json/api/macros/json_brace_init_copy_semantics.md

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

!!! note "Background"

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

```cpp
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)).

!!! warning "Opt-in only"

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

!!! tip "Workaround without the macro"

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

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

Examples

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

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

```cpp
#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
}
```

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

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

```cpp
#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
• basic_json(initializer_list_t) - the affected constructor

Version history

• Added in version 3.12.x.