Invalid use of 'this' in non-member function

I had working on a class and started writing everything in the same .cpp file. However, after a while I could see the class getting bigger and bigger so I decided to split it into a .h and a .cpp file.

gaussian.h file:

class Gaussian{ private: double mean; double standardDeviation; double variance; double precision; double precisionMean; public: Gaussian(double, double); ~Gaussian(); double normalizationConstant(double); Gaussian fromPrecisionMean(double, double); Gaussian operator * (Gaussian); double absoluteDifference (Gaussian); }; 

gaussian.cpp file:

#include "gaussian.h" #include <math.h> #include "constants.h" #include <stdlib.h> #include <iostream> Gaussian::Gaussian(double mean, double standardDeviation){ this->mean = mean; this->standardDeviation = standardDeviation; this->variance = sqrt(standardDeviation); this->precision = 1.0/variance; this->precisionMean = precision*mean; } //Code for the rest of the functions... double absoluteDifference (Gaussian aux){ double absolute = abs(this->precisionMean - aux.precisionMean); double square = abs(this->precision - aux.precision); if (absolute > square) return absolute; else return square; } 

However, I can't get this to compile. I try running:

g++ -I. -c -w gaussian.cpp 

But I get:

gaussian.cpp: In function ‘double absoluteDifference(Gaussian)’: gaussian.cpp:37:27: error: invalid use of ‘this’ in non-member function gaussian.h:7:16: error: ‘double Gaussian::precisionMean’ is private gaussian.cpp:37:53: error: within this context gaussian.cpp:38:25: error: invalid use of ‘this’ in non-member function gaussian.h:6:16: error: ‘double Gaussian::precision’ is private gaussian.cpp:38:47: error: within this context 

Why can't I use this?? I am using it in the fromPrecisionMean function and that compiles. Is it because that function returns a Gaussian? Any extra explanation will be really appreciated, I am trying to learn as much as I can! Thanks!

1 Answer

You forgot to declare absoluteDifference as part of the Gaussian class.

Change:

double absoluteDifference (Gaussian aux){ 

to this:

double Gaussian::absoluteDifference (Gaussian aux){ 

Side Note: It might be better to pass by reference rather than by value:

double Gaussian::absoluteDifference (const Gaussian &aux){