Default Values

There are several scenarios where a default value will be used to initialize a field during deserialization:

1. The field is not mandatory and is not present in the serialized data.
2. The field is present in the serialized data, but it has some issues trying to deserialize it and the validate attribute is set to fallback.
3. The field is skipped during serialization and it has to be defaulted during deserialization.

In these scenarios, FlatMessage will do one of the following:

1. Use the default value for the type if it is available (this implies that the type implements the Default trait).
2. If the attribute default is specified, it will use the value of the attribute.

Example:

1. Use the default value:

   #![allow(unused)]
   fn main() {
   #[derive(FlatMessage)]
   struct Test {
       #[flat_message_item(skip = true)]
       a: u32,
   }
   }

   In this case, the field a will be initialized to 0 (the default value for u32).
2. Use the value of the attribute default:

   #![allow(unused)]
   fn main() {
   #[derive(FlatMessage)]
   struct Test {
       #[flat_message_item(skip = true, default = 10)]
       a: u32,
   }
   }

   In this case, the field a will be initialized to 10.

Custom Default Values

When using the attribute default, you can specify a custom default value for the field in the following ways:

1. A constant value (e.g. default = 10 or default = MY_CONSTANT).
2. A string representation of the value (e.g. default = "10" ). In this case the value is parsed and adjusted to fit the actual type of the field.
3. A raw string representation of the value (e.g. default = r#"foo(1,2,3)"#). In this case the value is use exactly as is. This is in particular useul if you want to use exprssior or a call to a functon to initialize the value.

String representations

When using a string representation: default = "..." the following steps are checked:

1. if the type is &str the value of the default attribute is kept as it is.
2. if the type is String th value of the default attribute is converted into a String
3. if the type is NOT a string the quotes (") are removed and the actual value will be used
4. if the type is on option ( Option<T> ) then:
   • if the value of the default attribute is None then the field is set to None
   • if the value of the default attribute is Some(T) then the field is set to Some(T)
   • otherwise the value of the default attribute is converted into a Some<value>

Examples

Remark: If you need to be 100% sure that the value is converted into the correct type, you can use the raw string representation (e.g. default = r#"Some(1+2+3)"#).