Why not just use a wrapper class?

A wrapper class would require this:

const struct {
} myFlashData PROGMEM = { ... };

void func()
   FlashString fs(myFlashData);

Easy enough, just create a wrapper every time you need to use the data.

It does get a bit clunky though when you have a lot of strings, lists of strings or where strings are used in several places.

How about global/static wrappers

Like this:

FlashString fs(myFlashData);

void func()

Each wrapper uses 4 bytes of RAM. We also need to bear in mind that at startup objects are being initialised so we cannot necessarily guarantee that our FlashString wrappers have been created by the time they’re needed in other code. Result: hang/crash.

How about a macro to create the wrapper?

Something like this:

void func()

To be honest, I hadn’t really considered that one, but it is worth exploring.

Can’t I put the wrapper in PROGMEM?

A wrapper requires a constructor, each of which needs an initialiser which gets called at startup.

Which is why FlashString isn’t a wrapper.

FlashStrings can be used anywhere you can use PROGMEM data, including globally initialised classes.

There is a disadvantage to this approach: You can’t make copies of FlashString objects. They must always be accessed as references or pointers.

Reasons to use FlashString

  • Easy to use, treat like read-only String object

  • Use methods of FlashString instead of calls to memcpy_P, strcpy_P, etc.

  • Blocks of data stored in flash can be passed around easily as the length is known

  • Can be passed to functions instead of a String parameter, but without having to load it into RAM first

  • Can be streamed directly using FlashMemoryStream

  • Can be read randomly using FlashString::read()

  • Aligned read and copy operations provide excellent performance

  • Fast equality comparisons using length field to short-circuit comparison

  • Data can be imported and linked directly into the program image from a local file, and accessed as a FlashString

  • Custom structures can be defined and accessed as a FlashString

  • Operates transparently with ArduinoJson (patched as from version 6.13.0)

Reasons not to use FlashString

  • Storing lots of small strings can be inefficient because of length and alignment, and because a NUL terminator is always appended by DEFINE_FSTR macros even though it’s not generally required. (This is because of how C requires string arrays to be declared.)

    For example, “A” requires 8 bytes:

   01 00 00 00 // Length
   41 00 00 00 // "A\0" padded to word boundary

However, this disadvantage can be overcome by storing such strings in a single block
and accessing them using a :source:`Sming/Core/Data/CStringArray`.


Some counted string implementations store the length field at offset -4 so we always point to the actual data. In this case, that doesn’t offer any advantages as directly accessing the data is discouraged. Therefore, the length field always comes first.

Structure packing

When using Arrays with 64-bit types (including double) this is what we define:

struct {
   Array<int64_t> object;
   int64_t data[5];
} x;

The object is a structure containing a single 32-bit value. Data arrays with uint8, uint16 or uint32 elements will start on the next 32-bit boundary, which is what we want.

With 64-bit values this is what the compiler does:

struct {
   Array<int64_t> object;
   uint32_t; // Packing added by compiler
   int64_t values[5];
} x;

Which messes things up of course. Therefore Array classes and data are packed.

This is currently only an issue for Array types, but it also means that if you want to use Array with custom data structures then they should also be packed. That means you need to pay careful attention to member alignment and if packing is required then add it manually.