clone class in constructor

Question

I need something like

public class  object_t 
{

    public object_t ( string config, object_t default_obj )
    {
         if( /*failed to initialize from config*/ )
         { 
            this = default_obj; // need to copy default object into self
         }
    }
 }

I know this is not correct. How to implement this constructor ?

Answer 1

The most common is probably to use a static factory method:

public class object_t 
{    
    public static object_t CreateObjectT(string config, object_t default_obj)
    {
         object_t theconfiguredobject = new object_t();

         //try to configure it

         if( /*failed to initialize from config*/ )
         { 
             return default_obj.Clone();
         }
         else
         {
             return theconfiguredobject;
         }
    }
}

---

A better way to do the above would be to create a copy constructor:

public object_t (object_t obj)
    : this()
{
    this.prop1 = obj.prop1;
    this.prop2 = obj.prop2;
    //...
}

and a method that tries to create your object from the config string:

private static bool TryCreateObjectT(string config, out object_t o)
{
    //try to configure the object o
    //if it succeeds, return true; else return false
}

then have your factory method call the TryCreateObjectT first, and if it fails, the copy constructor:

public static object_t CreateObjectT(string config, object_t default_obj)
{
     object_t o;
     return TryCreateObjectT(config, out o) ? o : new object_t(default_obj);
}

Answer 2

You should copy each field from the default object to the new one within the constructor:

public class  object_t 
{ 
    int A, B;

    public object_t ( string config, object_t default_obj )
    {
         if( /*failed to initialize from config*/ )
         { 
            this.A = default_obj.A; 
            this.B = default_obj.B; 
            //...
         }
    }
}

However you should remember, if this class' fields are reference types, you'll have to Clone them as well, not just assign a refernce to.

This approach does create a copy of the default object, while if you only need to return the default object itself, you should use the factory approach, mentioned by lc.