json/api/macros/json_has_filesystem.md

JSON_HAS_FILESYSTEM / JSON_HAS_EXPERIMENTAL_FILESYSTEM

#define JSON_HAS_FILESYSTEM /* value */
#define JSON_HAS_EXPERIMENTAL_FILESYSTEM /* value */

When compiling with C++17, the library provides conversions from and to std::filesystem::path. As compiler support for filesystem is limited, the library tries to detect whether <filesystem>/std::filesystem (JSON_HAS_FILESYSTEM) or <experimental/filesystem>/std::experimental::filesystem (JSON_HAS_EXPERIMENTAL_FILESYSTEM) should be used. To override the built-in check, define JSON_HAS_FILESYSTEM or JSON_HAS_EXPERIMENTAL_FILESYSTEM to 1.

Default definition

The default value is detected based on the preprocessor macros #!cpp __cpp_lib_filesystem, #!cpp __cpp_lib_experimental_filesystem, #!cpp __has_include(<filesystem>), or #!cpp __has_include(<experimental/filesystem>).

!!! info "Known compiler/stdlib exclusions"

Even when the feature-test macro indicates filesystem support is available, the library disables it on the following broken toolchains:

- **MinGW + GCC 8** — disabled entirely (broken `std::filesystem` implementation; [MinGW-w64 bug 737](https://sourceforge.net/p/mingw-w64/bugs/737/))
- **GCC (non-Clang) < 8** — disabled (no filesystem support)
- **Clang < 7** — disabled (no filesystem support)
- **MSVC < 19.14** — disabled (no filesystem support)
- **iOS < 13** — disabled (no filesystem support)
- **macOS < Catalina (10.15)** — disabled (no filesystem support)

If `JSON_HAS_FILESYSTEM` or `JSON_HAS_EXPERIMENTAL_FILESYSTEM` is `0` despite `__cpp_lib_filesystem` being defined, one
of the exclusions above likely applies to your toolchain.

Notes

• Note that older compilers or older versions of libstdc++ also require the library stdc++fs to be linked to for filesystem support.
• Both macros are undefined outside the library.

Examples

??? example

The code below forces the library to use the header `<experimental/filesystem>`.

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

...
```

Version history

• Added in version 3.10.5.