92 lines
4.6 KiB
Markdown
92 lines
4.6 KiB
Markdown
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# Frequently Asked Questions (FAQ)
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## Limitations
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### Relaxed parsing
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!!! question
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- Can you add an option to ignore trailing commas?
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For the same reason this library does not support [comments](#comments), this library also does not support any feature which would jeopardize interoperability.
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### Parse errors reading non-ASCII characters
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!!! question "Questions"
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- Why is the parser complaining about a Chinese character?
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- Does the library support Unicode?
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- I get an exception `[json.exception.parse_error.101] parse error at line 1, column 53: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '"Testé$')"`
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The library supports **Unicode input** as follows:
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- Only **UTF-8** encoded input is supported which is the default encoding for JSON according to [RFC 8259](https://tools.ietf.org/html/rfc8259.html#section-8.1).
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- `std::u16string` and `std::u32string` can be parsed, assuming UTF-16 and UTF-32 encoding, respectively. These encodings are not supported when reading from files or other input containers.
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- Other encodings such as Latin-1 or ISO 8859-1 are **not** supported and will yield parse or serialization errors.
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- [Unicode noncharacters](http://www.unicode.org/faq/private_use.html#nonchar1) will not be replaced by the library.
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- Invalid surrogates (e.g., incomplete pairs such as `\uDEAD`) will yield parse errors.
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- The strings stored in the library are UTF-8 encoded. When using the default string type (`std::string`), note that its length/size functions return the number of stored bytes rather than the number of characters or glyphs.
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- When you store strings with different encodings in the library, calling [`dump()`](https://nlohmann.github.io/json/classnlohmann_1_1basic__json_a50ec80b02d0f3f51130d4abb5d1cfdc5.html#a50ec80b02d0f3f51130d4abb5d1cfdc5) may throw an exception unless `json::error_handler_t::replace` or `json::error_handler_t::ignore` are used as error handlers.
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In most cases, the parser is right to complain, because the input is not UTF-8 encoded. This is especially true for Microsoft Windows where Latin-1 or ISO 8859-1 is often the standard encoding.
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### Key name in exceptions
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!!! question
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Can I get the key of the object item that caused an exception?
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No, this is not possible. See <https://github.com/nlohmann/json/issues/932> for a longer discussion.
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## Serialization issues
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### Number precision
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!!! question
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- It seems that precision is lost when serializing a double.
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- Can I change the precision for floating-point serialization?
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The library uses `std::numeric_limits<number_float_t>::digits10` (15 for IEEE `double`s) digits for serialization. This value is sufficient to guarantee roundtripping. If one uses more than this number of digits of precision, then string -> value -> string is not guaranteed to round-trip.
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!!! quote "[cppreference.com](https://en.cppreference.com/w/cpp/types/numeric_limits/digits10)"
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The value of `std::numeric_limits<T>::digits10` is the number of base-10 digits that can be represented by the type T without change, that is, any number with this many significant decimal digits can be converted to a value of type T and back to decimal form, without change due to rounding or overflow.
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!!! tip
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The website https://float.exposed gives a good insight into the internal storage of floating-point numbers.
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## Compilation issues
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### Android SDK
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!!! question
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Why does the code not compile with Android SDK?
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Android defaults to using very old compilers and C++ libraries. To fix this, add the following to your `Application.mk`. This will switch to the LLVM C++ library, the Clang compiler, and enable C++11 and other features disabled by default.
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```ini
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APP_STL := c++_shared
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NDK_TOOLCHAIN_VERSION := clang3.6
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APP_CPPFLAGS += -frtti -fexceptions
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```
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The code compiles successfully with [Android NDK](https://developer.android.com/ndk/index.html?hl=ml), Revision 9 - 11 (and possibly later) and [CrystaX's Android NDK](https://www.crystax.net/en/android/ndk) version 10.
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### Missing STL function
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!!! question "Questions"
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- Why do I get a compilation error `'to_string' is not a member of 'std'` (or similarly, for `strtod` or `strtof`)?
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- Why does the code not compile with MinGW or Android SDK?
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This is not an issue with the code, but rather with the compiler itself. On Android, see above to build with a newer environment. For MinGW, please refer to [this site](http://tehsausage.com/mingw-to-string) and [this discussion](https://github.com/nlohmann/json/issues/136) for information on how to fix this bug. For Android NDK using `APP_STL := gnustl_static`, please refer to [this discussion](https://github.com/nlohmann/json/issues/219).
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