Question

I have this member function

std::ostream& operator<<(std::ostream &stream) const

in Histogram<T>.

then in another class I have

static void write(const RGBHistogram<T> &hist, Output &output)
{
    std::cout << hist.redHist << std::endl;
}

redHist, greenHist and blueHist are Histogram.

Why it complains that no operator found which takes a right-hand operand of type Histogram?

Answer 1

You should pass referece of you class and it should be friend not member function.

friend std::ostream& operator<<(std::ostream &ostream, const RGBHistogram<T> &stream)
{

  // do something.
  return ostream;
}