[Qt-interest] dereferencing pointer '<anonymous>' does break strict-aliasing rules error

[Thiago Macieira]

On Wednesday, 11 de May de 2011 12:29:45 Konrad Rosenbaum wrote:
> On Tuesday 10 May 2011 14:38:23 mmiacca wrote:
> > While i try with Qt 2010.05 (Qt 4.7.0 32 bits Windows) to compile
> > C:\Qt\2010.05\qt\examples\qtestlib\tutorial4 i get this error
> > ..\..\..\include/QtTest/../../src/testlib/qtestspontaneevent.h:77:
> > warning: dereferencing pointer '<anonymous>' does break strict-aliasing
> > rules
> 
> It's just a warning. Unless there is more (i.e. a real error) the
> application should still build and be executable.

That doesn't mean it will work right. Then again, there's warning-free code 
that doesn't work right either :-)

The C and C++ languages forbid aliasing of types (except for aliasing with 
char). So the following code is malformed:

	int i = 42;
	ushort u = (ushort *)&i;
	u = 43;

	printf("%d", i);

The types "i" and "u" alias each other and this is not allowed.

Undefined behaviour means anything could happen. The possible outcomes are 
(assuming little-endian machine):

1) it prints "43" because it did modify the memory located by i.

2) it prints "42" because the compiler concludes that the variable i didn't 
get modified between its assignment and its use. The two intermediary lines are 
considered dead code and eliminated.

3) it calls your ex-mother-in-law and gives her your new girlfriend's phone 
number.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Senior Product Manager - Nokia, Qt Development Frameworks
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358
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