json/doc/mkdocs/docs/home/faq.md

Frequently Asked Questions (FAQ)

Known bugs

Brace initialization yields arrays

!!! question

Why does

```cpp
json j{true};
```

and

```cpp
json j(true);
```

yield different results (`#!json [true]` vs. `#!json true`)?

This is a known issue, and -- even worse -- the behavior differs between GCC and Clang. The "culprit" for this is the library's constructor overloads for initializer lists to allow syntax like

json array = {1, 2, 3, 4};

for arrays and

json object = {{"one", 1}, {"two", 2}}; 

for objects.

!!! tip

To avoid any confusion and ensure portable code, **do not** use brace initialization with the types `basic_json`, `json`, or `ordered_json` unless you want to create an object or array as shown in the examples above.

Limitations

Relaxed parsing

!!! question

Can you add an option to ignore trailing commas?

This library does not support any feature which would jeopardize interoperability.

Parse errors reading non-ASCII characters

!!! question "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.

Wide string handling

!!! question

Why are wide strings (e.g., `std::wstring`) dumped as arrays of numbers?

As described above, the library assumes UTF-8 as encoding. To store a wide string, you need to change the encoding.

!!! example

```cpp
#include <codecvt> // codecvt_utf8
#include <locale>  // wstring_convert

// encoding function
std::string to_utf8(std::wstring& wide_string)
{
    static std::wstring_convert<std::codecvt_utf8<wchar_t>> utf8_conv;
    return utf8_conv.to_bytes(wide_string);
}

json j;
std::wstring ws = L"車B1234 こんにちは";

j["original"] = ws;
j["encoded"] = to_utf8(ws);

std::cout << j << std::endl;
```

The result is:

```json
{
  "encoded": "車B1234 こんにちは",
  "original": [36554, 66, 49, 50, 51, 52, 32, 12371, 12435, 12395, 12385, 12399]
}
```

Exceptions

Parsing without exceptions

!!! question

Is it possible to indicate a parse error without throwing an exception?

Yes, see Parsing and exceptions.

Key name in exceptions

!!! question

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

Yes, you can. Please define the symbol JSON_DIAGNOSTICS to get extended diagnostics messages.

Serialization issues

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.

!!! quote "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.

See this section on the library's number handling for more information.

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

!!! question "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.