# JSON_BRACE_INIT_COPY_SEMANTICS

```
#define JSON_BRACE_INIT_COPY_SEMANTICS /* value */
```

When defined to `1`, single-element brace initialization of a `basic_json` value is treated as a copy/move of the element rather than wrapping it in a single-element array.

## Default definition

The default value is `0` (disabled — existing behavior is preserved).

```
#define JSON_BRACE_INIT_COPY_SEMANTICS 0
```

## Notes

Background

C++ always prefers the `initializer_list` constructor over the copy/move constructor for brace initialization. This means that code like

```
json obj = {{"key", "value"}};
json j{obj};
```

creates a single-element **array** `[{"key":"value"}]` instead of a copy of `obj`. This behavior is compiler-dependent for older compilers (GCC wrapped, Clang did not), but starting from Clang 20, both compilers behave the same way.

Enabling this macro opts into copy/move semantics for this case (see [#5074](https://github.com/nlohmann/json/issues/5074)).

Opt-in only

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

Workaround without the macro

To explicitly create a single-element array without enabling this macro, use `json::array()`:

```
json j = json::array({obj});  // always creates [obj]
```

## Examples

Default behavior (macro not defined)

Without the macro, single-element brace initialization wraps the value in an array:

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

using json = nlohmann::json;

int main()
{
    json obj = {{"key", "value"}};

    json j{obj};
    // j is [{"key":"value"}]  -- single-element array, NOT a copy of obj
}
```

Opt-in copy semantics (macro defined to 1)

With the macro, single-element brace initialization copies/moves the value:

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

using json = nlohmann::json;

int main()
{
    json obj = {{"key", "value"}};

    json j{obj};
    // j is {"key":"value"}  -- copy of obj
}
```

## See also

- [FAQ: Brace initialization yields arrays](https://json.nlohmann.me/home/faq/#brace-initialization-yields-arrays)
- [**basic_json(initializer_list_t)**](https://json.nlohmann.me/api/basic_json/basic_json/index.md) - the affected constructor

## Version history

- Added in version 3.12.x.