CODEWITHSUNDEEP
c++avrcompile-time-constant
mmm
mmm

Reputation: 1297

Common constants for an AVR / Linux GCC C++ project

I'm creating software for an Linux + AVR Arduino project. Obviously the whole work is split in several projects in Eclipse (I'm not using Arduino IDE). I'd like to use common, mostly string, constants for all those projects. I also have to spare microcontroller's RAM so compile-time constants needed. How do I best implement it? My idea is to create a separate, header-only, project for those constants.

Using:

class A {
public:
    static const char * const STRING;
    static const unsigned char BOOL;
};

is not good enough, because I'd like to be able to concatenate string constants like this:

class A {
public:
    static const char * const STRING_PART1;
    static const char * const STRING_PART2;
    static const unsigned char BOOL;
};
const char * const A::STRING_PART1 = "PART1_";
//const char * const A::STRING_PART2 = A::STRING_PART1 + "PART2"; //obviously won't compile
//const char * const A::STRING_PART2 = strcat("PART2", A::STRING_PART1); //this is not compile-time

I also don't want to use define. I'd like to use:

class A {
public:
    static const std::string STRING_PART1;
    static const std::string STRING_PART2;
}

which allows for string concatenation and is (AFAIK) compile-time, but std::string is not available in avr projects - or I'm wrong here and just don't know how to use it.

Any help appreciated.

Upvotes: 4

Views: 1127

Answers (2)

hackotronic
hackotronic

Reputation: 46

Compile time for me means it is stored in ROM (eg. microcontroller's flash), where as runtime variables are stored in RAM. In my case, it's RAM I have to spare. The compiler decides where to put variables basing on many rules. Defines are one example of compile-time constants, they clearly don't count up to RAM usage. An other example should be static class constants - but in this case my compiler decides otherwise. Of course, I may be confusing things.

I think that you're in fact confusing things:

  • defines are not stored in ROM - they are not part of your program at all as they get evaluated by the preprocessor already.
  • the difference between "compile time" and "run time" you're making applies to evaluation, not storage.

If you want to use program memory (= AVR flash) for constant strings, use the PSTR macro from avr/pgmspace.h.

Upvotes: 1

iammilind
iammilind

Reputation: 70100

You can continue with the current idea of using const char* const (if std::string is not usable). I would suggest to use #define for assignment purpose only. Example:

class A {
public:
    static const char * const STRING_PART1;
    static const char * const STRING_PART2;
    static const unsigned char BOOL;
};
#define PART1_ "PART1_"  // <--- for value assignent
#define PART2_ "PART2_"
const char * const A::STRING_PART1 = PART1_;
const char * const A::STRING_PART2 = PART1_ PART2_;  // <--- ok! concatenation by compiler

Upvotes: 3

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