On Fri, Jan 30, 2015 at 1:28 PM, Sturla Molden <sturla.mol...@gmail.com> wrote:
> in Python. It actually corresponds to
>
> with Foo() as bar:
> <suite>
The problem with with statements is that they only handle the case of
RAII with stack allocated variables, and can't handle transfer of
ownership cleanly.
Consider the case of a function that opens a file and returns it:
def myfunction(name, stuff):
f = open(name)
f.seek(stuff) # or whatever
return f
def blahblah():
with myfunction('hello', 12) as f:
....
This code is wrong, because if an error occurs during seek in
myfunction, the file is leaked.
The correct myfunction is as follows:
def myfunction(name, stuff)
f = open(name)
try:
f.seek(stuff)
except:
f.close()
raise
Or whatever. (I would love a close_on_error context manager, BTW.)
With RAII, the equivalent C++ looks nearly exactly like the original
(bad) Python approach, except it uses unique_ptr to store the file,
and isn't broken. ("Modern") C++ makes this easy to get right. But
then, this isn't the common case.
-- Devin
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