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#1
May 25th, 2003, 05:03 AM
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Clean up problem
Hi,
I'm making a String class for educational purpose (new to c++). My problem is that multiple strings can reference the same buffer, to keep the string as light as possible. This means I have a copy constructor that looks like this: Code:
String::String(const String& str)
{
this->length = str.length;
this->data = str.data;
}
The problem is when the string is destructed. I don't know how the buffer has been created. It could have created using malloc, in which case I should invoke free, or it could have been created from a simple char array. So how can I free the buffer? The other problem is that I don't know if the buffer is referenced from another string. If the buffer would be freed, every string referencing it would be corrupted. How can I know when to free the buffer? Sort of like this: Code:
String::~String()
{
if(numberOfReferencesToBuffer == 0)
{
if(isBufferOnHeap(buffer))
{
free(buffer);
}
}
}
Thanks in advance, Nille
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#2
May 25th, 2003, 05:07 PM
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If sounds like horrible program design to me.
Is the "buffer" an object of your string class? If it is, you could create another variable in your string class to increment a counter everytime another string makes a copy of the string. Then only destroy a string object if the counter is zero. If it's a separate entity, you can still set up a similar variable or array of variables for multiple buffer counters. You're going to have to deal with the complexity of making a copy of a string that is a copy of the buffer: they'll all point to the same area in memory. As for how you will know whether to free up memory for your buffer, you could have a flag variable that you set to true or 1 to indicate that you had to allocate memory for it. Last edited by 7stud : May 25th, 2003 at 05:14 PM.
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#3
May 26th, 2003, 06:20 AM
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Quote:
It looks better in my head  The concept is that the String is an immutable "view", if you will, of the underlying character buffer. Since I come from Java, I am used to using resources carefully. I usually pass references, but in the case of strings it's sometimes easier to just pass it as a copy (I overload const char*). I just want to copy the "view", not that data. It looks like this inside my head:Code:
_____buffer_______
| | |
String String String
I just need to know how to delete the buffer when it is no longer referenced. I posted a new topic concerning memory. I hope I will get a response there.
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#4
May 27th, 2003, 08:47 PM
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Sounds like you're trying to implement a reference counted string implementation. IIRC, the ANSI standards committee specified this in one of their standards, and some implementations of the STL *do* use reference counted strings. So does Microsoft's CString class incidentally (proof here: http://msdn.microsoft.com/library/d...ings_in_mfc.asp)
To see some examples, see these links: http://www.michaelmoser.org/string.htm (gives you some reference counting basics) http://www.gotw.ca/gotw/043.htm (3 part series) http://www.cse.ucsc.edu/~pohl/Winter01/LL4/ (look to the bottom of the page) To see some problems with various implementations, see: http://www.sgi.com/tech/stl/string_discussion.html IIRC, there was a discussion about problems in some string implementations in Dr. Dobbs Journal, a while ago. Either that, or this Newcastle beer is really beginning to kick in .Hope this helps
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