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Merge pull request #2363 from nlohmann/issue2360
Fix and extend documentation of discarded values
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commit
b9d76516a2
36
doc/examples/parse__allow_exceptions.cpp
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36
doc/examples/parse__allow_exceptions.cpp
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#include <iostream>
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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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// an invalid JSON text
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std::string text = R"(
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{
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"key": "value without closing quotes
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}
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)";
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// parse with exceptions
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try
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{
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json j = json::parse(text);
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}
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catch (json::parse_error& e)
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{
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std::cout << e.what() << std::endl;
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}
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// parse without exceptions
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json j = json::parse(text, nullptr, false);
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if (j.is_discarded())
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{
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std::cout << "the input is invalid JSON" << std::endl;
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}
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else
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{
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std::cout << "the input is valid JSON: " << j << std::endl;
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}
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}
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1
doc/examples/parse__allow_exceptions.link
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doc/examples/parse__allow_exceptions.link
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<a target="_blank" href="https://wandbox.org/permlink/2TsG4VSg87HsRQC9"><b>online</b></a>
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2
doc/examples/parse__allow_exceptions.output
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doc/examples/parse__allow_exceptions.output
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[json.exception.parse_error.101] parse error at line 4, column 0: syntax error while parsing value - invalid string: control character U+000A (LF) must be escaped to \u000A or \n; last read: '"value without closing quotes<U+000A>'
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the input is invalid JSON
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@ -1,4 +1,4 @@
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# basic_basic_json::exception
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# basic_json::exception
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```cpp
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class exception : public std::exception;
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@ -52,7 +52,7 @@ Deserializes a given input to a JSON value using the BSON (Binary JSON) serializ
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## Return value
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deserialized JSON value; in case of a parse error and `allow_exceptions` set to `#!cpp false`, the return value will be
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`value_t::discarded`.
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`value_t::discarded`. The latter can be checked with [`is_discarded`](is_discarded.md).
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## Exception safety
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@ -59,7 +59,7 @@ Deserializes a given input to a JSON value using the CBOR (Concise Binary Object
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## Return value
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deserialized JSON value; in case of a parse error and `allow_exceptions` set to `#!cpp false`, the return value will be
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`value_t::discarded`.
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`value_t::discarded`. The latter can be checked with [`is_discarded`](is_discarded.md).
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## Exception safety
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@ -52,7 +52,7 @@ Deserializes a given input to a JSON value using the MessagePack serialization f
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## Return value
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deserialized JSON value; in case of a parse error and `allow_exceptions` set to `#!cpp false`, the return value will be
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`value_t::discarded`.
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`value_t::discarded`. The latter can be checked with [`is_discarded`](is_discarded.md).
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## Exception safety
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@ -52,7 +52,7 @@ Deserializes a given input to a JSON value using the UBJSON (Universal Binary JS
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## Return value
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deserialized JSON value; in case of a parse error and `allow_exceptions` set to `#!cpp false`, the return value will be
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`value_t::discarded`.
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`value_t::discarded`. The latter can be checked with [`is_discarded`](is_discarded.md).
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## Exception safety
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@ -1,4 +1,4 @@
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# basic_basic_json::invalid_iterator
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# basic_json::invalid_iterator
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```cpp
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class invalid_iterator : public exception;
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constexpr bool is_discarded() const noexcept;
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```
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This function returns true if and only if the JSON value was discarded during parsing with a callback function (see
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[`parser_callback_t`](parser_callback_t.md)).
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This function returns `#!cpp true` for a JSON value if either:
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- the value was discarded during parsing with a callback function (see [`parser_callback_t`](parser_callback_t.md)), or
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- the value is the result of parsing invalid JSON with parameter `allow_exceptions` set to `#!cpp false`; see
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[`parse`](parse.md) for more information.
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## Return value
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`#!cpp true` if type is discarded, `#!cpp false` otherwise.
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@ -21,6 +24,30 @@ Constant.
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## Notes
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!!! note
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Discarded values are never compared equal with [`operator==`](operator_eq.md). That is, checking whether a JSON
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value `j` is discarded will only work via:
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```cpp
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j.is_discarded()
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```
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because
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```cpp
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j == json::value_t::discarded
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```
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will always be `#!cpp false`.
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!!! note
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When a value is discarded by a callback function (see [`parser_callback_t`](parser_callback_t.md)) during parsing,
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then it is removed when it is part of a structured value. For instance, if the second value of an array is discared,
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instead of `#!json [null, discarded, false]`, the array `#!json [null, false]` is returned. Only if the top-level
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value is discarded, the return value of the `parse` call is discarded.
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This function will always be `#!cpp false` for JSON values after parsing. That is, discarded values can only occur
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during parsing, but will be removed when inside a structured value or replaced by null in other cases.
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@ -12,11 +12,10 @@ bool operator==(ScalarType lhs, const const_reference rhs) noexcept;
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Compares two JSON values for equality according to the following rules:
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- Two JSON values are equal if (1) they are from the same type and (2) their stored values are the same according to
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their respective `operator==`.
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- Two JSON values are equal if (1) they are not discarded, (2) they are from the same type, and (3) their stored values
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are the same according to their respective `operator==`.
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- Integer and floating-point numbers are automatically converted before comparison. Note that two NaN values are always
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treated as unequal.
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- Two JSON null values are equal.
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## Template parameters
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## Notes
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- Floating-point inside JSON values numbers are compared with `json::number_float_t::operator==` which is
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`double::operator==` by default. To compare floating-point while respecting an epsilon, an alternative
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[comparison function](https://github.com/mariokonrad/marnav/blob/master/include/marnav/math/floatingpoint.hpp#L34-#L39)
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could be used, for instance
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!!! note
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- NaN values never compare equal to themselves or to other NaN values.
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- JSON `#!cpp null` values are all equal.
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- Discarded values never compare equal to themselves.
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!!! note
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Floating-point numbers inside JSON values numbers are compared with `json::number_float_t::operator==` which is
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`double::operator==` by default. To compare floating-point while respecting an epsilon, an alternative
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[comparison function](https://github.com/mariokonrad/marnav/blob/master/include/marnav/math/floatingpoint.hpp#L34-#L39)
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could be used, for instance
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```cpp
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template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>
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inline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept
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}
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```
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- NaN values never compare equal to themselves or to other NaN values.
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## Example
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??? example
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# basic_basic_json::other_error
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# basic_json::other_error
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```cpp
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class other_error : public exception;
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# basic_basic_json::out_of_range
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# basic_json::out_of_range
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```cpp
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class out_of_range : public exception;
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@ -61,7 +61,7 @@ static basic_json parse(IteratorType first, IteratorType last,
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## Return value
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Deserialized JSON value; in case of a parse error and `allow_exceptions` set to `#!cpp false`, the return value will be
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`value_t::discarded`.
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`value_t::discarded`. The latter can be checked with [`is_discarded`](is_discarded.md).
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## Exception safety
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@ -79,7 +79,7 @@ super-linear complexity.
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## Examples
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??? example
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??? example "Parsing from a charater array"
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The example below demonstrates the `parse()` function reading from an array.
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--8<-- "examples/parse__array__parser_callback_t.output"
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```
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??? example
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??? example "Parsing from a string"
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The example below demonstrates the `parse()` function with and without callback function.
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--8<-- "examples/parse__string__parser_callback_t.output"
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```
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??? example
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??? example "Parsing from an input stream"
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The example below demonstrates the `parse()` function with and without callback function.
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--8<-- "examples/parse__istream__parser_callback_t.output"
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```
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??? example
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??? example "Parsing from a contiguous container"
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The example below demonstrates the `parse()` function reading from a contiguous container.
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--8<-- "examples/parse__contiguouscontainer__parser_callback_t.output"
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```
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??? example "Effect of `allow_exceptions` parameter"
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The example below demonstrates the effect of the `allow_exceptions` parameter in the ´parse()` function.
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```cpp
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--8<-- "examples/parse__allow_exceptions.cpp"
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```
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Output:
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```json
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--8<-- "examples/parse__allow_exceptions.output"
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```
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## Version history
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- Added in version 1.0.0.
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# basic_basic_json::parse_error
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# basic_json::parse_error
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```cpp
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class parse_error : public exception;
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# basic_basic_json::type_error
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# basic_json::type_error
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```cpp
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class type_error : public exception;
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