如何在不引用C语言结构实例的情况下访问变量？

Question

If I have a struct in C

typedef struct _a {
    int aval;
} a;

a a_inst;

void main() {
    a_inst.aval = 5;
}

how can I access "aval" without having to type as a_inst.aval for this example ?? Is this possible ?

Answer 1

a_inst.aval refers to "the aval part of the structure a_inst ".

aval - as you want to type - would refer to " aval ...?", at which point the compiler will become grumpy and refuse to cooperate.

Answer to your question: No.

(Unless, of course, you start wiggling about with #define / typedef / pointers as a means of shorthand notation, at which point you are crossing into dangerous waters of making your code less readable.)

Answer 2

Apart from #define or using a pointer, no. So you could do

#define AVAL a_inst.aval

or

int * pAval= a_inst.aval

But experienced programmers will not do this.

Answer 3

One possible way is to use a pointer on this structure field. But you will need

void main() {
  int *p_aval = &a_inst.aval;
  *p_aval=...; //access
}

personnally I will prefer to type the structure name. It is explicit, doesn't harm (like few more characters to type) and is less prone to bugs.

Answer 4

Since the structure is composed by only one int you can set aval in this way:

*((int *) &a_inst) = 5;
printf("AVAL %d\n", a_inst.aval);

result:

AVAL 5

but i reccomend to NOT USE this kind of structure access

Answer 5

Just presenting this as a simple, easy to understand example.

I am assuming you want to be able to do this:

typedef struct _a {
    int aval;
} a;

a a_inst;

void main() {
    aval = 5; // <-- Access aval without typing a_inst.aval
}

However, that is in no reasonable way possible.

Why?

Because what if you have:

typedef struct _a {
    int aval;
} a;

a a_inst;
a b_inst;  // Another instance of the a struct

void main() {
    aval = 5; // <-- How would the compiler know if aval refers to aval in a_inst or in b_inst?
}