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From: brianjparker (brianjparker_at_[hidden])
Date: 2002-02-19 23:50:17

--- In boost_at_y..., "Fernando Cacciola" <fcacciola_at_g...> wrote:
> >
> >template<typename T1, typename T2>
> >auto min(T1 t1, T2 t2)
> >{
> > if (t1 < t2) return t1;
> > else return t2;
> >}
> This example won't behave as intended.
> The type of foo() (the return type), is determined at runtime, not
> time.
> 'auto' here would actually mean 'boost::any', but we are
considering a
> compile-time type deduction, not a dynamic typing.
> This example, however, pops up an important thing:
> The type of the result of a function can ONLY be determined by its
> expression. As I pointed out, it cannot be determined by the
> arguments becuase there is no reason why a function should return a
> associated with the type of its arguments.
> But a return expression can only have a fixed known-at-compile-time
type. It
> doesn't matter which sort of type deduction is used, the type must
be fixed,
> otherwise, we are talking about true dynamic typing as in 'min'
above, but
> we don't want that.
> Therefore, 'auto' for a return type will allow implicit conversions
to an
> implicit type.
> The only case when it will guarantee no implicit conversions is
when a
> single auto variable is used for the return expresion:
> auto foo()
> {
> auto r = some();
> return r ;
> }
> If the return expression includes anything else, including more
than one
> auto variables, it could involve an implicit conversion.
> auto foo()
> {
> auto a = someA();
> auto b = someB();
> return choose_one() ? a : b ; // posible implicit conversion
> }
> Just as I wear the hat of 'don't use implicit conversions', I would
wear the
> hat of "don't use auto return types" if they were allowed.
> I do however, support the 'auto' for variable declaration because
there is
> not a conversion involved even though the type is implicit.

The auto in the min() function example would be a well-defined
compile-time-deduced type- it would be the common conversion type of
all return paths (which is just extended version of the type
conversion rules of operator ?: ). A min function that takes
arbitrary arguments *must* have as a return type the common
conversion type of the arguments, so one way or another you will need
to deduce this common type and cast the arguments to it; "auto" (or
my preferred template<typename R> syntax) just makes this convenient
and obvious.

As you say, in the case where there is a single return path, or all
return paths return the same type then the function return will be of
that type; and the case where different convertible types are
returned then one or more of them will involve an implicit conversion
to the common conversion type (just like ?:).

This is a feature- computing the common conversion type of several
types is essential for writing fully generic functions that take
arbitrary arguments.

,Brian Parker

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