Can doing this result in any UB or segmentation fault

What type does ptr point to? Does it have a dtor?

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9 месяцев назад

Manav | avoid unnecessary messaging me
Well assuming delete ptr itself is not UB, it shou...

It is UB according to the standard. Only comparison of pointers within an array (one past the end of the array) or a pointer to an object and a pointer just past the object or a nullptr can be compared with each other (unless you are using std::less). A deleted pointer is none of these though it may have been one of these before the deletion

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9 месяцев назад

Sahand 🏔️ Автор вопроса

delete ptr2; if (ptr1 == ptr2) { ptr1 = nullptr; } I wanna make ptr1 nullptr, in case it points to a deleted block memory.

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9 месяцев назад

No this is Undefined behavior. You have to compare them before deletion

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9 месяцев назад

VD
No this is Undefined behavior. You have to compare...

depends on what they are pointing to

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9 месяцев назад

Sahand 🏔️
delete ptr2; if (ptr1 == ptr2) { ptr1 = nullpt...

Why do you have 2 pointers to the same thing?

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9 месяцев назад

Sahand 🏔️ Автор вопроса

Azadi
What type does ptr point to? Does it have a dtor?

It does have a dtor, but dtor is reliable in any situation. My dtor deallocates every allocated object. In case they are already deallocated, nothing new happens.

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9 месяцев назад

Thomas
depends on what they are pointing to

Irrespective of what they were initially pointing to, this is still Undefined Behavior as ptr2 becomes an invalid pointer after the delete expression. The standard says using an invalid pointer in any way other than assigning it a new value is UB

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9 месяцев назад

VD
Irrespective of what they were initially pointing ...

link?

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9 месяцев назад

VD
Irrespective of what they were initially pointing ...

Compilers don't have the power to statically track lifetimes like that, so it will never actually become observable UB, it will always generate working code

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9 месяцев назад

Sahand 🏔️ Автор вопроса

Thomas
Why do you have 2 pointers to the same thing?

Assume I have an erase(). Well, for std erase(), it may take two iterators as arguments. But for my implementation, it takes two 'ints' as indices. erase() will first find the corresponding objects as the indices refer to (the second ptr to the obj), and also, I have already a pointer to the object (the first ptr to the obj).

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9 месяцев назад

Sahand 🏔️
Assume I have an erase(). Well, for std erase(), i...

okay?

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9 месяцев назад

Sahand 🏔️ Автор вопроса

Thomas
okay?

Now I have a remove() that deletes the second ptr. I need to make the first pointer to the object, if the second pointer (that is gonna be freed up) points to the same object the first one does.

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9 месяцев назад

Sahand 🏔️
Assume I have an erase(). Well, for std erase(), i...

You have an indice into a vec / array of... pointers?

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9 месяцев назад

Sahand 🏔️ Автор вопроса

Thomas
You have an indice into a vec / array of... pointe...

My container is a linked list

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9 месяцев назад

basic.std.dynamic.deallocation If the argument given to a deallocation function in the standard library is a pointer that is not the null pointer value (4.10), the deallocation function shall deallocate the storage referenced by the pointer, rendering invalid all pointers referring to any part of the deallocated storage. The effect of using an invalid pointer value (including passing it to a deallocation function) is undefined.

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9 месяцев назад

He's not using an invalid pointer *value*

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9 месяцев назад

Thomas
He's not using an invalid pointer *value*

When comparing two pointers, you are indeed using the value. Value here refers to the address the pointer points to

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9 месяцев назад

VD
When comparing two pointers, you are indeed using ...

Why would that be UB?

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9 месяцев назад

Thomas
Why would that be UB?

Because the standard says so

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9 месяцев назад

Madhu

VD
basic.std.dynamic.deallocation If the argument gi...

It is not Undefined Behavior but Implementation defined behavior

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9 месяцев назад

Madhu
It is not Undefined Behavior but Implementation de...

Errm, I just quoted the standard.

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9 месяцев назад

Madhu

VD
Errm, I just quoted the standard.

You quoted an old version of the standard. Here is the latest: If the argument given to a deallocation function in the standard library is a pointer that is not the null pointer value (4.10), the deallocation function shall deallocate the storage referenced by the pointer, rendering invalid all pointers referring to any part of the deallocated storage. Indirection through an invalid pointer value and passing an invalid pointer value to a deallocation function have undeﬁned behavior. Any other use of an invalid pointer value has implementation-deﬁned behavior.

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9 месяцев назад

VD
Errm, I just quoted the standard.

What link do you quote from? This is the version I visit occasionally: https://eel.is/c++draft/basic.stc.dynamic

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9 месяцев назад

Azadi
What link do you quote from? This is the version I...

I have the draft copies downloaded as PDF. I quoted from the wrong standard as pointed out. I must remember to sort the draft copies by date before opening up the one on top.

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9 месяцев назад

VD
I have the draft copies downloaded as PDF. I quote...

I think if there was such a statement in standard it'd surely exist in that up to date version I linked. Of course I found that statement on isocpp but with a old data, like 2012 or so. So I guess standard may not have any statement regarding that issue. But not sure yet.

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9 месяцев назад

Azadi
I think if there was such a statement in standard ...

You are referring to my statement or the one posted by Madhu? The one that Madhu posted is on CPPReference even. https://en.cppreference.com/w/cpp/language/pointer Check out the section on Pointer (2nd paragraph there) Since C++17, this piece on invalid pointers has been moved to another section of the standard. So don't search for it in deallocation

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9 месяцев назад

VD
You are referring to my statement or the one poste...

I was looking for that statement generally, ofc on the standard.

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9 месяцев назад