Bug 12118 – Modify immutable data using throw

It is possible to throw an immutable Exception and catch it as mutable. Using this immutable data can be modified. Example Code: class Dummy: Exception { int[] data; @safe pure nothrow this(immutable int[] data) immutable { super("Dummy"); this.data = data; } } @safe pure void modifyImmutable(immutable int[] data) { try { immutable Dummy e = new immutable Dummy(data); throw e; } catch(Dummy e) { e.data[1] = 42; } } @safe pure void main() { immutable int[] data = [1,2,3]; assert(data == [1,2,3]); modifyImmutable(data); assert(data == [1,42,3]); }

Interesting.. but I don't see how this can be solved except disallowing throwing non-mutable objects. Otherwise you'd end up in a situation where "try { } catch (Throwable) { }" could let an Exception through because it's non-mutable.

Also since Exceptions can be chained I don't think it makes sense to allow throwing non-mutable objects.

(In reply to comment #0) At least, defining immutable constructor in derived classes of Throwable should be disallowed. > class Dummy: Exception > { > int[] data; > @safe pure nothrow this(immutable int[] data) immutable > { > super("Dummy"); --> during immutable object construction, mutable super constructor should not be invoked. > this.data = data; > } > }

@ntrel created dlang/dmd pull request #14706 "Fix Issue 12118 - Modify immutable data using throw" fixing this issue: - Fix Issue 12118 - Modify immutable data using throw https://github.com/dlang/dmd/pull/14706

comment #0: > It is possible to throw an immutable Exception and catch it as mutable Also, the exception is mutated when thrown so that is another safety violation if immutable: https://github.com/dlang/dmd/blob/v2.101.1/druntime/src/rt/deh.d#L13-L21 comment #3: > At least, defining immutable constructor in derived classes of Throwable should be disallowed. That wouldn't disallow: throw new immutable Exception("");

dlang/dmd pull request #14706 "Fix Issue 12118 - Modify immutable data using throw" was merged into master: - 5e7700af332f69bcb887b558b421f29cfd4da21e by Nick Treleaven: Fix Issue 12118 - Modify immutable data using throw https://github.com/dlang/dmd/pull/14706