static in different languages

问题:

I've heard there are differences between languages about the meaning of the keyword static, but I've not found a good list that consolidates those differences.

Here's what I know about the meaning of static in C++:

• For local static variables within a function, the variable is initialized at startup and the value is saved across function calls.
• Static data members are shared among all instances of a class. In other words, there is only one instance of a static data member. Static data members must be initialized at file scope.
• Static member functions have access only to static members.
• In recursive code, a static object or variable is guaranteed to have the same state in different instances of a block of code.
• Static objects and variables defined at file scope only have internal linkage. No other files may use them.

How does the meaning of static change in other languages?

回答1:

C

---

• The keyword can change either the linkage or the duration (lifetime) of an object.
• Variables are always initialized to 0
• Functions have internal linkage.
• If declared in file level scope: variables have internal linkage and static duration (i.e. exists throughout the lifetime of the program)
• If declared in block scope: variables have no linkage but static duration
• There can multiple declarations of the same static variable in a translation unit. However, note that they must be the same. E.g: at file-level scope:

---

int a;        // a has external linkage

static int a; // a now has static linkage
              // same as if you wrote: static int a = 0;

//...

static int b; // static linkage

extern int b; // extern loses its meaning, b still has internal linkage

//...

extern int b; // b has external linkage

static int b; // error

//...

void func() {
  static int x; // automatic linkage, static duration
                // same as if you wrote: static int x = 0;
}

C++

---

• At file level scope the usage has been deprecated for both variables and members in favor of anonymous namespaces. Exists only as compatibility
• Variables still get default initialized (as in C) to 0
• "6.7 The zero-initialization (8.5) of all local objects with static storage duration (3.7.1) or thread storage duration (3.7.2) is performed before any other initialization takes place [...] "
• Variables have static storage duration unless accompanied by a thread_local specifier (from C++0x onwards)
• There can be only one definition of a static in a translation unit
• Member variables/functions mean they are properties of the class and not the instances Legal access syntax: instance.property or Class::property
• Static member functions can only access only static member variables No this pointer for such functions
• Non-static members can however access any static member
• At file level objects have internal linkage except for class members which have a class scope
• Class members need to be defined either in the class declaration or outside explicitly via class name and scope resolution operator
• Cannot use this in a static method

---

ActionScript

---

• Class methods as in C++
• cannot use this or super in a static method
• Accessed only through class name and not instance name
• Not inherited
• Derived classes however have access to bases' static properties
• Variables that are declared with both the static and const keywords must be initialized at the same time as you declare the constant

---

Object Oriented Design

---

• The Singleton design pattern is considered by many as a glorified static object
• Used in Factory design pattern

---

I may have missed a lot of other things -- feel free to chip in.

回答2:

In Delphi the static keyword is used exclusively for defining class methods. In Delphi a normal class method can be declared virtual and overridden in a subclass. Additionally Delphi has a self variable, similar to the this pointer in other languages. However in a class method the self points to the class in which the method is called instead of an instance.

Declaring a class method static means:

1. It cannot be overridden in a subclass
2. It does not have a self pointer

This means a static class method can only access class members in the class it was defined in, while a normal class method can access overridden class members in derived classes.

There are other informal uses of static in the Delphi documentation usually referring to a feature unchangability (is that a word?). For instance a static array vs a dynamic array. All instance methods in Delphi are static unless declared otherwise.

回答3:

In VB.NET, a Static variable is just like a C++ local static variable.

However, there is no class-wide Static; use Shared instead.

回答4:

In C# there are 3 ways a static keyword can be used:

• On class definition, which means the class will only expose static members and cannot be instanciated
• On a class member, which means that the member can be called without having to instanciate the class.
• On a constructor, which means that the static constructor will allways be called before a static member is called. (Those are mostly performance bottlenecks and therefore not recommended)

Hope this helps.

回答5:

In C# it pretty much always means: "related to a type rather than an instance of the type".

回答6:

Python has the decorator @staticmethod, which when applied to a class member, makes the method available on the class rather than instances, and passes no automatic arguments to the method. The @classmethod decorator performs a similar function, but passes the class as the first argument, making it much more useful.

回答7:

In C, static flags a function or global variable as local to the file its located in.

It's kinda like private in other languages. Sorta.

If it's in a function, then static preallocates that variable in the data section of the binary, rather than on the stack at run time.

回答8:

In VB.NET static is used procedure level to mean the variable is associated with all executions of the procedure (it's preserved from call to call). That's a bit of an arcane usage in an Object-Oriented application though.

The counterpart is "Shared" which means the method or member is type level (you don't need an instance to access it).

回答9:

Wikipedia summarizes many different meanings of static:

Static Methods, static variables, static typing.