Re: Constrained Forwarding(R-Value Reference)
In article <esojh4$9c6$1@aioe.org>, "Grizlyk" <grizlyk1@yandex.ru>
wrote:
I can repeat again:
Sorry for making you repeat.
if you will struggle with "compile time attributes", you
always will get warning about "undefined behaviour" and never will get the
warnings in opposite case of correct design. Coreect desing can be like
this:
A a1
if (foo())
{
A @a2(a1);
...
}
else
{
// Has a1 been moved from here?
A @a3(a1); // error or not?
}
Now all OK.
I'm not convinced that my original code was incorrect. The if clause
might have ended with:
throw_my_exception();
I know that this function is never going to return, thus the "else"
isn't needed. But the compiler has no way to know that short of whole
program analysis (which might have to be delayed until run time if
throw_my_exception() lives in a "plug-in".
You must take in account, that "moveable" is specific and
_limited_ data type, as "const", we can not use limited type in free manner.
We can not write for example like this:
Hmm... so can't move from just any type? In generic code I would like
to move things around without knowing much about the type. Maybe
something like:
template <class It> It& increment(It&);
template <class It, class T>
It
foo(It first, It last, const T& x)
{
It i = first;
while (increment(i) != last)
{
if (!(*i == x))
{
*first = std::move(*i);
++first;
}
}
return first;
}
At the time I write this generic code, I don't know the type of *i, but
I want the code to treat *i as an rvalue because I know that I'll never
need the value stored at *i again. But I'm not sure how the compiler
will know that. To the compiler it may look like I'm moving from the
same lvalue over and over. Is my above example code:
1. A compile time error?
2. A warning about undefined behavior?
3. Not possible to express with your design?
4. Requires another keyword such as "runtime" that you mentioned?
5. Incorrect design?
If the answer is 4 (requires "runtime") I fear that much (most?) of the
code I write will require this keyword. If I have to nearly always
write "runtime" anyway I might get lulled into believing that my
double-move compile time errors are always just noisy false positives.
-Howard
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