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Frequently Asked Questions (FAQ)

Limitations

Comments

Questions

  • Why does the library not support comments?
  • Can you add support for JSON5/JSONC/HOCON so that comments are supported?

This library does not support comments. It does so for three reasons:

  1. Comments are not part of the JSON specification. You may argue that // or /* */ are allowed in JavaScript, but JSON is not JavaScript.
  2. This was not an oversight: Douglas Crockford wrote on this in May 2012:

    I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.

    Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.

  3. It is dangerous for interoperability if some libraries would add comment support while others don't. Please check The Harmful Consequences of the Robustness Principle on this.

This library will not support comments in the future. If you wish to use comments, I see three options:

  1. Strip comments before using this library.
  2. Use a different JSON library with comment support.
  3. Use a format that natively supports comments (e.g., YAML or JSON5).

Relaxed parsing

Question

  • Can you add an option to ignore trailing commas?

For the same reason this library does not support comments, this library also does not support any feature which would jeopardize interoperability.

Parse errors reading non-ASCII characters

Questions

  • Why is the parser complaining about a Chinese character?
  • Does the library support Unicode?
  • 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$')"

The library supports Unicode input as follows:

  • Only UTF-8 encoded input is supported which is the default encoding for JSON according to RFC 8259.
  • 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.
  • Other encodings such as Latin-1 or ISO 8859-1 are not supported and will yield parse or serialization errors.
  • Unicode noncharacters will not be replaced by the library.
  • Invalid surrogates (e.g., incomplete pairs such as \uDEAD) will yield parse errors.
  • 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.
  • When you store strings with different encodings in the library, calling dump() may throw an exception unless json::error_handler_t::replace or json::error_handler_t::ignore are used as error handlers.

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.

Key name in exceptions

Question

Can I get the key of the object item that caused an exception?

No, this is not possible. See https://github.com/nlohmann/json/issues/932 for a longer discussion.

Serialization issues

Order of object keys

Questions

  • Why are object keys sorted?
  • Why is the insertion order of object keys not preserved?

By default, the library does not preserve the insertion order of object elements. This is standards-compliant, as the JSON standard defines objects as "an unordered collection of zero or more name/value pairs".

If you do want to preserve the insertion order, you can specialize the object type with containers like tsl::ordered_map (integration) or nlohmann::fifo_map (integration).

Number precision

Question

  • It seems that precision is lost when serializing a double.
  • Can I change the precision for floating-point serialization?

The library uses std::numeric_limits<number_float_t>::digits10 (15 for IEEE doubles) 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.

cppreference.com

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.

Tip

The website https://float.exposed gives a good insight into the internal storage of floating-point numbers.

Compilation issues

Android SDK

Question

Why does the code not compile with Android SDK?

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.

APP_STL := c++_shared
NDK_TOOLCHAIN_VERSION := clang3.6
APP_CPPFLAGS += -frtti -fexceptions

The code compiles successfully with Android NDK, Revision 9 - 11 (and possibly later) and CrystaX's Android NDK version 10.

Missing STL function

Questions

  • Why do I get a compilation error 'to_string' is not a member of 'std' (or similarly, for strtod or strtof)?
  • Why does the code not compile with MinGW or Android SDK?

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 and this discussion for information on how to fix this bug. For Android NDK using APP_STL := gnustl_static, please refer to this discussion.