using opAssign or having my own assign function

[Martin d Anjou]

Hi,

I am trying to build a data type where a key characteristic is that the 
data holding capacity is fixed and never changing, and the declaration 
looks like this:

auto a = BitVector(4);   // "a" can hold unsigned numbers from 0 to 15
auto b = BitVector(10);  // 10 bits of data
auto c = BitVector(79);  // etc.

Say I now want to add b and c, and store the result in a, while preserving 
the property that "a" has to have a capacity of 4 bits only. Adding and 
truncating the upper bits to fit in "a" is the easy part. The problematic 
part is stating that the result of the addition goes to variable "a".

I was first tempted to simply say "a=b+c", but b+c will return a new 
instance of BitVector, scrap the original "a", and loose the holding 
capacity of "a". I thought of overloading opAssign(), but I can't do 
"BitVector opAssign(BitVector other) { ... }".

So I came to the conclusion the least non-intuitive thing to do would be 
"a.assign(b+c)", which preserves the storage capacity declared for 
variable "a", and does not create a new instance for "a".

Question: Is there a better way to do this?

Question: Is there a chance opAssign could be used for this kind of 
thing at all in the future?

Thanks,
Martin