Edit: If you want to use the function dimAll() as an interrupt handler function, then it cannot be a (non-static) member of a class. All class functions have a hidden this pointer as the first parameter in the argument list, since it has to know which DimmerImplementor's members to modify. Thus you have to declare the function as static. And as soon as you do that, you cannot call dimAllDimmers(), because that functions accesses an instance of DimmerImplementor's variables. Luckily, you have a global object called implementor, so you could use that:

static void DimmerImplementor::dimAll() {
    implementor.dimAllDimmers();
}

But it's usual to make that variable a static member of the class itself:

class DimmerImplementor {
    .
    .
public: // Static variables
    static DimmerImplementor implementor;
    .
    .
}; // DimmerImplementor

You'd initialise that as follows:

DimmerImplementor DimmerImplementor::implementor(dimmers, _DIMMER_pinOpto);

In C++, you need to distinguish between two separate copies of an object, or having a reference to an existing object. @chrisl got it right when he identified that you were trying to create a second copy of DimmerGlider, so he suggested using a reference - and then showed using a pointer (*) instead.

He was right: Dimmer should hold a reference to the DimmerGlider object, but that is done with &. You need to make the following changes to the given code:

In the .h:

class Dimmer {

public:
    Dimmer(DimmerGlider &glider, int outputPin, int t3);
.
.
.
private:
    DimmerGlider &_glider;
.
.

And in the final .cpp:

DimmerGlider::DimmerGlider(int min, int max, int increase) :
              _min(min),
              _max(max),
              _increase(increase) {
}

Dimmer::Dimmer(DimmerGlider &glider, int outputPin, int t3) :
        _glider(glider),
        _outputPin(outputPin),
        _t3(t3) {
}

The syntax for the Dimmer constructor had to change. You were assigning the class members the values from the constructor parameters. References need to be initialised, not assigned. This is the syntax you need (note how they’re before the {?)

I fixed the DimmerGlider constructor code while I was at it...

In C++, you need to distinguish between two separate copies of an object, or having a reference to an existing object. @chrisl got it right when he identified that you were trying to create a second copy of DimmerGlider, so he suggested using a reference - and then showed using a pointer (*) instead.

He was right: Dimmer should hold a reference to the DimmerGlider object, but that is done with &. You need to make the following changes to the given code:

In the .h:

class Dimmer {

public:
    Dimmer(DimmerGlider &glider, int outputPin, int t3);
.
.
.
private:
    DimmerGlider &_glider;
.
.

And in the final .cpp:

DimmerGlider::DimmerGlider(int min, int max, int increase) :
              _min(min),
              _max(max),
              _increase(increase) {
}

Dimmer::Dimmer(DimmerGlider &glider, int outputPin, int t3) :
        _glider(glider),
        _outputPin(outputPin),
        _t3(t3) {
}

The syntax for the Dimmer constructor had to change. You were assigning the class members the values from the constructor parameters. References need to be initialised, not assigned. This is the syntax you need (note how they’re before the {?)

I fixed the DimmerGlider constructor code while I was at it...

Also, note the difference between using a reference versus using a pointer. A pointer needs to be dereferenced, so you need to use *ptr and ptr->_min to refer to the original object. A reference is already dereferenced, so you can simply use it like the original object: ref and ref._min.