H. S. Teoh:
What if templates were modified so that assumptions about T
have to be
stated up front? We don't necessarily have to introduce
typeclasses as a
separate thing, but we could have the compiler reject template
bodies
that try to access internals of T that aren't stated up front.
E.g.:
auto myFunc(T)(T t) {
return t.fun(); // error: T wasn't stated to have a member
named .fun
}
would produce a compile error, whereas:
auto myFunc(T)(T t)
if (hasMember!(T, "fun"))
{
return t.fun(); // OK: we stated that we expect T to have a
.fun member
}
would be accepted. The stated assumptions can themselves be
templates
too, which are expanded by the compiler before checking the
template
body. (Presumably, hasMember will eventually expand to some
compiler
intrinsic that tells the compiler what assumptions are being
made about
T.) So we can still continue using things like isInputRange!T,
etc.,
except that now the compiler will catch unstated assumptions.
What's the difference between this and Concepts (lite)? :-)
Bye,
bearophile
