Re: Implicit Instantiations - was: Re: Why is Ada NOT a good choice for a beginner to programming?

[Hyman Rosen <hyrosen@mail.com>]

Richard Riehle wrote:
> Implicit type conversions, along with implicit type promotions run
> counter to the Ada philosophy.

True, but irrelevant to the Unit C++ code, which does not involve
any implicit type conversions. (I should have declared my constructors
explicit, and special-cased Unit<Type,0,0,0> to allow an implicit
conversion from Type. But my example was just a rough outline.)

> C++ does not provide the same kind of capability for declaring new
> numeric types one has in Ada.   This, in spite of the entertaining
> example from Mr. Rosen.

But it does have the capability for declaring new class types,
and that's what the Unit code uses.

> Typedef does not do the same thing.   We are still left with predefined
> types.

I was typedefing class types, not numeric types. My typedefs were
just mnemonic shorthand for the full Unit<...> types.

> The whole idea of Ada is to declare types so they will be checked by
> the compiler and so mismatches will be caught.   If there is a problem
> with that, in the current standard, it needs to be fixed, but implicit
> conversions or promotions is simply a bad idea for this language.

And that is precisely what the C++ code is doing. At compile time,
and with zero run-time size or space overhead, the compiler will
catch attempts to add or subtract mismatched units, or to assign
one unit type to a different one. Multiplication or division of
units automatically produces a correct result type, and you can
have arbitrarily complex expressions, all checked by the compiler.

There are absolutely no implicit conversions or promotions involved.

> Actually, I don't think there is really a problem here wrt Ada, even
> after reading your paper.

Except that you can't do this in Ada, and people want to.