在非成员函数中无效使用“this”

问：

我正在做一个类，并开始在同一个 .cpp 文件中编写所有内容。然而，过了一会儿，我看到这个类越来越大，所以我决定把它拆分为一个 .h 和一个 .cpp 文件。

gaussian.h 文件：

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);
};

高斯 .cpp 文件：

#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;
}

但是，我无法编译它。我尝试运行：

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

但我得到：

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

为什么我不能用这个？？我在fromPrecisionMean函数中使用它并编译。是因为该函数返回高斯函数吗？任何额外的解释将不胜感激，我正在尽可能多地学习！谢谢！