Undefined behavior and temporaries

Question

1) Is it undefined behavior to return a reference to a temporary, even if that reference is not used? For example, is this program guaranteed to output \"good\":

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Answer 1

I don't see any rules that forbid case 1 and 2 nor can I find a relevant defect report either.

All we really have from the draft C++ standard is from section 8.3.2 [dcl.ref]:

[...]A reference shall be initialized to refer to a valid object or function. [ Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” obtained by indirection through a null pointer, which causes undefined behavior.[...]

which does not apply to case 1 since we are not initialing a reference and neither case 2 since the object is valid when we initialize the reference.

This does seem to apply to case 3. So what does valid object mean is subject of the following defect report. The defect report which covers this topic is still open and therefore we can only get a a feel for current thinking which is that this should be undefined behavior.

If we look at defect report 453: References may only bind to “valid” objects , which deals with what it means to bind a reference to an invalid object. The current proposed resolution says:

[...]If an lvalue to which a reference is directly bound designates neither an existing object or function of an appropriate type (8.5.3 [dcl.init.ref]), nor a region of storage of suitable size and alignment to contain an object of the reference's type (1.8 [intro.object], 3.8 [basic.life], 3.9 [basic.types]), the behavior is undefined. [...]

So we can say the current thinking is that this should be undefined behavior but currently this is defect and so we can't say for sure until this defect report is resolved. I would err on the side of caution and assume it is undefined behavior.