# nlohmann::basic_json::parse ```cpp // (1) template 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 static basic_json parse(IteratorType first, IteratorType 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 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 (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` - 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::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`](parser_callback_t.md) 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 ## 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`](is_discarded.md). ## Exception safety Strong guarantee: if an exception is thrown, there are no changes in the JSON value. ## Exceptions - Throws [`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token, or empty input like a null `FILE*` or `char*` pointer. - Throws [`parse_error.102`](../../home/exceptions.md#jsonexceptionparse_error102) if `to_unicode` fails or surrogate error. - Throws [`parse_error.103`](../../home/exceptions.md#jsonexceptionparse_error103) if `to_unicode` fails. ## 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. ## 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](accept.md) - check if the input is valid JSON - [operator>>](../operator_gtgt.md) - 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](../../features/assertions.md) in case of `FILE*` null pointers to exception in version 3.12.0. - Added `ignore_trailing_commas` in version 3.12.1. !!! 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.