Why AVR-GCC compiler throws an error when overloading with the same variables but as PROGMEM?

Question

My question is more like: Why the compiler "thinks" that a "PROGMEM variable" is the same as a "plain Variable"? is it because PROGMEM-keyword is "just" a macro and nothing more? or is it for some other reason too? and is there any workaround..?

ISSUE demonstration:

Lets consider the below example:

class object {
public:
  object(int* variable);
  object(int* variable PROGMEM);
};

it throws error: 'object::object(int*)' cannot be overloaded as if it is the same.

sketch_jul31a:4:3: error: 'object::object(int*)' cannot be overloaded

   object(int* variable PROGMEM)

   ^~~~~~

sketch_jul31a:3:3: error: with 'object::object(int*)'

   object(int* variable)

   ^~~~~~

exit status 1
'object::object(int*)' cannot be overloaded

Outro:

I came across this issue a while ago when i was developing a library, I've asked about it on the arduino-forum but i had not any answer and so i've thought after a long period to ask about it once again, here.

That annotation doesn't make sense for a function parameter that's a plain value. Overloading on pointer to progmem versus pointer to data memory would make some sense, but that would have to be part of the type of the pointer - can't remember if there's an annotation for that. — Mat
– Mat, Commented Jul 31, 2021 at 17:32
PROGMEM is information for the linker where to store the variable. it has nothing to do with the variable type. — Juraj
– Juraj, Commented Jul 31, 2021 at 17:36
@Juraj that's quite sad.. and it's what i was expecting as an answer actually... — Giorgos Xou
– Giorgos Xou, Commented Jul 31, 2021 at 17:54

kuroi neko · Accepted Answer · 2021-07-31 18:49:19Z

2

You can't expect the compiler to treat linker sections as type qualifiers, but you can define an overload for a const int*, which is pretty close to the semantics of PROGMEM (a ROM location).

I wonder what you plan on doing with a const int* though. All you will ever be able to do is read it, so it's basically equivalent to a plain constant int value, with the added cost of two bytes of ROM.

answered Jul 31, 2021 at 18:49

kuroi neko

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2 Comments

Giorgos Xou Over a year ago

That's a great one! it makes sense for something like const int* variable PROGMEM since PROGMEM, is constant anyways. That was just a "silly" example of mine, in my case i had to store a banch of weights and biases (for readonly purposes if PROGMEM), but at the end i got away with MACROS..

kuroi neko Over a year ago

I see. Glad to have been of help :)

3CEZVQ · Accepted Answer · 2021-07-31 17:34:39Z

0

__attribute__((progmem)) is a compiler feature, not a C++ language feature, so it does not participate in the overload resolution. The both object(int variable); and object(int variable PROGMEM); looks like the double declaration of object(int variable); in terms of C++.

answered Jul 31, 2021 at 17:34

3CEZVQ

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