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155 lines
4.8 KiB
Markdown
155 lines
4.8 KiB
Markdown
# <small>nlohmann::basic_json::</small>is_discarded
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```cpp
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constexpr bool is_discarded() const noexcept;
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```
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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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## Exception safety
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No-throw guarantee: this member function never throws exceptions.
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## Complexity
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Constant.
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## Notes
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!!! note "Comparisons"
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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 "Removal during parsing with callback functions"
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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 discarded,
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instead of `#!json [null, discarded, false]`, the array `#!json [null, false]` is returned. If the top-level value
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itself is discarded by the callback, the `parse` call returns a `#!json null` value.
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After a successful parse, this function always returns `#!cpp false`: discarded values can only occur during parsing and
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are either removed when inside a structured value or replaced by `#!json null` at the top level. The exception is parsing
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with `allow_exceptions` set to `#!cpp false`: a parse error then yields a discarded value for which this function returns
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`#!cpp true` (see [`parse`](parse.md)).
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## Examples
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??? example "Example: `is_discarded()` for ordinary JSON values"
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The following code exemplifies `is_discarded()` for all JSON types.
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```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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// create JSON values
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json j_null;
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json j_boolean = true;
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json j_number_integer = 17;
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json j_number_unsigned_integer = 12345678987654321u;
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json j_number_float = 23.42;
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json j_object = {{"one", 1}, {"two", 2}};
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json j_array = {1, 2, 4, 8, 16};
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json j_string = "Hello, world";
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json j_binary = json::binary({1, 2, 3});
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// call is_discarded()
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std::cout << std::boolalpha;
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std::cout << j_null.is_discarded() << '\n';
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std::cout << j_boolean.is_discarded() << '\n';
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std::cout << j_number_integer.is_discarded() << '\n';
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std::cout << j_number_unsigned_integer.is_discarded() << '\n';
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std::cout << j_number_float.is_discarded() << '\n';
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std::cout << j_object.is_discarded() << '\n';
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std::cout << j_array.is_discarded() << '\n';
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std::cout << j_string.is_discarded() << '\n';
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std::cout << j_binary.is_discarded() << '\n';
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}
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```
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Output:
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```json
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false
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false
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false
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false
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false
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false
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false
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false
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false
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```
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??? example "Example: discarded values from parsing"
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The following code shows the two situations in which a discarded value can be observed: parsing invalid JSON with
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`allow_exceptions` set to `#!cpp false`, and a parser callback that discards the top-level value (which is replaced
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by `#!json null` and therefore does *not* remain discarded).
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```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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// parsing invalid JSON without exceptions yields a discarded value
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json j_invalid = json::parse("[1,2,3", nullptr, false);
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// a callback that discards the top-level value does not leave it
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// "discarded" -- it is replaced by null instead
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json j_discarded_by_callback = json::parse("[1,2,3]", [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)
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{
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return event != json::parse_event_t::array_start;
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});
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std::cout << std::boolalpha;
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std::cout << "j_invalid.is_discarded() = " << j_invalid.is_discarded() << '\n';
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std::cout << "j_discarded_by_callback = " << j_discarded_by_callback << '\n';
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std::cout << "j_discarded_by_callback.is_discarded() = " << j_discarded_by_callback.is_discarded() << '\n';
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}
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```
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Output:
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```json
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j_invalid.is_discarded() = true
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j_discarded_by_callback = null
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j_discarded_by_callback.is_discarded() = false
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```
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## Version history
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- Added in version 1.0.0.
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