What EXCACTLY is pure pragma in object declaration?

[Stefan_Salewski]

[https://nim-lang.github.io/Nim/manual.html#pragmas-pure-pragma](https://nim-lang.github.io/Nim/manual.html#pragmas-pure-pragma)
    
    
    type
      O {.pure.} = object
        i: int
    
    var o: O
    echo sizeof(o)
    
    
    Run

Output is 8 with and without pure pragma for 64 bit Linux.

And I am not absolutely sure if sizeof() gives us size including hidden fields 
used for inheritance.