How to properly pass member function as argument in this situation in C++?

Question

I want to pass a member function of my C++ class to another member function of the same class. I did some research and found these similar questions on SO.

Passing a member function as an argument in C++

Function pointer to member function

They don't cover my specific case in an identical manner, but I wrote my code and would think that I adjusted the right parts to make it work in my situation. However, the compiler seems to disagree with me on that...

I have the following setup in my C++ class:

CutDetector.h

class CutDetector {
   double thresholdForFrameIndex(int frameIndex, vector<double> diffs, int steps, double (CutDetector::*thresholdFunction)(vector<double>diffs)); // should take other functions as args
   double calcMean(vector<double> diffs); // should be passed as argument
   double calcMeanMinMax(vector<double> diffs); // should be passed as argument
   double calcMedian(vector<double> diffs); // should be passed as argument
}

CutDetector.h

double thresholdForFrameIndex(int frameIndex, vector<double> diffs, int steps, double (CutDetector::*thresholdFunction)(vector<double>diffs)) {
    vector<double> window = ... init the window vector ;
    double threshold = thresholdFunction(window);
    return threshold;
}

However, passing the thresholdFunction as an argument like this doesn't work. The compiler complains with the following error:

error : called object type 'double (CutDetector::*)(vector<double>)' is not a function or function pointer

Can anyone see why my setup doesn't work and suggest how I can make it so that it works? Basically what I want is to be able to pass any member function that calculates a threshold (ie calcMean , calcMeanMinMax , calcMedian ) to the other member function thresholdForFrameIndex .

Answer 1

To invoke a pointer to member function, you need to supply an object:

double threshold = (this->*thresholdFunction)(window);
                   ^^^^^^^^                 ^

Answer 2

You can't call a member function without an instance of the class. You need to do something like this:

CutDetector cd;
double threshold = (cd.*thresholdFunction)(window);

Or if you have a CutDetector pointer somewhere:

double threshold = (pcd->*thresholdFunction)(window);

Or if thresholdForFrameIndex is a member function:

double threshold = (this->*thresholdFunction)(window);

Answer 3

I think it would be easier for you here to make calcMean , calcMeanMinMax and calcMedian static functions and treat like all others non-member functions. Others answers are correct, but in your case i guess that would be better for class design.