An FSTR::Object is a class template with array-like behaviour, though it is not used directly.

Instead, use one of the four classes in the library:

Each type has its own set of macros for easy data construction, and creation of the appropriate Object class which may then be used directly.

Macros follow the same pattern:


Creates a static data structure with an associated Object reference. The _LOCAL variant makes the reference static constexpr.


Use this in a header to declare an Object reference so it can be used across translation units.

Created symbols are C++ and adopt any enclosing namespaced.

Reading Object content

To read parts of an Object, use the FSTR::Object::read() method.

If the data isn’t used very often, use the FSTR::Object::readFlash() method instead as it avoids disrupting the cache. The FSTR::Stream class (alias FlashMemoryStream) does this by default.

Object Internals

This section provides some examples of how structures are created, but in normal use you should use the provided macros as they simplify the task and include structure validity checks.

FSTR::ObjectBase is a non-template POD base class, and looks like this (methods omitted):

class ObjectBase {
   uint32_t flashLength_;
   // uint8_t data[];


flashLength_ must not be accessed directly; use the length() method instead.

Data structures are created like this:

constexpr const struct {
   ObjectBase object;
   char data[8];
} flashHelloData PROGMEM = {

The object field may then be cast to a reference of the required type, like this:

auto& str = flashHelloData.object.as<FSTR::String>();

If you want to access it as an array, do this:

auto& arr = str.as<FSTR::Array<char>>();

References are an efficient and convenient way to access an Object, and should not consume any memory themselves as the compiler/linker resolve them to the actual object.

However, in practice the Espressif compiler stores a full pointer to most things to support relative addressing, and if the references aren’t declared PROGMEM they’ll consume RAM.

Copy behaviour

Whilst references are the preferred way to access flash Objects, they can also be created dynamically:

FSTR::String emptyString;
FSTR::String stringCopy(FS("Inline string"));

Such instances are stored in RAM but only consume 4 bytes as they simply keep a pointer to the real flash Object.


Don’t try to copy ObjectBase!

Here’s a somewhat contrived example to demonstrate:

DEFINE_FSTR_DATA_LOCAL(flashHelloData, "Hello");
auto myCopy = flashHelloData.object;
Serial.print("myCopy.length() = ");

In debug builds, this will throw an assertion. In release builds, you’ll get a zero-length object.

Aggregate initialization

We use aggregate initialization to set up the structures so the data is fixed at link time without any constructor or initialiser functions.

This means classes cannot have:

  • user-provided constructors

  • brace-or-equal-initializers for non-static data members

  • private or protected non-static data members

  • virtual functions

  • base classes (until C++17)

This is why FSTR::ObjectBase is used to define data structures.

Classes created using the FSTR::Object template ensures the necessary constructors are available to do this:

auto myCopy = flashHelloData.object.as<FSTR::String>();
Serial.print("myCopy.length() = ");

The macros create an appropriate Object& reference for you.

Structure checks

The construction macros include a sanity check to ensure the initialization is truly just Plain Old Data, without any hidden initialisers.

You may encounter one of the following errors during compilation:

  • The value of ‘X’ is not usable in a constant expression

  • FSTR structure not POD

This generally means one or more of the arguments in the initialisation data is not constexpr. Most compilers are quite relaxed about this but GCC 4.8.5 is particularly thick.

In testing, this happens with references for global Objects, which of course cannot be constexpr. To fix it, the offending Object either needs to be redefined LOCAL, or if the Object data is in scope (i.e. defined in the same source file) then you can get a direct pointer to it using the FSTR_PTR() macro.



Declare a global Object reference.

  • name

  • ObjectType

DEFINE_FSTR_REF(name, ObjectType, object)

Define a reference to an object.

  • name – Name for reference

  • ObjectType – Fully qualified typename of object required, e.g. FSTR::String, FlashString, FSTR::Vector<int>, etc.

  • object – Object instance to cast


Provide internal name for generated flash string structures.


Given an Object& reference, return a pointer to the actual object.

However, some older compilers such as GCC 4.8.5 requires such references to be declared constexpr. For example, this fails with FSTR structure not POD:

    DEFINE_FSTR(globalStringRef, "This creates a global reference");
    DEFINE_VECTOR(myVector, FSTR::String, &globalStringRef);

Global references cannot be declared constexpr, so changing DEFINE_FSTR to DEFINE_FSTR_LOCAL will fix the problem.

Another solution is to get a direct pointer to the actual data structure:

    DEFINE_VECTOR(myVector, FSTR::String, FSTR_PTR(globalStringRef));

We can only do this of course if the data structure is in scope.

  • objref – When an Object pointer is required, such when defining entries for a Vector or Map, it is usually sufficient to use &objref.


Check structure is POD-compliant and correctly aligned.

IMPORT_FSTR_OBJECT(name, ObjectType, file)

Import an object from an external file with reference.




Can only be used at file scope

  • name – Name for the object

  • ObjectType – Object type for reference

  • file – Absolute path to the file containing the content

IMPORT_FSTR_OBJECT_LOCAL(name, ObjectType, file)

Like IMPORT_FSTR_OBJECT except reference is declared static constexpr.

Class Template

template<class ObjectType, typename ElementType>
class FSTR::Object : public FSTR::ObjectBase

Base class template for all types.



tparam ObjectType

The object type actually being instantiated

tparam ElementType

Public Functions

inline Object()

Creates a null object.

inline Object(const Object &obj)

Copy constructor.


Objects are usually passed around by reference or as a pointer, but for ease of use we need a working copy constructor.

inline size_t length() const

Get the length of the content in elements.

inline ElementType operator[](unsigned index) const

Array operator[].

inline size_t read(size_t index, ElementType *buffer, size_t count) const

Read content into RAM.

  • index – First element to read

  • buffer – Where to store data

  • count – How many elements to read


size_t – Number of elements actually read

inline size_t readFlash(size_t index, ElementType *buffer, size_t count) const

Read content into RAM,using flashmem_read()

  • index – First element to read

  • buffer – Where to store data

  • count – How many elements to read


size_t – Number of elements actually read

Public Static Functions

static inline const ObjectType &empty()

Return an empty object which evaluates to null.