Does the standard say anything about the following code?

vector<int> a(5);
...
a.push_back(a[0]);  // Is this dangerous?

The basic question is:  push_back may have to reallocate itself to a
larger size.  Must it take into account that it's argument may be a
reference to itself?  Are references into a invalidated at the point of
entry to push_back, or only at the point of exit, or unspecified by the
standard?

-Howard
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