Store RAM and PROGMEM strings in one variable

Question

I want to store a number of strings in an object. Currently I have:

class MyClass {
 int numStrings = 0;
 ??? strings[20];
 public:
 void addString(const char str[]);
 void print();
};
void MyClass::addString(const char str[]) {
 strings[numStrings] = str;
 numStrings++;
}
void MyClass::print() {
 for (int i = 0 ; i < numStrings ; i++) {
 Serial.println(strings[i]);
 }
}

Serial.println() works with strings in RAM and in PROGMEM alike.

Is there a type that I can give to my strings variable that allows me to store both types of strings?

MyClass s;
s.addString("This should work");
s.addString(F("This should work, too"));

Answer 1

Is there a type that I can give to my strings variable that allows me to store both types of strings?

No, but you could tag the pointer or add a vector with type information or ultimately use a class hierarchy (BaseString<-SRAMString, PROMEMString).

As a pointer is 16-bit (Arduino Uno) and there is limited amount of SRAM it is possible to add a magic number to pointers to determine the address space.

Obviously the print() member function will have to use the right type and you will also need a new addString for F() strings. Check how F() is defined.

Cheers!

• AndreKR: I don't know how to "tag a pointer", but when working with a magic number I would make that a void pointer?
• Mikael Patel: Here is an example of mapping EEPROM, SRAM and PROGMEM to a single address space: github.com/mikaelpatel/Arduino-Shell/blob/master/Shell.h#L1058
• Mikael Patel: The simplest scheme is to use the sign bit for PROGMEM, i.e. add 0x8000 to PROGMEM pointers.
• AndreKR: I don't get it. So when I get a __FlashStringHelper* I add 0x8000 to mark that it points to PROGMEM. But then I still can't store that in a const char* variable, can I?
• Mikael Patel: Tell the compiler what you want: uint16_t str = 0x8000U + (uint16_t) str_P; strings[numStrings++] = (const char*) str;