Class, constructor and inherance.

[Ali Çehreli via Digitalmars-d-learn]

On 10/11/2015 08:26 PM, holo wrote:
 > On Monday, 12 October 2015 at 02:30:43 UTC, Meta wrote:
 >> On Monday, 12 October 2015 at 02:14:35 UTC, holo wrote:
 >>> class credential
 >>> {
 >>>     auto accessKey = environment.get["AWS_ACCESS_KEY"];
 >>>     auto secretKey = environment.get["AWS_SECRET_KEY"];
 >>> }
 >>>
 >>> class sigv4 : credential
 >>> {
 >>>     private:
 >>>         const algorithm = "AWS4-HMAC-SHA256";
 >>>
 >>>         auto currentClock = Clock.currTime(UTC());
 >>>         auto currentDate = cast(Date)currentClock;
 >>>         auto curDateStr = currentDate.toISOString;
 >>>         auto currentTime = cast(TimeOfDay)currentClock;
 >>>         auto curTimeStr = currentTime.toISOString;
 >>>         auto xamztime = curDateStr ~ "T" ~ curTimeStr ~ "Z";
 >>> }
 >>
 >> You should set these in the constructor rather than in the class body
 >> (the only one that's okay to intialize here is `algorithm`, as it's a
 >> const string).
 >
 > I rewrite it to something like this:

What Meta is saying is that the expressions used for default values 
class bodies must be known at compile time. For example, you cannot get 
current time at run time. Besides, if it did work, there is the question 
of "should every sigv4 object get the same default value currentClock, 
or should every object get their own time?"

 > class credential
 > {
 >      auto accessKey = environment.get["AWS_ACCESS_KEY"];
 >      auto secretKey = environment.get["AWS_SECRET_KEY"];
 > }

As mentioned, that cannot compile.

 > class sigv4 : credential
 > {
 >      this(string URI = "/", string queryStr =
 > "Action=DescribeInstances&Version=2013-10-15", string headerStr =
 > "host:" ~ host ~ "\n" ~ "x-amz-date:" ~ xamztime ~ "\n", string headers
 > = "host;x-amz-date")
 >      {
 >          auto currentClock = Clock.currTime(UTC());

That is a known error, which is usually caused by copy-paste: Because 
you used 'auot' on the left-hand side, that is the definition of a local 
variable. 'currentClock' above is not the member of this class. It is 
adviced to use fully qualify members with this. in constructors:

     this.currentClock = // ...

 >          auto currentClock = Clock.currTime(UTC());

Just define the members without a default value; they will be 
initialized in the constructor:

     SysTime currentDate;

 > My question was too if now i will inherit that class, will that default
 > values be in child class available?

Yes, every variable will obey its default value. When in doubt, test 
with simple examples. :)

class Base {
     int i = 42;
}

class Derived : Base {
     int j;

     this() {
         this.j = 100;
     }
}

void main() {
     auto d = new Derived();

     assert(d.i == 42);     // set by default value
     assert(d.j == 100);    // set in the constructor
}

Ali