Circular-buffer used in ISR, declared volatile, gives error. Why ? How to fix this?

Question

There is an Arduino library called 'ByteBuffer' (found here), which is a circular buffer implementation. I modified it slightly (calling it 'ByteBufferPro'), by chopping off support for all non-byte data-types, and adding few convenience methods. I intend to use this in my Interrupt-Service-Routine, that is already doing some heavy-lifting. To relieve some bit of workload on the ISR, I intend to let the ISR push bytes of information on the circular-buffer (since capturing/storing them is time-critical), but actual processing of that information is not as time-critical, which I do in the main loop().

Following the logic that any variable that can be updated in ISR should have the 'volatile' qualifier in declaration, I did declare the buffer as volatile, but now I am seeing these compilation errors:-

ByteBufferProExample.cpp: In function 'void setup()':
ByteBufferProExample:12: error: passing 'volatile ByteBufferPro' as 'this' argument of 'void ByteBufferPro::init(unsigned int)' discards qualifiers
ByteBufferProExample.cpp: In function 'void loop()':
ByteBufferProExample:24: error: passing 'volatile ByteBufferPro' as 'this' argument of 'void ByteBufferPro::clear()' discards qualifiers

Wondering as to what is the reason, and how I might fix this ? Taking the cribbage being about 'this' pointer, I have a feeling that if I turn the implementation of ByteBuffer from C++ to a C one (OO to procedural), that should be an easy way to fix it, although I really like the OO semantics. Would do so, if there is no other way.

Answer 1

All of your functions should have volatile qualifier, just like const qualifier needed for constant objects. Here is an example:

class A
{
public:
    A(unsigned int a)
    {
    }

    void init() volatile
    {
        cout << "A::init()" << endl;
    }
};