Niels Lohmann
63c10a51fc
* Review and extend the documentation, and check it in CI A review of all documentation pages found factual errors, dead links, missing cross-references, and gaps in examples. This fixes them and adds checks so the same problems are caught automatically. Fixes: - wrong signatures and version histories (operator!= C++20 member, binary() subtype type, get<PointerType>(), JSON_NO_THREAD_LOCAL, ...) - stale descriptions (number parsing since #5283, UBJSON table, SAX example that no longer compiled, tsl::ordered_map advice) - dead internal and external links; repology.org badges (the domain is suspended) replaced by badges that query the registries directly - deprecation notes link the migration guide; the guide itself fixed Additions: - "See also" sections, cross-references, 25 runnable examples, 12 Mermaid diagrams, new API pages for json_pointer::operator<=> and byte_container_with_subtype::operator==/!= - landing page, guides for untrusted input and performance - "unreleased" badge after versions newer than the latest release Checks: - strict documentation build (broken links/anchors fail it); CI and the publish workflow fetch the full history the build needs - weekly external link check, Mermaid syntax check in CI - check_structure.py: example titles, heading levels, alt texts, header links, docset index coverage; its unused-example check works again - all examples produce the same output on every platform Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Keep the customer links that could not be fixed A dead link on the customers page is still the evidence of where the use of the library was documented. Keep the original URLs of the entries without a working replacement (Marne, Cisco Webex Desk Camera, Philips Hue, CyberArk) and exclude exactly these URLs from the link check. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Correct the duplicate-key recipe's claim about SAX positions The SAX interface's key() receives no position either; only parse_error() does. Also note that the recipe does not report the path to the repeated key (see discussion #5085). Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Say the library is available as a single header and mention json_fwd.hpp Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Correct documentation errors found while hunting for bugs - patch/patch_inplace: list the JSON pointer errors parse_error.106-109 and out_of_range.402/404, and quote the actual parse_error.105 message. - unflatten: list parse_error.106/107/108 and out_of_range.404. - to_bson: list out_of_range.415 (binary subtype above 255) and note that 412 and 415 are new in 3.13.0. - to_string: state that string_t must be convertible to std::string, also in the StringType requirements table. - JSON Lines: a `while (input >> j)` loop also throws after the last value for concatenated JSON values; show a loop that works for both. - BON8: a string gets 0xFF only if nothing follows it in the message; a string at the end of an array or object is ended by 0xFE. - custom_string_type.hpp: add operator+=(char), which the "Always required" list asks for (json_pointer::to_string, flatten, unflatten, and diff did not compile), and an ADL int_to_string for diff and items. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Cache the release headers with functools.lru_cache Codacy (Pylint) flagged the mutable default argument that header() used as its cache. functools.lru_cache keeps the same memoization without it. The script's output is unchanged. Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
3.8 KiB
nlohmann::basic_json::number_float_t
using number_float_t = NumberFloatType;
The type used to store JSON numbers (floating-point).
RFC 8259 describes numbers as follows:
The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.
This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is
known whether the number is a signed integer, an unsigned integer, or a floating-point number. Therefore, three different
types, number_integer_t, number_unsigned_t and number_float_t are
used.
To store floating-point numbers in C++, a type is defined by the template parameter NumberFloatType which chooses the
type to use.
Template parameters
NumberFloatType- the type to store floating-point numbers. Parsing and serialization are implemented in terms of
#!cpp std::strtof/#!cpp std::strtod/#!cpp std::strtoldand#!cpp std::snprintf, so the type must be#!cpp float,#!cpp double, or#!cpp long double. The binary formats additionally require#!cpp floator#!cpp double, because they have no encoding for#!cpp long double. See Template Parameter Requirements.
Notes
Default type
With the default values for NumberFloatType (double), the default value for number_float_t is #!cpp double.
Default behavior
- The restrictions about leading zeros are not enforced in C++. Instead, leading zeros in floating-point literals will
be ignored. Internally, the value will be stored as a decimal number. For instance, the C++ floating-point literal
01.2will be serialized to1.2. During deserialization, leading zeros yield an error. - Not-a-number (NaN) values will be serialized to
null.
Limits
RFC 8259 states:
This specification allows implementations to set limits on the range and precision of numbers accepted. Since software that implements IEEE 754-2008 binary64 (double precision) numbers is generally available and widely used, good interoperability can be achieved by implementations that expect no more precision or range than these provide, in the sense that implementations will approximate JSON numbers within the expected precision.
This implementation does exactly follow this approach, as it uses double precision floating-point numbers. Note values
smaller than -1.79769313486232e+308 and values greater than 1.79769313486232e+308 will be stored as NaN internally
and be serialized to null.
Storage
Floating-point number values are stored directly inside a basic_json type.
Examples
??? example
The following code shows that `number_float_t` is by default, a typedef to `#!cpp double`.
```cpp
--8<-- "examples/number_float_t.cpp"
```
Output:
```json
--8<-- "examples/number_float_t.output"
```
See also
- number_integer_t the type used to store JSON integer numbers
- number_unsigned_t the type used to store JSON unsigned integer numbers
- is_number_float checks whether the JSON value is a floating-point number
- Number Handling - the article on number handling
Version history
- Added in version 1.0.0.