json/docs/mkdocs/docs/api/macros/json_strict_binary_utf8.md

JSON_STRICT_BINARY_UTF8

#define JSON_STRICT_BINARY_UTF8 /* value */

When defined to 1, the binary writers to_cbor, to_ubjson, to_bjdata, and to_bson check every string value and object key for valid UTF-8 and throw type_error.316 for ill-formed UTF-8, like dump does. Without it, they write the bytes unchanged.

The macro does not affect:

• to_msgpack: the MessagePack specification allows a str value to contain bytes that are not valid UTF-8, so it always writes them unchanged.
• to_bon8: BON8 always checks, because the UTF-8 lead bytes mark where a string ends.
• The binary readers (from_cbor, from_msgpack, from_ubjson, from_bjdata, from_bson): none of these formats requires a decoder to reject ill-formed UTF-8, so they always return the bytes unchanged.

Default definition

The default value is 0 (disabled, the behavior of version 3.12.0 and earlier is preserved).

#define JSON_STRICT_BINARY_UTF8 0

Notes

!!! note "Background"

CBOR, UBJSON, BJData, and BSON all require strings to be UTF-8. Up to version 3.12.0, the writers did not check
this, so they could produce output that other decoders reject. Checking by default would break code that stores
other encodings (for instance ISO 8859-1) in a string and only ever writes it to a binary format, so this macro
offers the check as an opt-in ahead of version 4.0.0, where it is planned to become the default (see
[#5529](https://github.com/nlohmann/json/issues/5529) and [#5651](https://github.com/nlohmann/json/issues/5651)).

!!! warning "Opt-in only"

This macro must be defined **before** including `<nlohmann/json.hpp>`. Defining it after the include has no
effect.

!!! note "ABI compatibility"

The value of this macro is encoded in the [namespace](../../features/namespace.md) (tag `_sbu8`), resulting in
distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program
without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

Examples

??? example "Default behavior (macro not defined)"

Without the macro, the bytes are written unchanged:

```cpp
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    auto v = json::to_cbor(json("\xFF"));
    // v is {0x61, 0xFF}
}
```

??? example "Opt-in check (macro defined to 1)"

With the macro, ill-formed UTF-8 is rejected:

```cpp
#define JSON_STRICT_BINARY_UTF8 1
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    auto v = json::to_cbor(json("\xFF"));
    // throws type_error.316: invalid UTF-8 byte at index 0: 0xFF
}
```

See also

• to_cbor - create a CBOR serialization of a JSON value
• to_ubjson - create a UBJSON serialization of a JSON value
• to_bjdata - create a BJData serialization of a JSON value
• to_bson - create a BSON serialization of a JSON value
• error_handler_t - how dump treats ill-formed UTF-8

Version history

• Added in version 3.13.0.
• Planned to become the default (with the macro removed) in version 4.0.0.