Implicit cast of int to a class

[Bill Baxter]

torhu wrote:
> Matthias Walter wrote:
>> Hello,
>>
>> I have a class called mpz_class which in particular implements the 
>> following:
>>
>> 1. this (int other) { ... }
>> 2. opAssign (int other) { ... }
>>
>> I now want to write templated code with T = int, T = long and T = 
>> mpz_class.
>> My mpz_class should work and feel like the orginal integers, which 
>> mostly works.
>>
>> I'd like to know, how I can coax my class to implicitely convert 
>> integer literals to instances of it. There are 2 cases which don't 
>> compile for me and one which is semantically wrong:
>>
>> // 1.
>> void func (T) (T input = 0) // error:  cannot implicitly convert 
>> expression (0) of type int to gmp.mpz.mpz_class


For structs, I think this should work, but it doesn't.  I was going to 
file a bug about it, but never got around to it.  In my particular case 
I was trying to make an array-like struct, and got bitten by the fact 
that I could use my opAssign to use "MyStruct foo=null" as a default 
argument.   For classes, as torhu points out, there's no LHS object 
allocated, so it's not going to work without some bigger change to the 
language.


> I don't think it's possible to use defaults args like this.  It seems 
> that opAssign isn't used for default args.  You need to remove the 
> default, overload the function, specialize the template, or redesign the 
> whole thing.
> 
> 
>> // 2. T a = 1; // I guess, a points to 0x1 now and not to a result of 
>> mpz_class.this(1)
> 
> Even if it tried to call opAssign, there's no object allocated.  It 
> would just crash.  You have to use 'new' somewhere.  Maybe it's better 
> to use a struct with static opCall overloads, instead of a class?  A 
> struct is a better fit if you want it to behave like ints and longs, for 
> sure.



--bb