Hello all,
I found Herb Sutter's article regarding error handling in the most recent
CUJ to be very, very good. However, in trying to apply what I learned from
the article, I am left with some questions.
I have a function that takes a std::string (that represents a file name) as
a parameter. This function reads in the contents of the file and returns a
struct representing the file's content.
Two things could go wrong inside the function:
1. The function might find that the file does not exist
2. The function might find that the content of the file is invalid
(Some might consider 1 to be a special case of 2.)
The question is: Are these cases a violation of the function's preconditions
or postconditions? According to the error handling philosophy advocated by
Herb in his article, if it is a precondition violation, it should have been
reported as an error in the calling function. Since it wasn't, we should
report a program bug via an assert in the called function. If it is a
postcondition violation, it should be reported as an error in the called
function. Since whether the problem is a precondition or postcondition
problem determines where the error gets reported, it is something I need to
know to properly write my code.
I can reason that these are precondition violations since it seems
justifiable to say that a precondition for the function is that the file
both exist and have valid content.
However, I can also make a reasonable argument that it is a postcondition
violation since the function cannot construct a valid return value. In this
case, the precondition would be nothing more than that the string object
passed in be valid. If it's a valid std::string, the precondition is met,
period.
So, I need a little more guidance than that provided in the article as to
how to classify a problem as a precondition or postcondition violation. I'm
thinking that perhaps it is a subjective thing and that either way you go
could be OK as long as you're consistent. All help will be appreciated!
Thanks,
Dave 3 2375
* Dave: I found Herb Sutter's article regarding error handling in the most recent CUJ to be very, very good.
Haven't read that. Link?
However, in trying to apply what I learned from the article, I am left with some questions.
I have a function that takes a std::string (that represents a file name) as a parameter. This function reads in the contents of the file and returns a struct representing the file's content.
Two things could go wrong inside the function:
1. The function might find that the file does not exist 2. The function might find that the content of the file is invalid
(Some might consider 1 to be a special case of 2.)
The question is: Are these cases a violation of the function's preconditions or postconditions? According to the error handling philosophy advocated by Herb in his article, if it is a precondition violation, it should have been reported as an error in the calling function. Since it wasn't, we should report a program bug via an assert in the called function. If it is a postcondition violation, it should be reported as an error in the called function. Since whether the problem is a precondition or postcondition problem determines where the error gets reported, it is something I need to know to properly write my code.
I can reason that these are precondition violations since it seems justifiable to say that a precondition for the function is that the file both exist and have valid content.
However, I can also make a reasonable argument that it is a postcondition violation since the function cannot construct a valid return value.
In this case, the precondition would be nothing more than that the string object passed in be valid. If it's a valid std::string, the precondition is met, period.
So, I need a little more guidance than that provided in the article as to how to classify a problem as a precondition or postcondition violation. I'm thinking that perhaps it is a subjective thing and that either way you go could be OK as long as you're consistent. All help will be appreciated!
It's very easy: don't have condition X as a precondition unless the
caller _can_ ensure that condition X holds (and in that case, if you
choose to have X as precondition then a violation is a logic error).
When you don't have condition X as precondition, but X is needed to
return a useful result: if X can be checked for, then it is a design
issue -- what's most practical -- whether the function should
guarantee the useful result on successful execution, or whether it
should also have X-didn't-hold as a possible normal case result.
A guaranteed useful result means X violation should throw exception.
Otherwise, one way of reporting a possibly "null" result is by using a
wrapper object that either is empty or logically contains the useful
result. The case where the useful result is a simple or small value
type has its own little pattern name, which I forget...
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
"Alf P. Steinbach" <al***@start.no > wrote in message
news:40******** ********@news.i ndividual.net.. . * Dave: I found Herb Sutter's article regarding error handling in the most
recent CUJ to be very, very good.
Haven't read that. Link?
However, in trying to apply what I learned from the article, I am left with some questions.
I have a function that takes a std::string (that represents a file name)
as a parameter. This function reads in the contents of the file and
returns a struct representing the file's content.
Two things could go wrong inside the function:
1. The function might find that the file does not exist 2. The function might find that the content of the file is invalid
(Some might consider 1 to be a special case of 2.)
The question is: Are these cases a violation of the function's
preconditions or postconditions? According to the error handling philosophy advocated
by Herb in his article, if it is a precondition violation, it should have
been reported as an error in the calling function. Since it wasn't, we
should report a program bug via an assert in the called function. If it is a postcondition violation, it should be reported as an error in the called function. Since whether the problem is a precondition or postcondition problem determines where the error gets reported, it is something I need
to know to properly write my code.
I can reason that these are precondition violations since it seems justifiable to say that a precondition for the function is that the file both exist and have valid content.
However, I can also make a reasonable argument that it is a
postcondition violation since the function cannot construct a valid return value.
In this case, the precondition would be nothing more than that the string object passed in be valid. If it's a valid std::string, the precondition is
met, period.
So, I need a little more guidance than that provided in the article as
to how to classify a problem as a precondition or postcondition violation.
I'm thinking that perhaps it is a subjective thing and that either way you
go could be OK as long as you're consistent. All help will be appreciated!
It's very easy: don't have condition X as a precondition unless the caller _can_ ensure that condition X holds (and in that case, if you choose to have X as precondition then a violation is a logic error).
When you don't have condition X as precondition, but X is needed to return a useful result: if X can be checked for, then it is a design issue -- what's most practical -- whether the function should guarantee the useful result on successful execution, or whether it should also have X-didn't-hold as a possible normal case result.
A guaranteed useful result means X violation should throw exception.
Otherwise, one way of reporting a possibly "null" result is by using a wrapper object that either is empty or logically contains the useful result. The case where the useful result is a simple or small value type has its own little pattern name, which I forget...
-- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail?
As far as a link goes, there isn't one available at this time as the article
is in print. If anyone else knows of a link I didn't find, please post...
Thanks!
"Dave" <be***********@ yahoo.com> wrote in message Two things could go wrong inside the function:
1. The function might find that the file does not exist 2. The function might find that the content of the file is invalid
(Some might consider 1 to be a special case of 2.)
The question is: Are these cases a violation of the function's
preconditions or postconditions? According to the error handling philosophy advocated
by Herb in his article, if it is a precondition violation, it should have
been reported as an error in the calling function. Since it wasn't, we should report a program bug via an assert in the called function. If it is a postcondition violation, it should be reported as an error in the called function. Since whether the problem is a precondition or postcondition problem determines where the error gets reported, it is something I need
to know to properly write my code.
Precondition and postconditions should test constraints between the member
variables of the class, as far as I know. For the two things that could go
wrong, I don't see a straightforward way to fit them into this scheme. So
my design decision is to throw an exception.
These seem to be This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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