Regarding opCast(bool) class method

[bearophile]

I think that now and then it's good to underline some D issues, 
even old ones.

This little program shows the asymmetry in opCast(bool) between 
struct instances and class instances:


struct FooStruct {
     int x;
     this(int xx) { this.x = xx; }
     T opCast(T: bool)() {
         return this.x != 0;
     }
}

class FooClass {
     int x;
     this(int xx) { this.x = xx; }
     T opCast(T: bool)() {
         return this.x != 0;
     }
}

void main() {
     import std.stdio;
     enum int N = 0;

     auto fstruct = FooStruct(N);
     if (fstruct)
         writeln("fstruct true");
     else
         writeln("fstruct false"); //*

     if (cast(bool)fstruct)
         writeln("cast(bool)fstruct true");
     else
         writeln("cast(bool)fstruct false"); //*

     auto fclass = new FooClass(N);

     if (fclass)
         writeln("fclass true"); //*
     else
         writeln("fclass false");

     if (cast(bool)fclass)
         writeln("cast(bool)fclass true");
     else
         writeln("cast(bool)fclass false"); //*
}



The output:

fstruct false
cast(bool)fstruct false
fclass true
cast(bool)fclass false


Is this situation a problem?
If in your code you convert a struct to class or the other way, 
and it contains an opCast(bool), you will have troubles. And 
generally I don't like how opCast(bool) works in classes, I think 
it's a bit bug-prone. I think "if(fclass)" and 
"if(cast(bool)fclass)" should always have the same boolean value.

If this situation is a problem, then here are two of the possible 
solutions:
- Do not allow opCast(bool) in classes. How much often do you 
need cast(bool) on class instances?
- Keep cast(bool) in classes, and remove the asymmetry between 
structs and classes, if possible. So "if(fclass)" on a class 
instance calls opCast(bool). Then to test the value of the 
reference you use "if(fclass is null)".


See also my issue on this:
http://d.puremagic.com/issues/show_bug.cgi?id=3926

More info on related matters in C++/C++11:
http://en.wikipedia.org/wiki/C%2B%2B11#Explicit_conversion_operators
http://www.artima.com/cppsource/safeboolP.html
http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool

Bye,
bearophile