json/api/basic_json_view/number_format.md

nlohmann::basic_json_view::number_format

enum class number_format {
    shortest,
    source
};

This enumeration is used in dump to choose how numbers are written. Two values are differentiated:

shortest

integers are copied from the source text -- already canonical in JSON -- except that #!cpp -0 becomes #!cpp 0, the way BasicJsonType::parse() reads it; floats are written with the library's shortest round-trip conversion, exactly as BasicJsonType::dump() would (e.g. #!cpp 1.5, #!cpp 100.0, #!cpp 1e+100)

source

every number is copied exactly as it appears in the source text -- #!cpp 1.50, #!cpp 1E2, #!cpp -0, all digits of an integer literal with more digits than any number type holds -- something BasicJsonType cannot do, since parsing already reduces every number to its parsed value

Examples

??? example

The example below writes back a price list received from a supplier: with `number_format::shortest` (the
default), a trailing zero and scientific notation are normalized away and a long account number that overflows
every number type is rounded, the same way `#!cpp materialize().dump()` (or `basic_json::dump()`) would;
`number_format::source` keeps every number exactly as it was written in the source text instead.

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

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

int main()
{
    // a price list received from a supplier feed -- prices and account
    // numbers must be forwarded exactly, e.g. into an invoice
    const json_document doc = json_document::parse(R"(
      [{"sku": "A1", "price": 19.90, "account_id": 12345678901234567890123456},
       {"sku": "A2", "price": 1E2, "account_id": 98765432109876543210987654}]
    )");
    const json_view list = doc.root();

    // number_format::shortest (the default) writes numbers the way
    // basic_json::dump() would: "19.90" becomes "19.9", "1E2" becomes
    // "100.0", and each account number -- far beyond any 64-bit integer --
    // is rounded to the nearest double, exactly as materialize().dump()
    // (or a plain nlohmann::json) would round it
    std::cout << list.dump() << '\n';

    // number_format::source copies every number exactly as it was written
    // in the source text instead -- something basic_json cannot do at all,
    // since parsing already reduces every number to its parsed value
    std::cout << list.dump(-1, ' ', false, json_view::number_format::source) << '\n';
}

```

Output:

```json
[{"sku":"A1","price":19.9,"account_id":1.2345678901234568e+25},{"sku":"A2","price":100.0,"account_id":9.876543210987655e+25}]
[{"sku":"A1","price":19.90,"account_id":12345678901234567890123456},{"sku":"A2","price":1E2,"account_id":98765432109876543210987654}]

```

See also

• dump - serialize to a JSON-formatted string
• number_token - get a single number's token text without dumping the whole value
• BasicJsonType::error_handler_t - the analogous enumeration for BasicJsonType::dump's decoding-error behavior

Version history

• Added in version 3.13.0.