Thread: How to use extern...

Hello. I would like help in a problem I have using "extern".

I had a cpp file (say Font.cpp) containing a constant array like

Code:

const uint8_t SmallFont[1144] ={   ....}

and I wanted to use this in a main.cpp file

Code:

extern uint8_t SmallFont[]; 

someFunction(SmallFont);

However I got an error saying "Undefined symbol"

My question is how to correctly use extern to be able to use the array...


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I seldom use extern. I solved the problem above by programming the way I always do:
Copy paste all the data to a h file: Font.h and then including that file in main.cpp

Code:

#include "Font.h"

that solves everything but I am wishing to learn the other way (using extern)

thanks

variable declaration - When to use extern in C++ - Stack Overflow

Originally Posted by africanwizz

variable declaration - When to use extern in C++ - Stack Overflow

Yes, I already read that and it was very confusing. First they were using h files. Why h files, if I am using extern I shouldnt need the h files, right?

A part of that link that I found informative was the reply by Il Divin Codino in the end where he says:

In C++, a const variable has internal linkage by default (not like C).

Is this true? Perhaps that was my mistake??
Because I was using a const, therefore it was considered internal and therefore even though I used extern it remained an "undefined symbol"???

I am going to try to use then in Font.cpp

Code:

extern constuint8_t SmallFont[1144] ={   ....}

What do you guys think??

It doesn't make any sense to have an extern const.

I assume your font is built in, so declare it in the source file that you manage it in. And if you need to use it elsewhere, pass it around by pointer.

Originally Posted by Yarin

It doesn't make any sense to have an extern const.

Why?

I assume your font is built in, so declare it in the source file that you manage it in. And if you need to use it elsewhere, pass it around by pointer.

The thing is that for reasons of clarity ,order I want to keep main.cpp as small as possible. Also I am going to use this in main.cpp but declaring it there does not make sense since I want to write this Font once and not rewriting it everytime I want to use it.
And the font has to be const because ...well because it doesnt change but also because in that way it goes to Flash and not SRAM (developing for a small device)

Originally Posted by KansaiRobot

Why?

Correction: It's not a good idea. Global variables should be contained as much as possible.

Originally Posted by KansaiRobot

The thing is that for reasons of clarity ,order I want to keep main.cpp as small as possible. Also I am going to use this in main.cpp but declaring it there does not make sense since I want to write this Font once and not rewriting it everytime I want to use it.

Is it clear to try to manage a variable in a unit that it wasn't declared in? It sounds like you need to refactor your code. If you don't want to declare the font in main.cpp, then move your management code to font.cpp, and use it through a pointer or descriptor in main.cpp.

Originally Posted by KansaiRobot

And the font has to be const because ...well because it doesnt change but also because in that way it goes to Flash and not SRAM (developing for a small device)

Of course, I wasn't suggesting the font wouldn't be const.