109 lines
4.3 KiB
Markdown
109 lines
4.3 KiB
Markdown
# <small>nlohmann::basic_json_view::</small>number_token
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```cpp
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string_view_t number_token() const;
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```
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Returns the number exactly as it appears in the source text, without parsing or rounding it.
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## Return value
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The number's token text, as a [`string_view_t`](index.md#member-types) into the document's
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[`source()`](../basic_json_document/source.md) text -- for example `#!cpp "1.50"`, `#!cpp "1E2"`, `#!cpp "-0"`, or an
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integer literal with more digits than any number type holds (such as a 30-digit integer, which [`get<T>()`](get.md)
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and [`materialize()`](materialize.md) can only represent approximately, as a `number_float_t`).
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## Exception safety
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Strong guarantee: there are no changes to the view or the document it refers to.
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## Exceptions
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Throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if the value is not a number; example:
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`"type must be number, but is string"`.
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This exception does not carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no
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`BasicJsonType` value to point at, so the exception is created without one, even if `BasicJsonType` was built with
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`JSON_DIAGNOSTICS` enabled.
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## Complexity
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Constant.
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## Notes
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`BasicJsonType` has no counterpart to this function: once a number is parsed into `number_integer_t`,
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`number_unsigned_t`, or `number_float_t`, its original textual form (leading zeros aside, which are already rejected
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by the grammar; trailing zeros in the fraction; the case and sign of the exponent; ...) is gone. `number_token()` is
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useful precisely where that form must survive -- a price or an identifier that must be reproduced exactly, or a
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number too large for any of `BasicJsonType`'s number types to hold without loss.
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The returned [`string_view_t`](index.md#member-types) always points into the document's
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[`source()`](../basic_json_document/source.md) text -- numbers are never decoded into the document's separate string
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buffer -- and is valid exactly as long as that text is, see the [validity rules](index.md) of `basic_json_view`.
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## Examples
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??? example
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The example below keeps a price and an order ID exactly as they were written in an incoming order, where
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converting them with [`get<T>()`](get.md) would lose information: the price picks up floating-point rounding, and
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the order ID -- more digits than a 64-bit integer holds -- can only be approximated as a `#!cpp double` once
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`#!cpp materialize()`d.
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```cpp
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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using json_document = nlohmann::json_document;
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using json_view = nlohmann::json_view;
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int main()
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{
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// a price and an order id from an incoming order -- both need to be
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// reproduced exactly, e.g. for an invoice or an audit log
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json_document doc = json_document::parse(R"(
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{"price": 19.90, "order_id": 1234567890123456789012345, "quantity": 3}
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)");
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const json_view order = doc.root();
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// number_token() returns the number exactly as written in the source
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std::cout << order["price"].number_token() << '\n';
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std::cout << order["order_id"].number_token() << '\n';
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// get<double>() converts it instead -- the exact source text is gone:
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// "19.90" becomes the double closest to 19.9, printed without the
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// trailing zero, and the 25-digit order id -- far beyond any 64-bit
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// integer -- can only be approximated as a double
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std::cout << order["price"].get<double>() << '\n';
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std::cout << order.materialize()["order_id"].dump() << '\n';
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// an ordinary quantity has nothing to lose either way
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std::cout << order["quantity"].number_token() << " == " << order["quantity"].get<int>() << '\n';
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}
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```
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Output:
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```json
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19.90
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1234567890123456789012345
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19.9
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1.2345678901234568e+24
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3 == 3
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```
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## See also
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- [get](get.md) - convert the value to a given type
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- [get_string](get_string.md) - the string, without a copy
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- [`BasicJsonType::number_integer_t`](../basic_json/number_integer_t.md),
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[`number_unsigned_t`](../basic_json/number_unsigned_t.md), [`number_float_t`](../basic_json/number_float_t.md) - the
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number types `#!cpp get<T>()` and `materialize()` convert into
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## Version history
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- Added in version 3.13.0.
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