Return a better error when a user tries to cast a document that has already been accessed to a value

doc/basics.md

@@ -542,9 +542,7 @@ support for users who avoid exceptions. See [the simdjson error handling documen
 * **Tree Walking and JSON Element Types:** Sometimes you don't necessarily have a document
   with a known type, and are trying to generically inspect or walk over JSON elements.
   You can also represent arbitrary JSON values with
-  `ondemand::value` instances: it can represent anything except a scalar document (lone number, string, null or Boolean). You can check for scalar documents with the method `scalar()`.
-  You can query the type of a document or a value with the `type()` method.
-  The `type()` method does not consume or validate documents and values, but it tells you whether they are
+  `ondemand::value` instances: it can represent anything except a scalar document (lone number, string, null or Boolean). You can check for scalar documents with the method `scalar()`. You can cast a document that is either an array or an object to an `ondemand::value` instance immediately after you create the document instance: you cannot create a `ondemand::value` instance from a document that has already been accessed as it would mean that you would have two instances of the object or array simultaneously (see [rewinding](#rewinding)). You can query the type of a document or a value with the `type()` method. The `type()` method does not consume or validate documents and values, but it tells you whether they are
   - arrays (`json_type::array`),
   - objects (`json_type::object`)
   - numbers (`json_type::number`),

include/simdjson/error.h

@@ -42,7 +42,7 @@ enum error_code {
   INVALID_URI_FRAGMENT,       ///< Invalid URI fragment
   UNEXPECTED_ERROR,           ///< indicative of a bug in simdjson
   PARSER_IN_USE,              ///< parser is already in use.
-  OUT_OF_ORDER_ITERATION,     ///< tried to iterate an array or object out of order
+  OUT_OF_ORDER_ITERATION,     ///< tried to iterate an array or object out of order (checked when SIMDJSON_DEVELOPMENT_CHECKS=1)
   INSUFFICIENT_PADDING,       ///< The JSON doesn't have enough padding for simdjson to safely parse it.
   INCOMPLETE_ARRAY_OR_OBJECT, ///< The document ends early.
   SCALAR_DOCUMENT_AS_VALUE,   ///< A scalar document is treated as a value.
@@ -51,6 +51,13 @@ enum error_code {
   NUM_ERROR_CODES
 };
 
+/**
+ * It is the convention throughout the code that  the macro SIMDJSON_DEVELOPMENT_CHECKS determines whether
+ * we check for OUT_OF_ORDER_ITERATION. The logic behind it is that these errors only occurs when the code
+ * that was written while breaking some simdjson::ondemand requirement. They should not occur in released
+ * code after these issues were fixed.
+ */
+
 /**
  * Get the error message for the given error code.
  *

include/simdjson/generic/ondemand/document-inl.h

@@ -68,7 +68,17 @@ simdjson_inline simdjson_result<object> document::start_or_resume_object() noexc
 simdjson_inline simdjson_result<value> document::get_value() noexcept {
   // Make sure we start any arrays or objects before returning, so that start_root_<object/array>()
   // gets called.
-  iter.assert_at_document_depth();
+
+  // It is the convention throughout the code that  the macro `SIMDJSON_DEVELOPMENT_CHECKS` determines whether
+  // we check for OUT_OF_ORDER_ITERATION. Proper on::demand code should never trigger this error.
+#if SIMDJSON_DEVELOPMENT_CHECKS
+  if (!iter.at_root()) { return OUT_OF_ORDER_ITERATION; }
+#endif
+  // assert_at_root() serves two purposes: in Debug mode, whether or not
+  // SIMDJSON_DEVELOPMENT_CHECKS is set or not, it checks that we are at the root of
+  // the document (this will typically be redundant). In release mode, it generates
+  // SIMDJSON_ASSUME statements to allow the compiler to make assumptions.
+  iter.assert_at_root();
   switch (*iter.peek()) {
     case '[': {
       // The following lines check that the document ends with ].

include/simdjson/generic/ondemand/document.h

@@ -146,6 +146,12 @@ class document {
   /**
    * Cast this JSON value to a value when the document is an object or an array.
    *
+   * You must not have begun iterating through the object or array. When
+   * SIMDJSON_DEVELOPMENT_CHECKS is set to 1 (which is the case when building in Debug mode
+   * by default), and you have already begun iterating,
+   * you will get an OUT_OF_ORDER_ITERATION error. If you have begun iterating, you can use
+   * rewind() to reset the document to its initial state before calling this method.
+   *
    * @returns A value if a JSON array or object cannot be found.
    * @returns SCALAR_DOCUMENT_AS_VALUE error is the document is a scalar (see is_scalar() function).
    */
@@ -268,10 +274,15 @@ class document {
    */
   simdjson_inline operator bool() noexcept(false);
   /**
-   * Cast this JSON value to a value.
+   * Cast this JSON value to a value when the document is an object or an array.
+   *
+   * You must not have begun iterating through the object or array. When
+   * SIMDJSON_DEVELOPMENT_CHECKS is defined, and you have already begun iterating,
+   * you will get an OUT_OF_ORDER_ITERATION error. If you have begun iterating, you can use
+   * rewind() to reset the document to its initial state before calling this method.
    *
-   * @returns A value value.
-   * @exception if a JSON value cannot be found
+   * @returns A value value if a JSON array or object cannot be found.
+   * @exception SCALAR_DOCUMENT_AS_VALUE error is the document is a scalar (see is_scalar() function).
    */
   simdjson_inline operator value() noexcept(false);
 #endif