Foo default_foo; Foo foo = lexical_cast(str, default_foo);
(or whatever instead of lexical_cast) has no Foo::Foo() requirement.
and... how is lexical_cast going to come up with an object into which it can stream? Is it going to use default_foo? In that case, what if streaming fails partway through? Are you assuming copyability?
Yes, the copy-constructability requirement is still there. Currently lexicac_cast does if(interpreter << arg) { Target result; // DEFAULT-CONSTRUCTED if (interpreter >> result) return result; } throw_exception(bad_lexical_cast(typeid(Source), typeid(Target))); return Target(); // DEFAULT-CONSTRUCTED For the proposed "extended" interface I imagined something as simple/minimal as if(interpreter << arg) { Target result(failure_value); // COPY-CONSTRUCTED if (interpreter >> result) return result; } return failure_value; // COPY-CONSTRUCTED The likely reason I got so hooked up on extending lexical_cast (rather than branching off) is that it already implements 95% of what I need/do. With lexical_cast submitted for TR2 I had an impression that it was the preferred interface we should be utilizing rather than branching off on our own. Now I get the feeling that people are suggesting/implying we should leave lexical_cast behind as an interesting-but-not-quite-successful experiment (which I somewhat agree with) and start fresh free of its shackles. V.