nlohmann::basic_json_view::number_token

string_view_t number_token() const;

Returns the number exactly as it appears in the source text, without parsing or rounding it.

Return value

The number's token text, as a string_view_t into the document's source() text -- for example #!cpp "1.50", #!cpp "1E2", #!cpp "-0", or an integer literal with more digits than any number type holds (such as a 30-digit integer, which get<T>() and materialize() can only represent approximately, as a number_float_t).

Exception safety

Strong guarantee: there are no changes to the view or the document it refers to.

Exceptions

Throws type_error.302 if the value is not a number; example: "type must be number, but is string".

This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

Complexity

Constant.

Notes

BasicJsonType has no counterpart to this function: once a number is parsed into number_integer_t, number_unsigned_t, or number_float_t, its original textual form (leading zeros aside, which are already rejected by the grammar; trailing zeros in the fraction; the case and sign of the exponent; ...) is gone. number_token() is useful precisely where that form must survive -- a price or an identifier that must be reproduced exactly, or a number too large for any of BasicJsonType's number types to hold without loss.

The returned string_view_t always points into the document's source() text -- numbers are never decoded into the document's separate string buffer -- and is valid exactly as long as that text is, see the validity rules of basic_json_view.

Examples

??? example

The example below keeps a price and an order ID exactly as they were written in an incoming order, where
converting them with [`get<T>()`](get.md) would lose information: the price picks up floating-point rounding, and
the order ID -- more digits than a 64-bit integer holds -- can only be approximated as a `#!cpp double` once
`#!cpp materialize()`d.

```cpp
#include <iostream>
#include <nlohmann/json_view.hpp>

using json_document = nlohmann::json_document;
using json_view = nlohmann::json_view;

int main()
{
    // a price and an order id from an incoming order -- both need to be
    // reproduced exactly, e.g. for an invoice or an audit log
    json_document doc = json_document::parse(R"(
      {"price": 19.90, "order_id": 1234567890123456789012345, "quantity": 3}
    )");
    const json_view order = doc.root();

    // number_token() returns the number exactly as written in the source
    std::cout << order["price"].number_token() << '\n';
    std::cout << order["order_id"].number_token() << '\n';

    // get<double>() converts it instead -- the exact source text is gone:
    // "19.90" becomes the double closest to 19.9, printed without the
    // trailing zero, and the 25-digit order id -- far beyond any 64-bit
    // integer -- can only be approximated as a double
    std::cout << order["price"].get<double>() << '\n';
    std::cout << order.materialize()["order_id"].dump() << '\n';

    // an ordinary quantity has nothing to lose either way
    std::cout << order["quantity"].number_token() << " == " << order["quantity"].get<int>() << '\n';
}

```

Output:

```json
19.90
1234567890123456789012345
19.9
1.2345678901234568e+24
3 == 3

```

Version history

Added in version 3.13.0.

4.3 KiB Raw Blame History