Compile time opAssign/@property constraints

[Simen Kjærås]

On Friday, 4 January 2019 at 09:54:25 UTC, Jacob Shtokolov wrote:
> Hi,
>
> I'd like to implement some compile time constraints for a 
> struct (though not sure if that's possible).
>
> I already tried to place "static assert" or any kind of static 
> condition into a body of @property and opAssign(), but every 
> time it shows the error "variable cannot be read at compile 
> time".
>
> Is there any way to catch and validate assignments or struct 
> initialization at compile time?

Well, yes and no. The error message you're getting seems to 
indicate you're trying to do something impossible, but it could 
be you simply haven't understood the limits of what can and 
cannot be done.

If I were to venture a guess, I'd say you're trying to disallow 
certain values - something along the lines of an int with a 
limited range, like Ada's integer ranges. An example would be 
percentages:

struct Percentage {
    int value;
    void opAssign(int v) {
        assert(v >= 0, "Value is too low!");
        assert(v <= 100, "Value is too high!");
        value = v;
    }
}

D does not let you limit the set of valid values like this at 
compile-time, instead the tests must be implemented at run-time, 
like above. Attempting to use static assert above would give the 
exact error message you mention.

There are many things that can be tested at compile-time, so if 
your use case is not analogous with the above, it may well be 
possible to implement compile-time testing of it.

The thing is, compile-time tests like static if and static assert 
can only test values that are known at compile-time, and are for 
the most part useful only in templates.

--
   Simen