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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31.07.2023

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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31.07.2023

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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31.07.2023

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

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31.07.2023

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

depends on what they are pointing to

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31.07.2023

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

Why do you have 2 pointers to the same thing?

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31.07.2023

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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31.07.2023

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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31.07.2023

VD
Irrespective of what they were initially pointing ...

link?

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31.07.2023

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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31.07.2023

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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31.07.2023

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

okay?

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31.07.2023

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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31.07.2023

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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31.07.2023

Sahand 🏔️ Автор вопроса

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

My container is a linked list

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31.07.2023

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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31.07.2023

He's not using an invalid pointer *value*

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31.07.2023

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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31.07.2023

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

Why would that be UB?

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31.07.2023

Thomas
Why would that be UB?

Because the standard says so

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31.07.2023

Madhu

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

It is not Undefined Behavior but Implementation defined behavior

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31.07.2023

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

Errm, I just quoted the standard.

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31.07.2023

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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31.07.2023

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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01.08.2023

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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01.08.2023

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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01.08.2023

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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01.08.2023

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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01.08.2023