On Saturday, 13 June 2020 at 11:26:58 UTC, Johannes Loher wrote:
Why is it a compile error to set `_a` directly but calling `a`
just works fine?
If we prevent modifying members of rvalues directly, I would
also expect calling non-const member functions of rvalues to be
prevented.
1) Constructors and destructors are also member functions. Surely
you won't suggest making all constructors `const`, and not being
able to destruct a temporary? :)
2) The member functions may have useful side effects even on the
state of temporary.
3) The member functions may serve as factories.
The temporary exists until the end of full expression, or until
the end of enclosing statement. It is simply not an lvalue for
the caller, but it certainly exists, and so its interface must
function. Consider (this code will fail on older compilers, up to
2.067.0, due to a bug):
void main()
{
import std.stdio : File;
import std.range : iota;
with (File("test.txt", "w"))
{
foreach (n; iota(1, 100, 5))
writeln(n);
}
}
This will open the file, write numbers [1, 96] on their own lines
in that file, and close it. Both the File and the Iota range are
temporaries. File can't be `const` as it's a wrapper over FILE*.
The range can't be const since the foreach mutates it, and it is
effectively a factory of numbers.
