8

To me, the use of the keyword static in C and languages like C# and Java are "false friends" like "to become" in English and "bekommen" in German (= "to get" in English), because they mean different things.

In C static means, that the function or variable is only accessible via functions inside the same source file, comparable to private functions and members in C++, Java and C#.

In C++, Java and C# static means, that the methods are not members of a class instance, but effectively are more or less like C functions plus namespace.

IMHO these two concepts are quite different, so why did the designers of C++ and later Java and C# choose the static keyword for that behaviour? Is there a logical connection that I miss?

EDIT I know, that static in C does not govern accessability in a way similar to private in C++, but can be used in that way, see https://stackoverflow.com/a/1479639/124983

9

I have a book about the design of C++ by Bjarne Stroustrup (the inventor of C++). I don't have it right here so I can't lookup the exact quote right now, but in it he admits that when he added static to C++, he didn't fully understand what it meant in C. So that's why in C++ it has a different meaning than in C.

Java and C# inherited the meaning from C++.

  • Created an account on here just to upvote this answer! This is amazing information that I wouldn't ever wanna miss in my life as software developer! – Maverick283 Mar 11 '19 at 7:57
  • @Maverick283 It's this book: The Design and Evolution of C++ – Jesper Mar 11 '19 at 8:42
14

Notably, C++ has both uses of static, and I think there's a third somewhere.

Generally, I think that the C use of static does not correspond at all to any English usage of the word, whereas I think that static as a static member variable, for example, makes a lot more sense.

Remember that as a language designer, you have a fair incentive to introduce fewer keywords into the language, to disallow less code- especially when you're trying to be source-compatible with C, as in C++, and the static keyword already existed, so they couldn't break any C programs trying to compile as C++ by re-using it.

Edit: I knew there was a third. C has function-level static variables. The static member is just a scoped version of this functionality. Therefore, both uses of static originate from C.

8

In C++, static has the following four uses:

  • Global (file-level) function and variable declarations: with static, you are specifying internal linkage. That means that the symbol is only used in that one compilation unit (.cpp/cc/whatever file, not a header). This is what you are referring to as "private".

  • Local variables: Static storage duration specifies that the variable should retain its value between function calls.

  • Data members: Static data members are shared between the instances of the class (i.e. there is only one copy of the member). This is akin to C# and Java static.

  • Member functions: Static member functions are member functions that don't have an implicit this pointer; they can be invoked without an instance. This is also much like in C# or Java.

So, as you can see, they dropped two of the above meanings. For scoping it is quite understandable why: both C# and Java (and even C++) relies on namespaces to do that. C++ probably has the internal linkage feature for backwards compatibility. Ditching static local variables is probably two-folded: for once, it can be difficult to fully understand its consequences, and these languages aim for simplicity. Second, both Java and C# has a garbage collector which (I can imagine) causes difficulties to implement such behavior.

  • "C++ probably has the internal linkage feature for backwards compatibility." - one might add (IIRC) that the C++ Standard first deprecated this use of static (preferring anonymous namespaces) and then, in its current incarnation C++11 de-deprecated the use of static for this. (all IIRC) – Martin Ba Mar 28 '12 at 17:27
  • Really? I'm pretty sure that compilers didn't honor this deprecation then, because it worked with at least two mainstream ones (gcc and MSVC). – Tamás Szelei Mar 28 '12 at 17:48
  • No compiler threw it out as far as I know. Still I think the '98/'03 standard deprecated it or somesuch. What I actually wanted to say with my comment was that the internal linkage thing is a feature that is apparently also considered useful in C++, so it's not only there for backwards compatibility. – Martin Ba Mar 28 '12 at 18:28
  • The "c++-y" way of doing the same thing is to declare in an anonymous namespace. – Tamás Szelei Mar 28 '12 at 19:16
  • "is to declare in an anonymous namespace" - except that it is not the same thing technically. (I do apologize for not having more references, but I lack the time atm. to look it up. (I think it was mentioned in the comp.lang.c++.moderated NG, and maybe one can find some reference in the standard papers of WG21 ...) – Martin Ba Mar 29 '12 at 5:49
3

C++ took it from C, because C++ likes to reuse existing keywords instead of introducing new ones, to minimize breaking existing code.

Java and C# then took it from C++.

3

C static does not govern accessibility, and isn't analagous to private in the way you suggest.

At file scope, it governs availability of the name (linker symbol), not the thing - you can pass a pointer to a static function or global from the translation unit where it is defined to code in another file, and it will work fine.

The C++ way of doing this (as DeadMG says, the C way is still available too) is to use an anonymous namespace.

In function scope, it essentially replaces the local variable with a global which is only accessible (by name again!) inside that function - this is identical in C++ (so, there isn't a more C++y way of doing it).

As for why C++ used the static qualifier for class members; it's probably right to say that this is to minimize the number of new keywords introduced. (Note the recent re-use of auto for comparison).

Calling per-class data static (so implicitly per-instance data are dynamic) seems fairly intuitive to me, but since I don't remember the process of forming that intuition, I can't really comment objectively on whether it made sense when starting out.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.