... This doesn't look like a good interface due to using a casted value as a "null"-code. This code would fail when "-1" is a value of some-string. This could lead to very big problems! I would suggest something more like boost::optional<int> value = boost::lexical_cast(some-string,boost::no_throw); if(!value) conversion failed.
Well, "very big problems" sounds like a stretch. I've been using a lexical_cast equivalent in that fashion and in practice very often there are values that are well out our sensible range. If, say, -1 is no good then something like INT_MAX might well be. Quite often though I do not even need to look for a ""null"-code" as I provide the *default* value instead. I.e. a value inside the sensible range that I proceed with when the conversion fails. If all that is no good, then I'd expect the existing interface to come to the rescue. In addition, the interface above does not seem to address #2, i.e. the default-constructability requirement of the Target which for my a big hassle. Best, V.