CODEWITHSUNDEEP
cstaticstructmember
user707549
user707549

Reputation:

The member variable of a static struct in C

I have a question about the member variables of static struct in C language.

Someone said we can declare a static struct, but in C, struct do not have the static members like class in C++, what does this mean? If I declare a static struct, what is the status of the members variable? can some one help me on this?

Upvotes: 7

Views: 23364

Answers (3)

justin
justin

Reputation: 104718

I have a question about the member variables of static struct in C language.

Someone said we can declare a static struct

Correct/legal:

// (global scope)
static struct t_struct {
    int a;
} THE_STATIC_VARIABLE;

but in C, struct do not have the static members like class in C++

// (global scope)
struct t_ill_struct {
    static int a; // << ill-formed in C, but valid in C++
};

what does this mean? If I declare a static struct, what is the status of the members variable? can some one help me on this?

Using the above example, it means that THE_STATIC_VARIABLE will have static storage. The following are equivalent:

A

// (global scope)
static struct t_struct {
    int a;
} THE_STATIC_VARIABLE;

B

// (global scope)
struct t_struct {
    int a;
};

static struct t_struct THE_STATIC_VARIABLE;

That is to say, every translation which sees THE_STATIC_VARIABLE's definition will get its own copy.

If you want the same effect as a static C++ member, you will have to declare it in another scope -- outside of a struct's declaration:

// file.h

// (global scope)
struct t_struct {
    int a;
};

extern struct t_struct THE_GLOBAL_VARIABLE;

// file.c
struct t_struct THE_GLOBAL_VARIABLE;

Now we really have exactly one, like in C++.

Upvotes: 4

Lundin
Lundin

Reputation: 215350

static in C means:

  • There shall only be one instance of this variable in the program.
  • The variable should be valid throughout the whole duration of the program execution.
  • The variable shall be initialized before main() is executed. If the programmer initializes a variable explicitly, then it should be set to this value. Otherwise it should be set to zero.
  • The variable (or static function) shall only be accessible from the scope it is declared inside. If declared at file scope (global), then it is only accessible in the same .c file.

In C++, there is also one additional meaning:

  • A static member variable or member function shall only be allocated once, no matter how many instances of a class that are allocated.

Upvotes: 4

user529758
user529758

Reputation:

Note that a static struct itself is different from a static member of a struct. While you can declare a static struct variable:

static struct MyStruct s;

you can't define a struct type with a static member:

struct MyStruct {
    static int i; // <- compiler error
};

The reason for this is that in C, a struct is a type - declaring a type of which a member variable is always the same instance (i. e. static) for multiple instances of that type is simply nonsense. In C++, structs are in reality classes (they only differ in the default visibility scope of members), and in C++ the static keyword means something else in this case. It means a class method - but since C doesn't have classes and methods, this doesn't make sense and is invalid in C.

Lesson learned: C++ is not C.

Upvotes: 9

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