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nlohmann
138 lines
4.1 KiB
Markdown
138 lines
4.1 KiB
Markdown
# nlohmann::basic_json::accept
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```
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// (1)
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template<typename InputType>
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static bool accept(InputType&& i,
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const bool ignore_comments = false,
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const bool ignore_trailing_commas = false);
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// (2)
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template<typename IteratorType>
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static bool accept(IteratorType first, IteratorType last,
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const bool ignore_comments = false,
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const bool ignore_trailing_commas = false);
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```
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Checks whether the input is valid JSON.
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1. Reads from a compatible input.
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1. Reads from a pair of character iterators
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The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32.
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Unlike the [`parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md) function, this function neither throws an exception in case of invalid JSON input (i.e., a parse error) nor creates diagnostic information.
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## Template parameters
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`InputType` : A compatible input, for instance:
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```
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- an `std::istream` object
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- a `FILE` pointer (throws if null)
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- a C-style array of characters
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- a pointer to a null-terminated string of single byte characters (throws if null)
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- a `std::string`
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- an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
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```
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`IteratorType` : a compatible iterator type, for instance.
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```
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- a pair of `std::string::iterator` or `std::vector<std::uint8_t>::iterator`
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- a pair of pointers such as `ptr` and `ptr + len`
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```
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## Parameters
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`i` (in) : Input to parse from.
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`ignore_comments` (in) : whether comments should be ignored and treated like whitespace (`true`) or yield a parse error (`false`); (optional, `false` by default)
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`ignore_trailing_commas` (in) : whether trailing commas in arrays or objects should be ignored and treated like whitespace (`true`) or yield a parse error (`false`); (optional, `false` by default)
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`first` (in) : iterator to the start of the character range
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`last` (in) : iterator to the end of the character range
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## Return value
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Whether the input is valid JSON.
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## Exception safety
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Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
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## Exceptions
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Throws [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) in case of an empty input like a null `FILE*` or `char*` pointer.
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## Complexity
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Linear in the length of the input. The parser is a predictive LL(1) parser.
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## Notes
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A UTF-8 byte order mark is silently ignored.
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## Examples
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Example
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The example below demonstrates the `accept()` function reading from a string.
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```
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#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// a valid JSON text
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auto valid_text = R"(
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{
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"numbers": [1, 2, 3]
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}
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)";
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// an invalid JSON text
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auto invalid_text = R"(
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{
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"strings": ["extra", "comma", ]
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}
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)";
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std::cout << std::boolalpha
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<< json::accept(valid_text) << ' '
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<< json::accept(invalid_text) << '\n';
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}
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```
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Output:
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```
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true false
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```
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## See also
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- [parse](https://json.nlohmann.me/api/basic_json/parse/index.md) - deserialize from a compatible input
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- [sax_parse](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - parse input using the SAX interface
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- [operator>>](https://json.nlohmann.me/api/operator_gtgt/index.md) - deserialize from stream
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## Version history
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- Added in version 3.0.0.
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- Ignoring comments via `ignore_comments` added in version 3.9.0.
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- Changed [runtime assertion](https://json.nlohmann.me/features/assertions/index.md) in case of `FILE*` null pointers to exception in version 3.12.0.
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- Added `ignore_trailing_commas` in version 3.12.x.
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Deprecation
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Overload (2) replaces calls to `accept` 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 `accept({ptr, ptr+len}, ...);` with `accept(ptr, ptr+len, ...);`.
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You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated function.
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