07 03 default values

Some P4 types define a “default value,” which can be used to automatically initialize values of that type. The default values are as follows:

• For int, bit<N> and int<N> types the default value is 0.
• For bool the default value is false.
• For error the default value is error.NoError (defined in core.p4)
• For string the default value is the empty string ""
• For varbit<N> the default value is a string of zero bits (there is currently no P4 literal to represent such a value).
• For enum values with an underlying type the default value is 0, even if 0 is actually not one of the named values in the enum.
• For enum values without an underlying type the default value is the first value that appears in the enum type declaration.
• For header types the default value is invalid.
• For header stacks the default value is that all elements are invalid and the nextIndex is 0.
• For header_union values the default value is that all union elements are invalid.
• For struct types the default value is a struct where each field has the default value of the suitable field type – if all such default values are defined.
• For a tuple type the default value is a tuple where each field has the default value of the suitable type – if all such default values are defined.

Note that some types do not have default values, e.g., match_kind, set types, function types, extern types, parser types, control types, package types.