How do you define a char array as a constant?

Question

C/C++ noob here. I've defined this in a header file...

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

extern BitChar BitFont[];

and I have this in a 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 seemingly runs just fine. However, since it will never change, I thought it should be marked as a constant. How do I mark it as such? What I expected, adding 'const', throws compile errors so I'm stumped. Here's the error...

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

Answer 1

Just add const. This

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 perfectly fine in C. (Assuming that your compiler knows what these B00000000 identifiers mean.)

This will also work perfectly fine in C++. The only potential for error in the C++ version is based on C++-specific properties of const. If the definition does not see the declaration, then you have to specify the explicit extern in the definition as well

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

because in C++ const objects have internal linkage by default. However, if the declaration already contains the extern and the definition can see the declaration, then that extern in the definition is optional.

The error message you quoted suggests that somewhere in your code you are trying to initialize a reference of type BitChar & (aka unsigned char (&)[9]) with a const-qualified BitChar array. This will not work, since it violates the basic rules of const-correctness. The reference has to become const-qualified as well, i.e. it has to change to const BitChar & (aka const unsigned char (&)[9]).