Test discarded views with is_discarded() in the operator[] docs and examples

The explicit conversion to bool of basic_json_view is gone.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/operator[].md

@@ -93,7 +93,7 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 Unlike `BasicJsonType::operator[]`, which is undefined behavior (guarded by a
 [runtime assertion](../../features/assertions.md)) for a missing key on a **const** value, this operator returns a
 safe, testable result for a missing key or an index out of range: a [discarded](is_discarded.md) view, which is
-`#!cpp false` in a boolean context.
+tested with [`is_discarded`](is_discarded.md).
 There is also no non-const overload that inserts a missing key or extends an array -- a view never modifies the
 document.
 

docs/mkdocs/docs/examples/basic_json_view__operator[].cpp

@@ -22,17 +22,19 @@ int main()
         std::cout << user["name"].materialize().dump();
 
         // operator[] on a missing object key gives a discarded view -- test
-        // it with a plain "if". The const overload of json::operator[]
+        // it with is_discarded(). The const overload of json::operator[]
         // would instead be undefined behavior (guarded by an assertion) for
         // a missing key
-        if (const auto email = user["email"])
+        const auto email = user["email"];
+        if (!email.is_discarded())
         {
             std::cout << " <" << email.materialize().dump() << ">";
         }
 
         // the same holds for an array index past the end: a discarded view,
         // not undefined behavior
-        if (const auto first_tag = user["tags"][0])
+        const auto first_tag = user["tags"][0];
+        if (!first_tag.is_discarded())
         {
             std::cout << " #" << first_tag.materialize().dump();
         }

docs/mkdocs/docs/examples/basic_json_view__operator[]_json_pointer.cpp

@@ -25,7 +25,8 @@ int main()
     // a missing key or an out-of-range index along the path gives a
     // discarded view, exactly where const json::operator[] would be
     // undefined behavior for the same pointer
-    if (const auto missing = root[json_pointer("/region/servers/5/metrics/cpu")])
+    const auto missing = root[json_pointer("/region/servers/5/metrics/cpu")];
+    if (!missing.is_discarded())
     {
         std::cout << missing.materialize().dump() << '\n';
     }