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digitalmars.D.bugs - [Issue 3968] New: Some way to do certain implicit casts with operator overloading

http://d.puremagic.com/issues/show_bug.cgi?id=3968

           Summary: Some way to do certain implicit casts with operator
                    overloading
           Product: D
           Version: 2.041
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc


--- Comment #0 from bearophile_hugs eml.cc 2010-03-15 10:29:34 PDT ---
Here an implicit cast to uint or int can be handy, to avoid the cast (to be
able to create a generic "fake int" struct, so for example it can be possible
to create a multiprecision integer that acts like a built-in integer), is this
possible?


struct Foo {
    int x;
    int opCast(T:int)() {
        return this.x;
    }
}
void main() {
    Foo f = Foo(5);
    auto a1 = new int[cast(int)f]; // OK
    auto a2 = new int[f];          // ERR
}


An implicit cast can be useful in other situations (but I don't know about its
possible bad side effects):


struct Foo {
    int x;
    int opCast(T:int)() {
        return this.x;
    }
}
void bar(int i) {}
void main() {
    Foo f = Foo(5);
    bar(f); // Error
}


Steven Schveighoffer has suggested some possible solutions, like:


struct Foo {
   int x;
   uint opCast(T:uint)() { return this.x; }
   int opCast(T:int)() { return this.x; }
   alias opCast this;
}
void main() {
    Foo f = Foo(5);
    int[] a = new int[f]; // line 9
}


Or:

struct Foo {
    uint x;
    uint castToUint() { return x; }
    alias castToUint this;
}


But they don't work. He says at least something like this should:

alias opCast!uint this;

I think that an implicit cast can also lead to bugs, so this has to be designed
with care.

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Mar 15 2010