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I have a question: let's say we have this function: (in C++)

int& f() {
    static int x = 0;
    return x;
} // OK

and 

int& h() {
    int x=0;
    return x;
} // ERROR

Why does h give an error? Is it because of the keyword static? I found static keyword lets my x variable live after my function is terminated. So I still can access at that memory location from the outside (another function or main? right?). Instead int x = 0 is lost after h terminates. Right? I'm not sure I really got it!

And what about Java? I read I cannot declare static variables in methods but only in classes.

Thank you.

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5
  • 1
    That isn't an error unless you use -Werror. But stackoverflow.com/questions/6441218/… Commented Jun 18, 2014 at 14:59
  • C++: yes. Java: one question at a time! Commented Jun 18, 2014 at 14:59
  • One at a time. Ask for C++ or Java. And yes, static keyword has a different meaning in Java. Commented Jun 18, 2014 at 14:59
  • Regarding static in C++ : The static keyword and its various uses in C++ Commented Jun 18, 2014 at 15:04
  • "Right?" Right. You've answered your own question. Commented Jun 18, 2014 at 15:05

4 Answers 4

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22

In C++, static is one of the most overloaded keywords of the language. The meaning you're using here is this:

A variable which is defined inside a function with the static specifier has static storage duration - it occupies the same space for the entire runtime of the program, and keeps its value between different calls to the function. So you can safely return a reference to it, as the variable is always there to back the reference.

A normal (non-static) function-local variable is destroyed when the function call returns, and so the reference becomes dangling - it doesn't refer to anything valid. Using it results in Undefined Behaviour.

Java simply doesn't have function-scope static variables (it doesn't have that meaning of the keyword static). That's why you can't declare it there.

Both C++ and Java have the "class-scope" meaning of the static keyword. When a member of a class is declared with the static keyword, it means the member is not bound to any instance of the class, but is just a global variable whose identifier lives in the class's scope.

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3 Comments

I would give a precision on C++ (or C) static variables. As the have only one space all along the program, they are non re-entrant : in multithreaded programs, they share a common value between all threads. And if you want to call the function in a recursive manner, they are neither pushed to stack nor popped.
@SergeBallesta What exactly does a "re-entrant variable" mean? I always understood code to be re-entrant or thread-safe, not data. But you're of course right that a static local variable is just a global variable with funny name access rules.
I agree with you an re-entrant variable does not mean anything. But when you have a static variable in a function, the function will have re-entrance problems ...
3

Static keyword is used for almost same purpose in both C++ and Java. There are some differences though.

1) Static Data Members: Like C++, static data members in Java are class members and shared among all objects.

2) Static Member Methods: Like C++, methods declared as static are class members and have following restrictions:

  • (i) They can only call other static methods.
  • (ii) They must only access static data.
  • (iii) They cannot access this or super

Like C++, static data members and static methods can be accessed without creating an object. They can be accessed using class name.

3) Static Block: Unlike C++, Java supports a special block, called static block (also called static clause) which can be used for static initialization of a class. This code inside static block is executed only once (first time you make an object of that class or the first time you access a static member of that class (even if you never make an object of that class)).

4) Static Local Variables: Unlike C++, Java doesn’t support static local variables. If used , Java program fails in compilation.

5) Static class: Classes can also be made static in Java.In java, we can’t make Top level class static. Only nested classes can be static.

  • Nested static class doesn’t need reference of Outer class, but Non-static nested class or Inner class requires Outer class reference.
  • Inner class(or non-static nested class) can access both static and non-static members of Outer class. A static class cannot access non-static members of the Outer class. It can access only static members of Outer class.

Ref : www.geeksforgeeks.org

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2 Comments

Verbatim repetition of this webpage: geeksforgeeks.org/static-keyword-in-java Are you the author of this article or have you plagiarized? If the former, say so in your post so we will know.
agreed - possible plagiarism
0

For the Java side you are correct. Static variables in Java must be declared at the class level not inside a method.

If you need to scope the static variables then you probably have some seriously broken architecture but you can do it to a certain extent by using inner classes to store the static variables in.

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0

Static variables in C++ in functions are persistant varibles in the scope of a function. In Java, you can't have static variables in methods. Static variables are variables of the class, not its instances.

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2 Comments

@jaunchopanza, it's sort-of right for the case of variables declared static at function scope. It's incomplete, however, since it ignores other uses of the heavily overloaded static keyword to qualify variables in other scopes.
@juanchopanza Sorry, I specified that that's only when declared in a function. Is that correct?

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