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I understand the this (or self or Me) is used to refer to the current object, and that it is a feature of object-oriented programming languages. The earliest language I could find which has such a concept was Smalltalk, which uses self but was wondering where and when (which programming language) the concept was first implemented?

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Simula 67 is generally considered the first object-oriented language and predates Smalltalk by a number of years.

It also used the this keyword for the same concept, which can be seen in this book chapter extract:

  class Linker;
  begin
     ref(Linker) Next, Sex, Employment;
     text ID;

     procedure Add_to_List(LHead); name LHead; ref(Linker) LHead;
     begin
        Next :- LHead;
        LHead :- this Linker
     end..of..Add..to..List;

     procedure Onto_Lists(Gender,Occupation);
         name Gender,Occupation;
         ref(Linker) Gender,Occupation;
     begin
        Sex :- Gender;
        Employment :- Occupation;
        Gender :- Occupation :- this Linker
     end..of..Onto..Lists;

     InImage;
     ID :- Copy(SysIn.Image);
     InImage;
  end--of--Linker;
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    $\begingroup$ Interesting! What's up with those end.. and end-- bits? Do they constitute part of the syntax? $\endgroup$
    – jogloran
    Commented Mar 7, 2020 at 21:31
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    $\begingroup$ @jogloran: The Vim syntax-highlighting rules for Simula says: "Text between the keyword 'end' and either a semicolon or one of the keywords 'end', 'else', 'when' or 'otherwise' is also a comment." [link] That's obviously not authoritative -- it just means that Vim would color the ..of..Onto..Lists and --of--Linker and so on as comments -- but judging from the examples in the book chapter and elsewhere, I think that they are indeed optional comments (albeit not wholly freeform). $\endgroup$
    – ruakh
    Commented Mar 7, 2020 at 22:22
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    $\begingroup$ FWIW, a book I have here called "Introduction to Simula 67" gives the same rule as the Vim rule set mentioned by @ruakh, i.e. comments can directly follow the keyword END but may not contain any of the following: ;, END, WHEN, OTHERWISE, ELSE. Comments in the block body need to start with the keyword COMMENT. $\endgroup$
    – njuffa
    Commented Mar 8, 2020 at 0:04
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    $\begingroup$ @njuffa You are correct --- Simula67 used the old Algol60 comment style. I haven't used Simula67 since the 1970s though. It was in some ways more advanced than C++ which was an attempt to implement as much of Simula67 as possible using C. But Simula67 had coroutines which did not make it into C++. $\endgroup$
    – Martin Ellison
    Commented Apr 2, 2020 at 7:44
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    $\begingroup$ Nitpick: Smalltalk-80 was the first generally available version but Smalltalk-71 was the first iteration: worrydream.com/EarlyHistoryOfSmalltalk $\endgroup$
    – user3810626
    Commented Apr 2, 2020 at 23:50

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