Document one use-case of rfl::num_fields

Author: bryceschober

docs/custom_parser.md

@@ -1,9 +1,10 @@
 # Custom parsers
 
-## `rfl::Reflector` 
+## `rfl::Reflector`
 
-If you absolutely do not want to make any changes to your original classes whatsoever,
-You can create a Reflector template specialization for your type:
+If you absolutely do not want to (or are unable to) make any changes to your
+original classes whatsoever, you can create a Reflector template specialization
+for your type:
 
 ```cpp
 namespace rfl {
@@ -13,7 +14,7 @@ struct Reflector<Person> {
     std::string first_name;
     std::string last_name;
   };
-  
+
   static Person to(const ReflType& v) noexcept {
     return {v.first_name, v.last_name};
   }
@@ -25,7 +26,27 @@ struct Reflector<Person> {
 }
 ```
 
-It's also fine to define just the `from` method when the original class is 
+One way to help make sure that your `ReflType` is kept up to date with your
+original class is to use the `rfl::num_fields<T>` utility to implement a compile-
+time assertion to verify that they have the same number of fields. The
+`rfl::num_fields<T>` utility can be used even in cases where the original
+class is too complex for `reflect-cpp`'s default reflection logic or
+`rfl::to_view()` to be able to handle.
+
+```cpp
+namespace rfl {
+template <>
+struct Reflector<Person> {
+  struct ReflType {
+    std::string first_name;
+    std::string last_name;
+  };
+  static_assert(rfl::num_fields<ReflType> == rfl::num_fields<Person>,
+    "ReflType and actual type must have the same number of fields");
+  // ...
+```
+
+It's also fine to define just the `from` method when the original class is
 only written, or `to` when the original class is only read:
 
 ```cpp
@@ -46,7 +67,7 @@ struct Reflector<Person> {
 ```
 
 Note that the `ReflType` does not have to be a struct. For instance, if you have
-a custom type called `MyCustomType` that you want to be serialized as a string, 
+a custom type called `MyCustomType` that you want to be serialized as a string,
 you can do the following:
 
 ```cpp
@@ -114,7 +135,7 @@ struct Person {
 };
 ```
 
-You can then write a helper struct: 
+You can then write a helper struct:
 
 ```cpp
 struct PersonImpl {

docs/docs-readme.md

@@ -28,11 +28,11 @@
 
 [Standard containers](standard_containers.md) - Describes how reflect-cpp treats containers in the standard library.
 
-[C arrays and inheritance](c_arrays_and_inheritance.md) - Describes how reflect-cpp handles C arrays and inheritance. 
+[C arrays and inheritance](c_arrays_and_inheritance.md) - Describes how reflect-cpp handles C arrays and inheritance.
 
-[rfl::Bytestring](bytestring.md) - Describes how reflect-cpp handles binary strings for formats that support them. 
+[rfl::Bytestring](bytestring.md) - Describes how reflect-cpp handles binary strings for formats that support them.
 
-[rfl::Binary, rfl::Hex and rfl::Oct](number_systems.md)- For expressing numbers in different formats. 
+[rfl::Binary, rfl::Hex and rfl::Oct](number_systems.md)- For expressing numbers in different formats.
 
 ## Validation
 
@@ -58,7 +58,7 @@
 
 [Custom classes](concepts/custom_classes.md) - For custom classes with private fields.
 
-[Custom parsers for your classes](custom_parser.md) - For custom classes with private fields that you want to leave absolutely untouched.
+[Custom parsers for your classes](custom_parser.md) - For custom classes with private fields that you want (or need) to leave absolutely untouched.
 
 ## Useful helper functions and classes