templating opEquals/opCmp (e.g. for DSL/expression templates)

[Atila Neves]

On Tuesday, 12 February 2019 at 11:35:28 UTC, Nicholas Wilson 
wrote:
> On Tuesday, 12 February 2019 at 10:37:22 UTC, Jonathan M Davis 
> wrote:
>> Having the meaning be different is precisely the problem. It's 
>> using the syntax for something _other_ than what it's intended 
>> for. It's just supposed to be a way to have user-defined types 
>> emulate the operations of built-in types. Walter has mentioned 
>> time and time again how he thinks that stuff like << in C++ 
>> where they made the operators mean something completely 
>> different than what they mean for built-in types was a big 
>> mistake, and personally, I agree.
>
> I agree using << for I/O is stupid.

I've never understood this argument. I've never met anyone who 
was confused by this. Personally, I read it as "shove". C++20 
ranges use `|` as a pipe operator instead of bitwise or, and not 
only does it make sense, I don't think anybody's complaining 
about that either.

> The use case is e.g. building a DB query predicate where == 
> still means equality not e.g. I/O, it just isn't evaluated 
> directly. The fact is you can already use it for arithmetic to 
> build e.g. linear algebra expression templates, and you can use 
> == predicates but not != or any of the ordering comparisons is 
> inconsistent arbitrary and annoying.

Agreed.