SetInfo

A field which carries informations about whether it was set or not

Some configurations may need to know which fields were set explicitly while keeping defaults. An example of this is a struct where at least one field needs to be set, such as the following:

public struct ProtoDuration
{
    public @Optional long weeks;
    public @Optional long days;
    public @Optional long hours;
    public @Optional long minutes;
    public           long seconds = 42;
    public @Optional long msecs;
    public @Optional long usecs;
    public @Optional long hnsecs;
    public @Optional long nsecs;
}

In this case, it would be impossible to know if any field was explicitly provided. Hence, the struct should be written as:

public struct ProtoDuration
{
    public SetInfo!long weeks;
    public SetInfo!long days;
    public SetInfo!long hours;
    public SetInfo!long minutes;
    public SetInfo!long seconds = 42;
    public SetInfo!long msecs;
    public SetInfo!long usecs;
    public SetInfo!long hnsecs;
    public SetInfo!long nsecs;
}

Note that SetInfo implies Optional, and supports default values.