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How do you define an array of characters as a constant?

C / C ++ noob here. I defined this in the header file ...

typedef unsigned char BitChar[9]; // 8 data bytes (chars) and one width byte (char)

extern BitChar BitFont[];

and I have it in the cpp file ...

BitChar BitFont[] = {
    B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,2, // 32 - Space
    B10000000,B10000000,B10000000,B10000000,B10000000,B00000000,B10000000,B00000000,1, // 33 - !
    ...
    B00000000,B00000000,B11100000,B11100000,B11100000,B00000000,B00000000,B00000000,3, // 127 - Unknown
};

It compiles and seems to work fine. However, since it will never change, I thought that it should be marked as a constant. How do I tag it as such? What I expected by adding const produces compilation errors, so I'm at a dead end. Here's the error ...

error: invalid initialization of reference of type 'unsigned char (&)[9]' from expression of type 'const unsigned char [9]'
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2 answers

Just add const. it

extern const BitChar BitFont[];
...
const BitChar BitFont[] = {
    B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,2, // 32 - Space
    B10000000,B10000000,B10000000,B10000000,B10000000,B00000000,B10000000,B00000000,1, // 33 - !
    ...
    B00000000,B00000000,B11100000,B11100000,B11100000,B00000000,B00000000,B00000000,3, // 127 - Unknown
};

should work fine in C. (Assuming your compiler knows what these identifiers mean B00000000.)

++. ++ ++- const. , extern

extern const BitChar BitFont[] = {
    B00000000
    ...

++ const . , extern, , extern .

, , , - BitChar & (aka unsigned char (&)[9]) BitChar const. , const-correctness. , .. const BitChar & (aka const unsigned char (&)[9]).

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, CONST - . (, Flash ). .

const uint8_t BitFont[] PROGMEM = {
    B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,2, // 32 - Space
    B10000000,B10000000,B10000000,B10000000,B10000000,B00000000,B10000000,B00000000,1, // 33 - !
    B00000000,B00000000,B11100000,B11100000,B11100000,B00000000,B00000000,B00000000,3, // 127 - Unknown
};
#define BITFONT_X_SIZE (sizeof(BitFont)/sizeof(BitFont[0]))

void setup() {
  Serial.println("");
  Serial.print(F("BitFont[] = "));
  for(int y = 0 ; y < BITFONT_X_SIZE ; y++) {
    Serial.print(pgm_read_byte_near( &(BitFont[1]) ) );
    Serial.print(F(","));
  }
  Serial.println("");
}

, . PROGMEM avr-gcc, . -, pgm_read_byte_near . ,

, F(), Serial.print(), . Serial.print .

#define BRICK_COLUMNS 9
const uint8_t BitFont[][BRICK_COLUMNS] PROGMEM = {
    {B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,B00000000,2}, // 32 - Space
    {B10000000,B10000000,B10000000,B10000000,B10000000,B00000000,B10000000,B00000000,1}, // 33 - !
    {B00000000,B00000000,B11100000,B11100000,B11100000,B00000000,B00000000,B00000000,3} // 127 - Unknown
};
#define BITFONT_X_SIZE (sizeof(BitFont)/sizeof(BitFont[0]))

void setup() {
  Serial.println("");

  for(int x = 0 ; x < BITFONT_X_SIZE ; x++) {
    Serial.print(F("BitFont["));
    Serial.print(x);
    Serial.print(F("][y] = "));
    for(int y = 0 ; y < BRICK_COLUMNS ; y++) {
      Serial.print(pgm_read_byte_near ( &(BitFont[1]) ));
      Serial.print(F(","));
    }
    Serial.println("");
  }
  Serial.println("");
}
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