Question

I’ve noticed in many examples/tutorials that people use the brace operator “()” for matrix access, which is confusing for me in some cases.

The question is:
Let’s assume we have a n*m matrix (3×4, for example) named M. What elements will be returned by accessing it via “M(0)”, or “M(1)”, or any other singular parameter inside the braces? I thought that you are supposed to specify both the row and the column (like “M(0,1)” or something like that).

Answer 1

This is from OpenCV 2.4.2

template<typename _Tp> inline _Tp& Mat_<_Tp>::operator ()(int i0, int i1)
{
    return ((_Tp*)(data + step.p[0]*i0))[i1];
}
template<typename _Tp> inline _Tp& Mat_<_Tp>::operator ()(int i0)
{
    return this->at<_Tp>(i0);
}
template<typename _Tp> inline _Tp& Mat_<_Tp>::operator ()(int i0, int i1, int i2)
{
    return this->at<_Tp>(i0, i1, i2);
}

and this is definition for Mat::at(int i0) which Mat_ publicly inherits from Mat

template<typename _Tp> inline const _Tp& Mat::at(int i0) const
{
    if( isContinuous() || size.p[0] == 1 )
        return ((const _Tp*)data)[i0];
    if( size.p[1] == 1 )
        return *(const _Tp*)(data + step.p[0]*i0);
    int i = i0/cols, j = i0 - i*cols;
    return ((const _Tp*)(data + step.p[0]*i))[j];
}