Ways to access structs

Projects Programming
1

Hi All
I am studying structs at the moment and according to the tutorial in the link below (from minute 14), there are a number of ways to access and modify structs.

https://www.youtube.com/watch?v=y4x3iyESFgs&list=PLfVsf4Bjg79CZ5kHTiQHcm-l2q8j06ofd&index=59)
however two of the ways he describe (the ones I have commented out) don't seem to work in arduino code. I just checking if this is normal or maybe I am missing something.

Also I am trying to use String instead of using a char array for the name type, but I get errors when using strcpy... yes I am aware that I can simply do clare.name = "Clare"; when using String rather than char but I wander... is strcpy only working with char arrays and not with strings ?? :thinking:

example:

struct students{		
char name[25];	
int age;		
int rollno;		
};

struct students john {"John", 24, 555};
struct students clare, bob;


// struct students mark { name : "Mark", age : 24, rollno : 566 };
// struct students mark { .name = "Mark", .age = 24, .rollno = 566 };


void setup() 
{
clare.age = 55;
strcpy(clare.name, "Clare");
}

void loop() {
}
2

What is not work in this code for you?

yes, the strcpy() finction is for char arrays only, and all other strxxx functions (strcmp, strlen... etc) as well.

3

the lines above do not compile

4

That's an open bug: designated initializer for char array by string constant. Found that by searching for the error message; first result was in StackOverflow. One reply says that it is fixed in a certain version of the compiler, but you really don't have much control over when that will trickle down to what your board is using.

There's a workaround mentioned in Bugzilla, but the Arduino language server doesn't like it, and marks it as an error if you have Real Time Diagnostics enabled.

You can just do

mark.name = "Mark";

in a separate statement. A little more typing; compiled code is nearly the same.

5

Thanks a lot for that!! I couldn't understand if it was me or if there is stuff in C that arduino doesn't like...

6

Just an addition to @kenb4 answer - the only char field name is a problem, you can easily use both syntaxes to initialize the other fields:

 struct students mark {  age : 24, rollno : 566 };
 struct students dave {  .age = 28, .rollno = 66 };

and then add the name as additional line.

7

I am not getting the result you are getting . I am using wokwi right now and getting this error:
sketch.ino:11:49: sorry, unimplemented: non-trivial designated initializers not supported
struct students mark { age : 24, rollno : 566 };

8

What the board did you selected while the compile the code? Arduino IDE uses a different compilers depending on board.

I used a arm-none-eabi-gcc v9.2.1 compiler for STM32F103 mcu

9

I am simulating with a UNO

10

Confirm, it is not compiled for Uno.
Sorry

11

Thanks!! :slightly_smiling_face:

12

Hello fluxia

Check and study this small example to gain the knowledge.

struct students {
  char name[25];
  int age;
  int rollno;
};

students john {"John", 24, 555};
students clare {"clare", 24, 555};
students bob {"bob", 24, 555};

void setup()
{
  Serial.begin(115200);
  
  clare.age = 55;
  strcpy(clare.name, "Clare");

  bob.age = 23;
  strcpy(bob.name, "Bob");

  Serial.print(john.name),  Serial.print(" - "), Serial.print(john.age),  Serial.print(" - "), Serial.println(john.rollno);
  Serial.print(clare.name),  Serial.print(" - "), Serial.print(clare.age),  Serial.print(" - "), Serial.println(clare.rollno);
  Serial.print(bob.name),  Serial.print(" - "), Serial.print(bob.age),  Serial.print(" - "), Serial.println(bob.rollno);
}

void loop() {
}

Have a nice day and enjoy coding in C++.

13

Note that this "designated initializer" feature is only useful for initializations, anyway. It's not generally useful for "access to a struct."

14

Thanks for this clarification.
I have also just discovered how to make a struct variable "read only"
const students john {"John", 24, 555}; but I must say I am getting frustrated with the fact that I am trying to improve my knowledge of C but keep getting compilation errors when trying stuff on Wokwi simulating an arduino uno... for example the line to assing a memory address to a pointer to a struct (ptr = &john;) will give an error... :face_with_head_bandage:

struct students *ptr;
ptr = &john;

error: 'ptr' does not name a type
ptr = &john;

15

I think the cause of the errot is that you put this line somewhere before setup(), outside the any function. Only initializations and defines can be put there.
The line ptr = &john; is not an initialization, it is an assignment. The assignment is allowed only inside the function.

Try to move it inside the setup()

struct students *ptr;

void setup() {
ptr = &john;
...
}

and it will compiled without errors.

The other way to put this lines together:

struct students *ptr = &john;
16

You are right! what a silly mistake :slight_smile:

17

That's because you're looking at the wrong language: Arduino code is C++, not C.

Designated initializers are a C++20 feature, while the Arduino Uno is still (artificially) stuck on C++11.

18

I am aware of this and I am keeping an eye on both languages when I find things that do not work. On this occasion I noticed that the syntax was correct in C++. But now we know what I did wrong... :blush: