Add FlashString abstract class to support extension of String and Print methods to core specific program memory implementation. The FlashString abstract class completely eliminates the dependency on the file pgmspace.h. ArduinoCore-API should never include any references to a specific implementation (e.g. AVR).

An example implementation is shown below, tested on a SAMD21 core. I have not had a chance to implement / test on an AVR-based Arduino, but should be easily implemented at avr core level.

classFlashStringTest :publicFlashString{public:FlashStringTest(uintptr_t address): _address(address) {}        StringtoString()const;size_tprintTo(Print& p)const;protected:uintptr_t _address;};StringFlashStringTest::toString()const{returnString((constchar *)_address);}size_tFlashStringTest::printTo(Print& p)const{size_t n =0;constchar* str = (constchar*)_address;while (1) {char c = *str++;if (c ==0)break;if (p.print(c)) n++;elsebreak;  }return n;}#ifdef F#undef F#endif#defineF(string_literal) (FlashStringTest((uintptr_t)(string_literal)))

Add FlashString abstract class to support extension of String and Pri…

ff9a16a

…nt methods to core specific program memory implementation.

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