Arduino compile error while using reference to a struct in another struct

Question

I'm on prgramming a menu for an 16x2 LCD for a small arduino project. I'm nearly finished, but i do not understand the last small issue.

The following (simplified) code generates the problem:

int var1=0;
int var2=0;

typedef struct {
    unsigned int item_value;
    char item_name[17];
} Select_Item;

typedef struct {
  char item_name[17];
  int* variable;
  Select_Item* list;
} Menu_Item;

Select_Item sel_one = { 0, "Selection 1" };
Select_Item sel_two = { 1, "Selection 2" };
Select_Item* sel_list[2] = { &sel_one, &sel_two };

Menu_Item menu_item1 = { "Item 1", &var1, NULL }; 
Menu_Item menu_item2 = { "Item 2", &var2, &sel_list }; 
Menu_Item* menu_list[2] = { &menu_item1, &menu_item2 };

It ends up with the following error:

 sketch_feb08a.ino:24:53: error: cannot convert 'Select_Item* (*)[2]' to 'Select_Item*' in initialization

In the code i'm accessing the values from the variables and show it in the display and once edited i can write it back to the variable. That was not the problem as long as i had just numbers to show/edit. Now for ease of use i wanted to add some kind of option menu, where the user can choose from the options. The item_name should be displayed, instead of the raw value, but of course the item_value should be used behind the scene. That is why i introduced the Select_Item struct.

I don't understand the error message. What's wrong here?

The error is pretty clear. &sel_list is a pointer-to-array, or Select_Item * (*)[2]. Whereas Menu_Item::list is a pointer-to-Select_Item or Select_Item *. — The Paramagnetic Croissant
– The Paramagnetic Croissant, Commented Feb 8, 2015 at 18:22
I want to have a pointer to the selection list sel_list in the menu_item. Or in short a way how i can make a list of options that i can access later in the code — Stefan P
– Stefan P, Commented Feb 8, 2015 at 19:04
Since arrays decay into pointers to their first element, you can use sel_list without the address-of (&) operator. — The Paramagnetic Croissant
– The Paramagnetic Croissant, Commented Feb 8, 2015 at 19:36

Jeff Loughlin · Accepted Answer · 2015-02-23 18:04:24Z

I think what you're trying to do is something like this:

int var1=0;
int var2=0;

typedef struct {
    unsigned int item_value;
    char item_name[17];
} Select_Item;

typedef struct {
  char item_name[17];
  int* variable;
  Select_Item* list;
} Menu_Item;

Select_Item sel_one = { 0, "Selection 1" };
Select_Item sel_two = { 1, "Selection 2" };
Select_Item sel_three = {2, "Selection 3" };
Select_Item sel_four = {3, "Selection 4" };

Select_Item* sel_list_1[] = { &sel_one, &sel_two };
Select_Item* sel_list_2[] = { &sel_three, &sel_four };

Menu_Item menu_item1 = { "Item 1", &var1, sel_list_1[0] }; // Note the syntax here
Menu_Item menu_item2 = { "Item 2", &var2, sel_list_2[0] }; // Note the syntax here
Menu_Item* menu_list[2] = { &menu_item1, &menu_item2 };

// Added for testing
int main()
{
    printf("Menu item '%s'\n", menu_list[0]->item_name);
    printf("    %d - %s\n", menu_list[0]->list[0].item_value, menu_list[0]->list[0].item_name);
    printf("    %d - %s\n", menu_list[0]->list[1].item_value, menu_list[0]->list[1].item_name);
    printf("Menu item '%s'\n", menu_list[1]->item_name);
    printf("    %d - %s\n", menu_list[1]->list[0].item_value, menu_list[1]->list[0].item_name);
    printf("    %d - %s\n", menu_list[1]->list[1].item_value, menu_list[1]->list[1].item_name);
    return 0;
}

Note the syntax for the initialization of menu_item1 and menu_item2.

I added main() just so I could test this with a normal C compiler (since I don't have an Arduino with an LCD handy). The output from my testing looks like this:

Menu Item 'Item 1'
    0 - Selection 1
    1 - Selection 2
Menu Item 'Item 2'
    3 - Selection 3
    4 - Selection 4

And I think that's what you're trying to achieve with your data structures. You'll just need to adapt this to your Arduino code and use these data structures to manage the menu selections on your LCD.

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