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As the title states, which method is recommended to calculate the size

assuming types are non -zero copy hence alignment calculation is not needed or is the type aligned per say

in this situation which method is preferred and why ? i have seen people saying to not use sizeof i dont understand what is the harm in it except for paying rent for a few more bytes.

Update : mem::sizeof also takes into account the padding done to align the struct, if so manually calculating the individual sizes and summing it up is wrong is it not ? but almost every program i read seems to do it manually if so why is it not necessary to take into account the padding added while coming up with the final size

further question :Also if i have type like Option how is the size calculated ? 1 for Option + 1 for Enum +1 if it has simple fields ? or the 1 allocated for Enum Covers it ?

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  • i am aware rust uses null pointer optimization but if my account struct does not have any types like Option or Enum, mem::sizeof should work fine right ? ref :
    – Nabeel Naveed
    Commented Jul 26, 2022 at 16:25
  • Also if i have type like Option<Enum with simple fields> how is the size calculated ? 1 for Option + 1 for Enum +1 if it has simple fields ? or the 1 allocated for Enum Covers it ?
    – Nabeel Naveed
    Commented Jul 26, 2022 at 16:26
  • If you have additional questions these should be added to the question itself or moved to a new question
    – Henry E
    Commented Jul 27, 2022 at 20:01
  • done added it to the question
    – Nabeel Naveed
    Commented Jul 28, 2022 at 8:53
  • Reading it more carefully now, this does seem like worth asking in a separate question perhaps
    – Henry E
    Commented Jul 28, 2022 at 10:07

1 Answer 1

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For zero_copy accounts, you can use mem::size_of(). But for non-zero_copy accounts, the alignment is off and size_of() will give you a larger, incorrect space value. Instead you can do

    let acc = Account::default();
    let size = x.try_to_vec().unwrap().len();`

Basically, you cannot use the mem::size_of for serialized accounts, you need to give the serialized size, which can be inferred from the Default trait of the account

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  • i think it would be wrong to say alignment is off, i think if am not wrong the way to think about it is, not to think about alignment if its not zero-copy but think in terms of how borsh will serialize it, which is independent of the alignment, is my observation correct ser ?
    – Nabeel Naveed
    Commented Jul 26, 2022 at 23:49
  • yes, exactly, it depends on how borsh will serialize it
    – hrs8
    Commented Jul 27, 2022 at 7:18
  • Can this method just be run once at compile time or will this code run every time you want to initialize an account with the correct amount of space?
    – Henry E
    Commented Jul 27, 2022 at 19:58

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