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Objects

Introduction

An :cpp:class:`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:

DEFINE_FSTR_*
Creates a static data structure with an associated Object reference. The _LOCAL variant makes the reference static constexpr.
DECLARE_FSTR_*
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 :cpp:func:`FSTR::Object::read` method.

If the data isn't used very often, use the :cpp:func:`FSTR::Object::readFlash` method instead as it avoids disrupting the cache. The :cpp:class:`FSTR::Stream` class (alias :cpp:type:`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.

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

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

Attention!

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 = {
   {5},
   "hello"
};

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.

Note

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() = ");
Serial.println(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 :cpp:class:`FSTR::ObjectBase` is used to define data structures.

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

auto myCopy = flashHelloData.object.as<FSTR::String>();
Serial.print("myCopy.length() = ");
Serial.println(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 :c:func:`FSTR_PTR` macro.

Macros

.. doxygengroup:: fstr_object
   :content-only:

Class Template

.. doxygenclass:: FSTR::Object
   :members: