Want to win a PS4? Go Premium and enter to win our High-Tech Treats giveaway. Enter to Win

x
?
Solved

When references, pointers, and iterators to a string become invalid

Posted on 2004-04-14
6
Medium Priority
?
289 Views
Last Modified: 2010-04-01
From the standard I gather that references, pointers, and iterators to a string become invalid after:

calling any non-const member functions (with the exception of [], at, begin, end, rbegin, rend)

calling data or c_str

the first call to non-const functions [], at, begin, end, rbegin, rend

However, what isn't immediately obvious is that the standard defines compare() as:

traits::compare(data(), str.data(), rlen);

And most string implementations use compare() in the comparison operators.

So can one add to that list all of the compare() overloads and comparison operators?

Is that a justifiable statement? And might there be any more exceptions?

Thanks,
-Sandra
0
Comment
Question by:Sandra-24
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
6 Comments
 
LVL 86

Expert Comment

by:jkr
ID: 10829240
>>So can one add to that list all of the compare() overloads and comparison operators?

Give me a reason why a comparison is not "const"  regarding the objects being compared and I will say "no" - I guess it is a "yes", though :o)

In other words: Do not rely on the 'const' keyword alone. But, saying that, I start to wonder why there are no 3 'const' overloads for all 4 combinations of comparison...
0
 
LVL 86

Accepted Solution

by:
jkr earned 2000 total points
ID: 10829251
>>Is that a justifiable statement?

Forgot to write "yes".
0
 
LVL 12

Expert Comment

by:stefan73
ID: 10830822
Hi Sandra-24,
> calling data or c_str

Why should calling c_str invalidate a pointer or reference? Or even an iterator?

Cheers,
Stefan
0
What does it mean to be "Always On"?

Is your cloud always on? With an Always On cloud you won't have to worry about downtime for maintenance or software application code updates, ensuring that your bottom line isn't affected.

 
LVL 9

Expert Comment

by:jasonclarke
ID: 10840909
> Why should calling c_str invalidate a pointer or reference?

Realistically it probably will not invalidate the pointer.

However, the standard allows for the fact that the string could be implemented such that an internal representation may not occupy contiguous storage - e.g. while building a string it could store different sections of the string in multiple places.

However, when the buffer returned by c_str/data must be contigous, so in such an implementation, at this point the class would have to reallocate some contiguous space for the string.
0
 
LVL 3

Author Comment

by:Sandra-24
ID: 10846898
Comparison operations SHOULD be const. Any string implementation with a data() function that invalidates pointers,references, or iterators SHOULD not call data() in their comparison operators as the standard seems to hint. However, as far as the standard is concerned, a string implementation could behave this way. Realisticaly it's probably only a concern to implementors of STL strings. I would have to think that any place where data() must be mutating in a way that invalidates references (and I really can't think of one!) that the programmers would have been clever enough to design a compare function that is non-mutating (i.e. doesn't use data() internally). However, the good folks that created the standard must have envisioned a possible scenario (I would hope they didn't just add the rule for fun) where data() could invalidate references, pointers and iterators.
0
 
LVL 9

Expert Comment

by:jasonclarke
ID: 10857656
>  However, the good folks that created the standard must have envisioned a possible scenario

There is quite a lot of evidence that they messed up in the design of the string class (e.g. see this reference: http://www.gotw.ca/gotw/084.htm ).

I think that were they starting again,  std::string might be more prescriptive in design, but would be a lot simpler...  This is the kind of thing that C++ detractors can point to as evidence of how pointlessly complicated the language is - this is only a string, after all.
0

Featured Post

Tech or Treat!

Submit an article about your scariest tech experience—and the solution—and you’ll be automatically entered to win one of 4 fantastic tech gadgets.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

When writing generic code, using template meta-programming techniques, it is sometimes useful to know if a type is convertible to another type. A good example of when this might be is if you are writing diagnostic instrumentation for code to generat…
Unlike C#, C++ doesn't have native support for sealing classes (so they cannot be sub-classed). At the cost of a virtual base class pointer it is possible to implement a pseudo sealing mechanism The trick is to virtually inherit from a base class…
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…
The viewer will learn how to pass data into a function in C++. This is one step further in using functions. Instead of only printing text onto the console, the function will be able to perform calculations with argumentents given by the user.

609 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question