deploy: e9c3985f0a

features/conversions.md

@@ -84,6 +84,49 @@ which forces the explicit `get` form and can catch unintended conversions at com
     j_null.get_to(opt);                                     // ✅ std::nullopt
     ```
 
+!!! warning "`static_cast` and `get<std::optional<T>>()` are not guaranteed equivalent"
+
+    `operator ValueType()` (used by `static_cast` and implicit conversions) intentionally excludes
+    `std::optional<T>` from delegating to `get<T>()`, to avoid a constructor ambiguity with
+    `std::optional<T>`'s own converting constructor from `basic_json`. As a result,
+    `static_cast<std::optional<T>>(json_value)` goes through `std::optional<T>`'s own converting
+    constructor rather than through `get<std::optional<T>>()`, which can behave differently -- for example,
+    with a custom `adl_serializer<std::optional<T>>` specialization. Prefer `get<std::optional<T>>()`/`get_to()`
+    over `static_cast` for optional types.
+
+!!! warning "Converting to a fixed-size `std::array` does not check length"
+
+    Converting a JSON array to `#!cpp std::array<T, N>` does not check that the JSON array's size matches `N`:
+    if the JSON array is longer, the extra elements are silently dropped; if it is shorter, the remaining
+    `std::array` elements are left default-constructed. No exception is thrown in either case.
+
+    ```cpp
+    json j = {1, 2, 3, 4, 5};
+    auto a = j.get<std::array<int, 3>>();  // {1, 2, 3} -- elements 4 and 5 silently dropped
+    ```
+
+## Omitting a field when serializing `std::optional`
+
+By default, `to_json` for `std::optional<T>` writes either the value or `#!json null` -- there is no built-in way
+to make a field disappear from the serialized object entirely when the `std::optional` is `std::nullopt`. Because
+a specialization of `adl_serializer<std::optional<T>>` only controls how the *value* is converted (it cannot
+prevent the containing object's `to_json` from inserting the key in the first place), omission has to be
+implemented in the *containing* type's `to_json`:
+
+```cpp
+struct person {
+    std::string name;
+    std::optional<int> age;
+};
+
+void to_json(json& j, const person& p) {
+    j = json{{"name", p.name}};
+    if (p.age) {
+        j["age"] = *p.age;  // key is only inserted when the optional has a value
+    }
+}
+```
+
 ## Putting values in
 
 The reverse direction works the same way: assigning or constructing a `json` from a C++ value converts it to JSON.