On Tuesday, 11 January 2022 at 10:57:28 UTC, forkit wrote:
On Monday, 10 January 2022 at 03:21:46 UTC, Paul Backus wrote:
Taking the address of a local variable is forbidden in @safe
code. Even though str is a ref variable that points to a
heap-allocated string, it is still considered a local variable
because it is declared inside the body of a function.
but strings[] is also a local variable declared in the body of
the same function, and yet within the foreach statement, @safe
lets me do:
pointers ~= &strings[i]; // safe
...but not this below, where str is just a reference to the
exact same memory as the statement above... is it not? How is
this below any more or less safe than the above statement.
pointers ~= &str; // not safe - ok, but why??
Because the compiler doesn't look at that much context, and it's
possible to write code where `str` points to memory that's on the
stack; for example:
string[3] strings = ["foo", "bar", "baz"];
foreach (ref str; strings) {
// ...
}
If you compile with -preview=dip1000, the compiler will actually
keep track of which pointers point to stack memory, and will
allow your original code. But -preview=dip1000 is still somewhat
experimental, and the documentation for it is pretty sparse, so
you may have an easier time just working around the limitations
of the default safety checks.
