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nlohmann
104 lines
3.1 KiB
Markdown
104 lines
3.1 KiB
Markdown
# nlohmann::basic_json::is_discarded
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```
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constexpr bool is_discarded() const noexcept;
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```
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This function returns `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`](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md)), or
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- the value is the result of parsing invalid JSON with parameter `allow_exceptions` set to `false`; see [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) for more information.
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## Return value
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`true` if type is discarded, `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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Comparisons
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Discarded values are never compared equal with [`operator==`](https://json.nlohmann.me/api/basic_json/operator_eq/index.md). That is, checking whether a JSON value `j` is discarded will only work via:
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```
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j.is_discarded()
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```
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because
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```
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j == json::value_t::discarded
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```
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will always be `false`.
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Removal during parsing with callback functions
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When a value is discarded by a callback function (see [`parser_callback_t`](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md)) during parsing, then it is removed when it is part of a structured value. For instance, if the second value of an array is discarded, instead of `[null, discarded, false]`, the array `[null, false]` is returned. If the top-level value itself is discarded by the callback, the `parse` call returns a `null` value.
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After a successful parse, this function always returns `false`: discarded values can only occur during parsing and are either removed when inside a structured value or replaced by `null` at the top level. The exception is parsing with `allow_exceptions` set to `false`: a parse error then yields a discarded value for which this function returns `true` (see [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md)).
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## Examples
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Example
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The following code exemplifies `is_discarded()` for all JSON types.
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```
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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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```
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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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## Version history
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- Added in version 1.0.0.
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