Clarification of use of structs with Arduino and storing structs in PROGMEM

Question

(Because of the limited memory available on most Arduino boards, I've sometimes run into problems using valid C/C++ code, so this question is specifically about any issues on using structs on Arduino.)

I've seen example code of using structs in Arduino but no discussion of memory reqs.

1. Is the size of a struct simply the sum of the data types of its fields?
2. Is storing structs in PROGMEM an option? Are there access speed issues?
3. Are the fields of a struct writable (e.g. example below s1.LED1.state = 0;) (though not if stored in PROGMEM, of course).
4. Can I define a field of a struct as another (different type) struct?
5. Is it possible to iterate through a struct using for..in or by index?

My use case is that I have 64 LEDs driven by a MAX7219 chip. Because of the requirements of the physical wiring layout, it would be convenient to organize the LED order in a more logical way using structs in order to make the programming easier/more coherent.

---

  typedef struct {
      byte row : 6;
      byte col : 128;
      byte state : 1;
    } LED;

---

 typedef struct {
      LED LED1 : {1,1,1};
      LED LED2 : {1,2,1};
      LED LED3 : {1,4,1};
      LED LED4 : {1,8,1};
      LED LED5 : {1,16,1};
      LED LED6 : {1,32,1};
      LED LED7 : {1,64,1};
      LED LED8 : {1,128,1};
    } LED_SECTION;

---

LED_SECTION s1;
s1.LED1.row = 1;

---

 s1.LED1.state = 0;

Answer 1

1. When using the semi-colon syntax, the size of the struct will be the sum of all its fields.

2. I think its possible using this syntax: (http://www.arduino.cc/en/Reference/PROGMEM)

   LED leds PROGMEM;
   

3. Yes they are, the syntax is as you wrote in your question.

4. Yes you may:

   typedef struct { struct otherStruct; };

5. Yes you can do that using masks. For example:

   for (int i = 0, byte cur = s1 & 1; ; i < numOfFieldsInStruct; i++, cur = (s1<<i)&1) {
       ....
   }
   

Regarding your last comment to this answer, let me propose the following solution:

Organize the leds in a way that addressing them won't take up memory (like in the question - the LED struct takes memory for addressing). Instead, you can address the leds using their position in an array and in the struct like this:

typedef struct {
    byte LED1 : 1;
    byte LED2 : 1;
    byte LED3 : 1;
    byte LED4 : 1;
    byte LED5 : 1;
    byte LED6 : 1;
    byte LED7 : 1;
    byte LED8 : 1;
} LED_ROW;

LED_ROW leds[256];

leds[0].LED1 = 1; // turn led at row 0, col 0 to 1
leds[0].LED5 = 1; // turn led at row 4, col 0 to 1
led[100].LED3 = 1; // turn led at row 2, col 100 to 1
...
// and so on

You may consider arranging the array differently, with 256 items in the struct and 8 items in the array so the rows will be refered to by [] and the cols after the dot, like so:

leds[0].LED3 = 1; // turn on led at row 0, col 2 to 1