Structure Array Initialization

Question

I'm trying to initialize a char array which is part of a structure as follows:

#define TRUE 1
#define FALSE 0

struct s_field_def { // structure for managing display field
  char f_txt[16]; // option text
  bool f_inhibit; // set this to inhibit updating of the field on the display
};

enum field_name {F_STATE, F_PULSE};

void setup() {

  // initialize display fields structure
  struct s_field_def fields[2]; // initialize the display fields
  fields[F_STATE] =  {"HALTED", FALSE}; // displays the current run state
  fields[F_PULSE] =  {"@", TRUE}; // displays indicator of system running   
}

void loop() {

}

The initialization of the structure is producing an error at these lines:

  fields[F_STATE] =  {"HALTED", FALSE}; // displays the current run state
  fields[F_PULSE] =  {"@", TRUE}; // displays indicator of system running   

no match for 'operator=' (operand types are 's_field_def' and '<brace-enclosed initializer list>')

What am I doing wrong here?

Edgar Bonet · Accepted Answer · 2018-02-07 16:17:08Z

4

As explained in Michel Keijzers' answer, you cannot use the = operator to assign something to a string (a character array). However, you can initialize a string using =. There is an important distinction between assignment, which you can do anywhere in a program, an initialization, which can only be done at the time you define a variable.

For example:

int i = 2;  // initialization

void setup() {
    i = 4;  // assignment
}

In your specific case, you can initialize fields as follows:

struct s_field_def fields[2] = {
    {"HALTED", FALSE},  // displays the current run state
    {"@", TRUE}         // displays indicator of system running   
};

answered Feb 7, 2018 at 16:17

Edgar Bonet

45.2k4 gold badges42 silver badges81 bronze badges

That reduces code size too, both in LOC and memory. And it's clearer: declaration and initialization is just one place. Maybe you can even declare data static.

user31481
– user31481

2018-02-07 16:21:30 +00:00
Commented Feb 7, 2018 at 16:21
Thanks for the quick feedback about the distinction between assignment and initialization. That makes it clear. Problem solved!

whalphen K8VFO
– whalphen K8VFO

2018-02-07 16:31:03 +00:00
Commented Feb 7, 2018 at 16:31
I worked too long in an environment where this was not used. Upvoted for learning something.

Michel Keijzers
– Michel Keijzers

2018-02-07 17:09:44 +00:00
Commented Feb 7, 2018 at 17:09

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Michel Keijzers · Accepted Answer · 2018-02-07 17:10:50Z

0

You can use this (but look at the accepted answer, it shows a much neater way).

  strcpy(fields[F_STATE].f_txt, "HALTED");
  fields[F_STATE].f_inhibit = FALSE;

  strcpy(fields[F_PULSE].f_txt, "@");
  fields[F_PULSE].f_inhibit = TRUE;

I don't have a compiler so I hope it's correct.

When using C++ you can use a constructor or assignment operator, but C does not have these object oriented features.

edited Feb 7, 2018 at 17:10

answered Feb 7, 2018 at 16:03

Michel Keijzers

13k7 gold badges43 silver badges59 bronze badges

I was hoping to avoid strcpy for the initialization.

whalphen K8VFO
– whalphen K8VFO

2018-02-07 16:32:31 +00:00
Commented Feb 7, 2018 at 16:32

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