json/home/faq/index.md

# Frequently Asked Questions (FAQ)

## Known bugs

### Brace initialization yields arrays

Question

Why does

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

and

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

yield different results (`[true]` vs. `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.

To explicitly create a single-element array, use `json::array({value})`:

```
json j = json::array({true});  // [true]
```

**Opt-in copy semantics (since version 3.12.0)**

If you define `JSON_BRACE_INIT_COPY_SEMANTICS` to `1` before including the library, single-element brace initialization is treated as copy/move instead of creating a single-element array:

```
#define JSON_BRACE_INIT_COPY_SEMANTICS 1
#include <nlohmann/json.hpp>

json obj = {{"key", "value"}};
json j{obj};   // -> {"key":"value"}  (copy, not array)
```

Without the macro (default behavior), `json j{obj}` creates `[{"key":"value"}]`. This opt-in macro fixes issue #5074 while preserving backwards compatibility for existing code.

## Limitations

### Relaxed parsing

Question

Can you add an option to ignore trailing commas?

This library does not support any feature that 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](https://tools.ietf.org/html/rfc8259.html#section-8.1).
- `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.
- The library will not replace [Unicode noncharacters](http://www.unicode.org/faq/private_use.html#nonchar1).
- 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()`](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.

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](#parse-errors-reading-non-ascii-characters), the library assumes UTF-8 as encoding. To store a wide string, you need to change the encoding.

Example

```
#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:

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

## Usage

### Thread safety

Question

Is `basic_json` thread-safe?

No. `basic_json` provides no built-in synchronization, the same as `std::map` or `std::vector`. Concurrent reads of the same value from multiple threads are safe, as are concurrent (non-overlapping) accesses to independent `json` objects. However, any concurrent write to a `json` object -- or a concurrent read while another thread writes to the same object -- is a data race and requires external synchronization (e.g., a `std::mutex`) by the caller.

### Schema validation

Question

Does this library support JSON Schema validation?

Not directly, but the companion project [json-schema-validator](https://github.com/pboettch/json-schema-validator) builds JSON Schema (draft 4, 6, 7, and 2019-09) validation on top of this library and is a common recommendation for this use case.

## Exceptions

### Parsing without exceptions

Question

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

Yes, see [Parsing and exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md).

### 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`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md) to get [extended diagnostics messages](https://json.nlohmann.me/home/exceptions/#extended-diagnostic-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 `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.

[cppreference.com](https://en.cppreference.com/w/cpp/types/numeric_limits/digits10)

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](https://json.nlohmann.me/features/types/number_handling/#number-serialization) on the library's number handling for more information.

### Using JSON values with `std::format` or `fmt`

Question

- Can I use `std::format("{}", j)` on a JSON value?
- Can I use `fmt::format("{}", j)` or `fmt::print("{}", j)` (the [{fmt}](https://github.com/fmtlib/fmt) library) on a JSON value?

`std::format` works out of the box since version 3.13.0, as long as the standard library provides `<format>` (see [`JSON_HAS_STD_FORMAT`](https://json.nlohmann.me/api/macros/json_has_std_format/index.md)); see [`std::formatter<basic_json>`](https://json.nlohmann.me/api/basic_json/std_formatter/index.md) for details, including the `"{:#}"` pretty-print spec, indent widths (`"{:2}"`), and custom indent characters (`"{:.>#}"`).

For `fmt`, the library ships [`format_as`](https://json.nlohmann.me/api/basic_json/format_as/index.md), a small customization point `fmt` looks for via argument-dependent lookup. It only has an effect on fmt 10.0.0 through 11.0.2 — from fmt 11.1.0 onwards, `fmt` no longer picks up a `format_as` overload that returns a `std::string`. On such versions (or any version, if you also want the same `"{:#}"`/width/fill-and-align spec support that `std::formatter<basic_json>` has), define your own `fmt::formatter` specialization; see [`format_as`](https://json.nlohmann.me/api/basic_json/format_as/index.md) for a recipe that mirrors it.

If you get ambiguous-overload errors when passing a JSON value to `fmt::format`/`fmt::print` without any `fmt::formatter<json>` specialization in scope, that's `fmt` picking up `basic_json`'s implicit `operator ValueType()` conversion operator (see [#964](https://github.com/nlohmann/json/issues/964) and [#958](https://github.com/nlohmann/json/issues/958)); disabling it via [`JSON_USE_IMPLICIT_CONVERSIONS 0`](https://json.nlohmann.me/api/macros/json_use_implicit_conversions/index.md) avoids the ambiguity.

## 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](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.

### 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](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).