Re: [PATCH] Fix PR libstdc++/19510: Uninitialized variable in some iterators

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

Chris Jefferson <caj@cs.york.ac.uk> writes:

| >But copying singular iterators is explicitly disallowed, isn't it?
| >
| >24.1/5:
| >   Iterators can also have singular values that are not associated with
| >   any container. [Example: After the declaration of an uninitialized
| >   pointer x (as with int* x;), x must always be assumed to have a
| >   singular value of a pointer. ] Results of most expressions are
| >   undefined for singular values; the only exception is an assignment of
| >   a nonsingular value to an iterator that holds a singular value. In
| >   this case the singular value is overwritten the same way as any other
| >   value.
| >
| >This explicitly mentions the case of uninitialised pointers being
| >singular.
| >
| >
| Just a quick note. 8.3.1/2, the line "pc = p", shows you can copy
| uninitalised pointers.

You can write any code you want, it does not mean they have valid
meaning.  As a matter of fact GCC would warn if you copy an
unintialized pointer.  And rightly so.  Being a pointer is no special
about that.  Please consult 4.1/1

  An lvalue (3.10) of a non-function, non-array type T can be
  converted to an rvalue. If T is an incomplete type, a program that
  necessitates this conversion is ill-formed. If the object to which
  the lvalue refers is not an object of type T and is not an object of
  a type derived from T, or if the object is uninitialized, a program
  that necessitates this conversion has undefined behavior. If T is a
  non-class type, the type of the rvalue is the cv-unqualified version
  of T. Otherwise, the type of the rvalue is T.   


The only exception is a char (and varints).  

-- Gaby