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Support zero-member types in NLOHMANN_DEFINE_TYPE_* macros (#4041) #5272

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Support zero-member types in NLOHMANN_DEFINE_TYPE_* macros (#4041) #5272

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d969eef
Support zero-member types in NLOHMANN_DEFINE_TYPE_* macros (#4041)
nlohmann Jul 11, 2026
9d83673
Fix CI failures in zero-member NLOHMANN_DEFINE_TYPE_* macros
nlohmann Jul 11, 2026
4dcd45b
Fix clang-tidy misc-const-correctness in unit-udt_macro.cpp
nlohmann Jul 12, 2026
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18 changes: 18 additions & 0 deletions docs/mkdocs/docs/features/arbitrary_types.md
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Original file line number Diff line number Diff line change
Expand Up @@ -215,6 +215,24 @@ For _derived_ classes and structs, use the following macros
nlohmann::ordered_json j = p; // keys appear in declaration order: name, address, age
```

!!! note "Zero-member types"

All 12 `NLOHMANN_DEFINE_TYPE_*`/`NLOHMANN_DEFINE_DERIVED_TYPE_*` macros (excluding the `WITH_NAMES` variants)
also accept types with no member variables to serialize, producing/accepting an empty JSON object `{}`
(or, for the derived-type macros, just the base class's own JSON representation):

```cpp
namespace ns {
struct marker {
bool operator==(const marker&) const { return true; }
NLOHMANN_DEFINE_TYPE_INTRUSIVE(marker)
};
}

ns::marker m{};
nlohmann::json j = m; // {}
```

!!! note "No macro for non-default-constructible types"

There is currently no `NLOHMANN_DEFINE_TYPE_*`-style macro for types that are not
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