Initializing all members (of the same, basic, data type) of a struct to one given value [duplicate]

Answer 1

One way to address this problem is using the X Macro concept link1, link 2.

The list of struct members would be

#define LIST_OF_STRUCT_MEMBERS \
    X(a) \
    X(b) \
    X(c)

Then one would declare the struct as:

#define X(name) int name;
struct myStruct {
     LIST_OF_STRUCT_MEMBERS
}
#undef X

Where int could be replaced by any basic data type.

One could then create a const variable initialized to the default value:

#define X(name) -1,
const struct myStruct myStruct_DEFAULT_VALUE = { LIST_OF_STRUCT_MEMBERS };
#undef X

Translated by the preprocessor into:

const struct myStruct myStruct_DEFAULT_VALUE = {-1, -1, -1,};

This can be copied every time a new variable has to be initialized.

The method can easily be verified defining in the same way a print function:

#define X(name) printf("%s = %d\n", #name, in->name);
void printAllMembers(struct myStruct* in) {
     LIST_OF_STRUCT_MEMBERS
}    
#undef X

(The printAllMembers function might need some modification when the underlying basic data type is changed).

Verified here.

---

This concept can generalize to a struct with fields of different data types if we include the data type in LIST_OF_STRUCT_MEMBERS, e.g.:

#define LIST_OF_STRUCT_MEMBERS \
    X(int, a) \
    X(float, b) \
    X(double, c)

#define X(type, name) type name;
struct myStruct {
     LIST_OF_STRUCT_MEMBERS
};
#undef X

#define X(type, name) -1,
const struct myStruct myStruct_DEFAULT_VALUE = { LIST_OF_STRUCT_MEMBERS };
#undef X

#define intFormatting "%d"
#define floatFormatting "%f"
#define doubleFormatting "%f"

#define X(type, name) printf(#name " = " type ## Formatting "\n", in->name);
void printMyStruct (struct myStruct* in) {
     LIST_OF_STRUCT_MEMBERS
}
#undef X

Tested here.