warning: left-hand operand of comma has no effect

Hi. I've got the above warning message roughly 130 times for a program I'm using. Analyzing this more carefully I've found that I'm only getting it 10 times, but multiple occurrences of the same locations. Presumably from multiple method calls.
I have no idea what causes these warnings, except that it's to do with my overloaded brackets operator.
Can anyone help me?

Code:

// Overloaded brackets operator
mType & Matrix::operator()(const int i, const int j) {
        return values[j * xsize + i];
 }

// Lines causing warnings
res(i, j) += (q,j) * m(i, q);
res(i, j) = (i, j) * f;
res(i,j) = (i, j) + m(i,j);
(i,j) = x;
res(i,j) = (j,i);
return Vec3D<mType>((0,0),(0,1),(0,2));	            // 3 x Warnings
return Vec2D<mType>((0,0),(0,1));                     // 2 x Warnings

It's very obvious that the warning only appears when the () is used with the implied "this" operator.
NB. The lines generating warnings that I have shown are not in this context. They are within seperate methods, but since the overall file is 160 lines long I chose to display them this way for now. More code is available if needed.

Thanks,
Simon

You cannot call it implicitly. Something like (*this)(q, j) and (*this)(0, 0) should work.

You cannot imply it. You want to do something like:

(*this)(q,j);

I don't know if that would work because I'm not much familiar with overloading brackets but that seems more right to me.

Thanks. I haven't tried that yet. Why is this a problem? I mean, (curse me for saying this) but it's only a warning? It's not a full blown error, and my program works exactly as I want it to do.
What is wrong with doing it this way that deserves a warning?
I'm reasonably new with C++ but I'm finding my way, and this will eventually be part of my dissertation so a reason or some background would be extremely helpful to me.
Thanks for the rapid response.
Simon

Basically, the parentheses in C++ are really put to work. They stand for grouping, function calls, casting, functional macro replacement - have I forgotten something?

Anyway, what they mean exactly depends on where in the syntax they are. Only when they immediately follow a callable entity do they actually mean function call, and only function call can be overloaded with operator().

Well, the problem is that something like:

Code:

res(i,j) = (j,i);

is equivalent to:

Code:

res(i,j) = j,i;

which is equivalent to:

Code:

res(i,j) = i;

Well, the problem is that something like:
Code:

Code:

res(i,j) = (j,i);

is equivalent to:
Code:

Code:

res(i,j) = j,i;

Those lines are not exactly equivalent. Consider this example:

Code:

int a, b, i = 1, j = 2;
a = (i,j);
b = i,j;
cout << a << " " << b << endl << endl;

Prints out "2 1".

Whoa, way to resurrect a months-old thread.

Yeah, since ,'s priority is even lower than ='s, the two lines are indeed not equivalent, but I really don't think it matters anymore.