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

JSON_USE_GLOBAL_UDLS

#define JSON_USE_GLOBAL_UDLS /* value */

When defined to 1, the user-defined string literals (UDLs) are placed into the global namespace instead of nlohmann::literals::json_literals.

Default definition

The default value is 1.

#define JSON_USE_GLOBAL_UDLS 1

When the macro is not defined, the library behaves as if it were defined to its default value.

Notes

!!! info "Future behavior change"

The user-defined string literals will be removed from the global namespace in the next major release of the library.

To prepare existing code, define `JSON_USE_GLOBAL_UDLS` to `0` and bring the string literals into scope where
needed. Refer to any of the [string literals](#see-also) for details.

See the [migration guide](../../integration/migration_guide.md#import-namespace-literals-for-udls) for how to update existing code.

!!! hint "CMake option"

The placement of user-defined string literals can also be controlled with the CMake option
[`JSON_GlobalUDLs`](../../integration/cmake.md#json_globaludls) (`ON` by default) which defines
`JSON_USE_GLOBAL_UDLS` accordingly.

!!! info "Leaving out the literals"

If [`JSON_NO_AUTOMATIC_UDLS`](json_no_automatic_udls.md) is defined, the literals are only declared where
`<nlohmann/json_literals.hpp>` is included; this macro then applies to that header.

Examples

??? example "Example: Default behavior"

The code below shows the default behavior using the `_json` UDL.

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

#include <iostream>

int main()
{
    auto j = "42"_json;

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

Output:

```json
42
```

??? example "Example: Namespaced UDLs"

The code below shows how UDLs need to be brought into scope before using `_json` when `JSON_USE_GLOBAL_UDLS` is
defined to `0`.

```cpp
#define JSON_USE_GLOBAL_UDLS 0
#include <nlohmann/json.hpp>

#include <iostream>

int main()
{
    // auto j = "42"_json; // This line would fail to compile,
                           // because the UDLs are not in the global namespace

    // Bring the UDLs into scope
    using namespace nlohmann::json_literals;

    auto j = "42"_json;

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

Output:

```json
42
```

See also

• operator""_json - user-defined string literal for JSON values
• operator""_json_pointer - user-defined string literal for JSON pointers
• JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals automatically
• :simple-cmake: JSON_GlobalUDLs - CMake option to control the macro
• Creating JSON values - the article on creating JSON values

Version history

• Added in version 3.11.0.