Difference between static in C and static in C++??

Question

What is the difference between the static keyword in C and C++?

Answer 1

The static keyword serves the same purposes in C and C++.

1. When used at file level (outside of a function), it sets the visibility of the item it's applied to. Static items are not visible outside of their compilation unit (e.g., to the linker). Their duration is the same as the duration of the program.
   
   These file-level items (functions and data) should be static unless there's a specific need to access them from outside (and there's almost never a need to give direct access to data since that breaks the central tenet of encapsulation).
   
   If (as your comment to the question indicates) this is the only use of static you're concerned with then, no, there is no difference between C and C++.

2. When used within a function, it sets the duration of the item. Again, the duration is the same as the program and the item continues to exist between invocations of that function.
   
   It does not affect the visibility of that item since it's visible only within the function. An example is a random number generator that needs to keep its seed value between invocations but doesn't want that value visible to other functions.

3. C++ has one more use, static within a class. When used there, it becomes a single class variable that's common across all objects of that class. One classic example is to store the number of objects that have been instantiated for a given class.

As others have pointed out, the use of file-level static has been deprecated in favour of unnamed namespaces. However, I believe it'll be a cold day in a certain warm place before it's actually removed from the language - there's just too much code using it at the moment. And ISO C have only just gotten around to removing gets() despite the amount of time we've all known it was a dangerous function.

And even though it's deprecated, that doesn't change its semantics now.

Answer 2

The use of static at the file scope to restrict access to the current translation unit is deprecated in C++, but still acceptable in C.

Instead, use an unnamed namespace

namespace {     int file_scope_x; } 

Variables declared this way are only available within the file, just as if they were declared static.

The main reason for the deprecation is to remove one of the several overloaded meanings of the static keyword.

Originally, it meant that the variable, such as in a function, would be given storage for the lifetime of the program in an area for such variables, and not stored on the stack as is usual for function local variables.

Then the keyword was overloaded to apply to file scope linkage. It's not desirable to make up new keywords as needed, because they might break existing code. So this one was used again with a different meaning without causing conflicts, because a variable declared as static can't be both inside a function and at the top level, and functions didn't have the modifier before. (The storage connotation is totally lost when referring to functions, as they are not stored anywhere.)

When classes came along in C++ (and in Java and C#) the keyword was used yet again, but the meaning is at least closer to the original intention. Variables declared this way are stored in a global area, as opposed to on the stack as for function variables, or on the heap as for object members. Because variables cannot be both at the top level and inside a class definition, extra meaning can be unambiguously attached to class variables. They can only be referenced via the class name or from within an object of that class.