) : iterator to end of character range

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

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

(1) A UTF-8 byte order mark is silently ignored.

!!! danger "Runtime assertion"

The precondition that a passed `#!cpp FILE` pointer must not be null is enforced with a
[runtime assertion](../../features/assertions.md).

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"
```

See also

  • accept - check if the input is valid JSON
  • 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.

!!! 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.
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