detect invalid reference
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best practices teach to NULL a pointer you just delete...
delete testVector;
testVector = NULL;what i suspect you was to expecting the last cout to print 0x00000000. but delete frees the memory pointed to by the pointer, nothing much.
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That is not the problem. I don't even reference the pointer. I reference an element of the vector that has been destroyed. I could leave the delete on the vector and it would still print nonsense. The reference to the integer stored in the vector has become invalid. I have this problem because i need to implement a wrapper for a map on wich multiple threads can operate concurrently, inserting, reading and deleting. Now one thread could delete an element another thread is currently working on. Since the objects are managed by the map itself i cannot find out when that happened.
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That is not the problem. I don't even reference the pointer. I reference an element of the vector that has been destroyed. I could leave the delete on the vector and it would still print nonsense. The reference to the integer stored in the vector has become invalid. I have this problem because i need to implement a wrapper for a map on wich multiple threads can operate concurrently, inserting, reading and deleting. Now one thread could delete an element another thread is currently working on. Since the objects are managed by the map itself i cannot find out when that happened.
what do you mean by "print non-sense" ? what do you expect ? [edit] oh, my bad, i read your post incorrectly (i read that you were printing out the address in the testVector pointer). AFAIK, a reference cannot be changed of address, so by clearing the vector, then deleting it, you invalidate the reference. the only way you have to know the reference may be invalid is not writing such code. i don't think we can know otherwise BTW, it seems that you need some synchronizing code around your vector... one should not read it until another thread finished to modify it, and such references being temporary shouldn't be stored [/edit]
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That is not the problem. I don't even reference the pointer. I reference an element of the vector that has been destroyed. I could leave the delete on the vector and it would still print nonsense. The reference to the integer stored in the vector has become invalid. I have this problem because i need to implement a wrapper for a map on wich multiple threads can operate concurrently, inserting, reading and deleting. Now one thread could delete an element another thread is currently working on. Since the objects are managed by the map itself i cannot find out when that happened.
Mr.Brainley wrote:
I reference an element of the vector that has been destroyed.
This would be impossible if you had assigned
NULLto the pointer after deleting it. It's a two-step process. AssignNULLto the pointer after deleting it, and check if the pointer is equal toNULLbefore referencing it.
"Approved Workmen Are Not Ashamed" - 2 Timothy 2:15
"Judge not by the eye but by the heart." - Native American Proverb
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what do you mean by "print non-sense" ? what do you expect ? [edit] oh, my bad, i read your post incorrectly (i read that you were printing out the address in the testVector pointer). AFAIK, a reference cannot be changed of address, so by clearing the vector, then deleting it, you invalidate the reference. the only way you have to know the reference may be invalid is not writing such code. i don't think we can know otherwise BTW, it seems that you need some synchronizing code around your vector... one should not read it until another thread finished to modify it, and such references being temporary shouldn't be stored [/edit]
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Synchronization is of course neccessary (bad seplling ?!), but i want to keep it minimal, since too much synchronization can make a multithreaded application almost sequential, this voiding the advantage of multithreading. It is also correct that the reference should not be stored, but experience tells us, that users always do whatever it takes to crash a program, or the library in that case. So i try to keep my code bulletproof. My approach now is to write a wrapper-class that behaves like a reference, but checks on every access weather the elemennt/vector actually still exists. I'm not sure though if that is the smartest way to do it, it seems like a waste of resources.
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Synchronization is of course neccessary (bad seplling ?!), but i want to keep it minimal, since too much synchronization can make a multithreaded application almost sequential, this voiding the advantage of multithreading. It is also correct that the reference should not be stored, but experience tells us, that users always do whatever it takes to crash a program, or the library in that case. So i try to keep my code bulletproof. My approach now is to write a wrapper-class that behaves like a reference, but checks on every access weather the elemennt/vector actually still exists. I'm not sure though if that is the smartest way to do it, it seems like a waste of resources.
Mr.Brainley wrote:
but i want to keep it minimal
don't keep it "minimal", keep it efficient. if you have too much synchronization, then there may have a design problem. for your vector, the need of the mutex is pretty neat...
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Consider the following code :
vector \*testVector = new vector(); testVector->push\_back(1); int &testRef = (\*testVector)\[0\]; cout << testRef << endl; // completeley destroy the vector and everything in it testVector->clear(); delete testVector; // now access the invalid reference cout << testRef << endl; // works, but prints nonsenseHow can i detect that the reference has become invalid ?
A try/catch around the dereference would do it, but it's cumbersome to have many of those throughout the app. In C++, it's your responsibility to not do that in the first place. :)
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Consider the following code :
vector \*testVector = new vector(); testVector->push\_back(1); int &testRef = (\*testVector)\[0\]; cout << testRef << endl; // completeley destroy the vector and everything in it testVector->clear(); delete testVector; // now access the invalid reference cout << testRef << endl; // works, but prints nonsenseHow can i detect that the reference has become invalid ?
Mr.Brainley wrote:
How can i detect that the reference has become invalid ?
You can not; a reference just refers to a memory block that is expected to contain data of a given type (the referenced data type). It is up to you, the programmer, to insure that the memory block is valid, as the language does not provide that for you. If there is a chance that the memory block will become invalid without you knowing it, bad design, then you could try using one of Microsoft’s “IsBad…” functions to check the address referred to by the reference:
IsBadWritePtr(&testRef,sizeof(int)). This method, of course, may not work if some other piece of code has allocated that same memory block between the time you freed it and the time you checked the address pointer; which will lead to very hard to find bugs. Another thing I would like to point out is that just adding another item to the ‘vector’ can cause your reference to become invalid if the ‘vector’ had to reallocate memory in order to expand the memory block size. Basically, any reference, of the type you provided, should be temporary and only exist in local scope. It should not be ‘static’ or ‘global’, unless the memory it refers to is not released until the application exits.INTP "Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra
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Consider the following code :
vector \*testVector = new vector(); testVector->push\_back(1); int &testRef = (\*testVector)\[0\]; cout << testRef << endl; // completeley destroy the vector and everything in it testVector->clear(); delete testVector; // now access the invalid reference cout << testRef << endl; // works, but prints nonsenseHow can i detect that the reference has become invalid ?
The best solution would be not to get in this situation. There are other solutions however. For example you could construct a container based on a
std::vectorthat uses something like Boost's Shared Container Iterator[^] and use iterators instread of usig ponters. Other smart pointer based techniques are also possible. Astd::vectoras it stands is not designed for this usage pattern (and doesn't pay the price that implementing it incurs).Steve
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A try/catch around the dereference would do it, but it's cumbersome to have many of those throughout the app. In C++, it's your responsibility to not do that in the first place. :)
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Michael Dunn wrote:
A try/catch around the dereference would do it, but it's cumbersome to have many of those throughout the app. In C++, it's your responsibility to not do that in the first place.
I don't think it would (do it). This approach is dangerous. The memory manager could have reused the address that used to contain the
vector's contents after thevectorhas been destroyed. This technique can not be relied upon.Steve
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Consider the following code :
vector \*testVector = new vector(); testVector->push\_back(1); int &testRef = (\*testVector)\[0\]; cout << testRef << endl; // completeley destroy the vector and everything in it testVector->clear(); delete testVector; // now access the invalid reference cout << testRef << endl; // works, but prints nonsenseHow can i detect that the reference has become invalid ?
Mr.Brainley wrote:
How can i detect that the reference has become invalid ?
You can't. BTW, vector objects are not meant to be created on the heap.