Question

Assuming I have a structure such as:

typedef struct
{
    char * string1;
    char * string2;
} TWO_WORDS;

such that all the fields are of the same type, and my main has

TWO_WORDS tw;

can I reference string1 with tw[0] and string2 with two[1]? If so:

• is this part of the c standard?
• do i have to cast the struct to an array first?
• what about fields which are different sizes in memory
• what about fields which are different types but the same size?
• can you do pointer arithmetic within a structure?

–

Answer 1

I got pretty close with this construct:

((char**)&tw)[0];

As an example:

int main()
{
    typedef struct
    {
        char * string1;
        char * string2;
    } TWO_WORDS;

    TWO_WORDS tw = {"Hello", "World"};

    printf("String1: %s\n", ((char**)&tw)[0]);
    printf("String2: %s\n", ((char**)&tw)[1]);

    return 0;
}

It is not guaranteed to work, as the compiler may add padding between fields. (Many compilers have a #pragma that will avoid padding of structs)

To answer each of your questions:

• is this part of the c standard? NO

• do i have to cast the struct to an array first? YES

• what about fields which are different sizes in memory
  This can be done with even more “evil” casting and pointer-math

• what about fields which are different types but the same size?
  This can be done with even more “evil” casting and pointer-math

• can you do pointer arithmetic within a structure?
  Yes (not guaranteed to always work as you might expect, but a structure is just a piece of memory that you can access with pointers and pointer-math)