Zero values, as in what would placehold the value when a key in a mapping is not yet assigned.
In Solidity docs they are called initial values:
0.0(presumably; this type is not fully supported)
enum: the first element of the enum
internal: empty function, returning initial values (if return is needed)
external: function that throws when called
mapping: empty mapping
struct: a struct where all members are set to initial values
- fixed-sized: an array of the fixed size where all elements are set to initial values
When you use the
delete keyword it will assign the initial value to the variable, except for mappings, where it doesn't have any effect. For structs the
delete keyword will recurse into the members, unless they are mappings.