Re: Shooting yourself in the foot with rvalue references
Am 15.05.2011 03:40, schrieb Matthias Hofmann:
[..]
However, even after having read all of the first and at least some of the
second article as well as a considerable number of explanations about
rvalue
references on various web sites, one question still remains open. Please
take a look at the following code:
void f( int&& rri )
{
g( rri );
}
void g( int& ri )
{
ri = 4;
}
int main()
{
int&& rri = 5;
rri = 4;
f( 5 );
return 0;
}
According to C++0x, an rvalue reference that has a name is an lvalue. So
in
my example, the first line of code in main() aims at your foot and the
second line bluntly pulls the trigger. The call to f() is a shot around
the
corner, which is more practical, but the bullet goes through the same
whole
in your foot.
Sorry, I don't see any reason for self-harming behaviour in this
example. Certainly, the literal 5 will not be mutated into an ugly
monster, even though you try hard to realize that. So what?
I have been desperately looking for a rule that says that C++0x does not
have such a severe security whole, but I haven't found one yet. Or is this
only in compliance with the first amendment to the C++ standard, which
says
that "the committee shall make no rule that prevents C++ programmers from
shooting themselves in the foot"?
I never have seen such a rule and it would clearly stay in conflict with
another (non-official) rule that says that a compiler shall not prevent
programmers to write reasonable code. Rvalue-references (including the
rules as defined in the FDIS) allow for a lot of very useful code (I
don't replicate the amounts of examples. You can study use-cases
provided by the original submission papers, if you want).
In other words: I still miss to recognize the actual problem that you
are implying by your colorful description: An rvalue reference is
supposed to refer to something that is not considered useful for the
assigner anymore, so you can do what you like to do with it, including
assigning new values.
HTH & Greetings from Bremen,
Daniel Kr?gler
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