string.rst

Flash Strings

Introduction

Strings are basically just arrays of char, but have additional methods to allow them to be used more easily. These methods are consistent with Wiring :cpp:class:`String`, so should be reasonably familiar.

• :cpp:func:`length() <FSTR::Object::length>` returns the number of characters in the String, excluding the NUL terminator
• :cpp:func:`size() <FSTR::String::size>` returns the number of bytes of storage used

For example, "123" is actually stored as { '1', '2', '3', '\0' } so the length is 3 and the size is 4. However, "1234" is stored as { '1', '2', '3', '4', '\0' } so the length is 4 and the size is 8.

Using Strings

Note

You can use FSTR::String or the Sming-provided FlashString alias to work with Strings.

Within a function:

DEFINE_FSTR_LOCAL(myFlashString, "This is my flash string");

Serial.println(myFlashString);
Serial.printf("myFlashString has %u chars and occupies %u bytes\n", myFlashString.length(), myFlashString.size());

To use Strings across translation units, we do this in the header:

DECLARE_FSTR(myFlashString);

And in a source file:

DEFINE_FSTR(myFlashString, "I am a flash string\0I've got a Naughty NUL.");

You can generally use a Flash String anywhere you can use a regular Wiring String as it has an implicit ::String() operator. Note that WString is used within the library for disambiguation.

Inline Strings

Use the :c:func:`FS` macro to create Flash Strings inline:

Serial.println(FS("A Flash String"));

Note

The macro makes use of FS_PTR() which creates the structure and returns a pointer to it. It behaves like a function call, although the compiler inlines the code.

Therefore FS() may only be used within functions. At file scope you'll get this error:

statement-expressions are not allowed outside functions nor in template-argument lists

The example above doesn't provide any improvement over :c:macro:`F` as there are no Flash String overloads available, so is equivalent to this:

String s = FS("A Flash String");
Serial.println(s);

However, it's rather different if you pass it to a function which recognises Flash Strings, like this:

FSTR::println(Serial, FS("A Flash String"));

This is equivalent to:

FS("A Flash String").printTo(Serial);
Serial.println();

:cpp:func:`FSTR::String::printTo` uses no heap and imposes no restriction on the string length.

Nested Inline Strings

It would be really useful to be able to use inline Strings this within nested structures, and this can be done provided those structures are in RAM.

Important

Inline Strings cannot be used when defining Vectors or Maps.

Here's is a simplified structure we will attempt to initialize:

static const struct {
   FlashString* string;
} flashData PROGMEM = { FS_PTR("Inline Flash String") };
Serial.println(*flashData.string);

The static flashData structure gets initialised at runtime on first use, as per C++ rules. This attempts to copy our pointer into the flashData structure which clearly it cannot do as it's in :c:macro:`PROGMEM`, so we get a LOAD/STORE error. We must remove PROGMEM.

Avoiding the heap

Instead of using a temporary Wiring String, you can use :c:func:`LOAD_FSTR` to load the content into a temporary stack buffer:

DEFINE_FSTR(globalTest, "This is a testing string");

void func()
{
   LOAD_FSTR(local, globalTest);
   printf("%s, %u characters, buffer is %u bytes\n", local, globalTest.length(), sizeof(local));
}

You can do this with inline Flash Strings using :c:func:`FSTR_ARRAY`:

FSTR_ARRAY(buffer, "text");

Is roughly equivalent to:

char name[] = "text";

Except the buffer is word aligned, so sizeof(name) may differ.

Macros

.. doxygengroup:: fstr_string
   :content-only:

String Class

.. doxygenclass:: FSTR::String
   :members: