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Hi, I’m developing a physical simulation in C++ and after some base approach, I’m trying to group my functions in libraries. With functions I didn’t got problems, but now I’m trying to implement a simple library file with a function and my main code didn’t recognize the class.
Those are the errors that I got when I try to compile my main code.

1. error: use of undeclared identifier ‘Material’
   Material::Material( double initialMass, double initialVolume, double initialAbundance, double initialMassnumber ) {
2. error: use of undeclared identifier ‘Material’
   double Material::getDensity( double New_Nuclei_produced, double Exhausted_Nuclei ) const

If it can help, those are my class library files

#pragma once 
#ifndef MATERIAL_H
#define MATERIAL_H
#include "Material.cpp"


class Material 
{
   private:
     double mass ;
     double volume ;
     double abundance ;
     double massnumber ;

   public:
     Material( double initialMass, double initialVolume, double initialAbundance, double initialMassnumber ) ;

     double getDensity(double New_Nuclei_produced, double Exhausted_Nuclei) const;

} ;


#endif

and

#include "NT_functions.h"
#include <iostream>
#include "Material.h"
// This class will evaluate the density of the material in terms of particle per cm3

Material::Material( double initialMass, double initialVolume, double initialAbundance, double initialMassnumber ) {
    mass = initialMass ;
    volume = initialVolume ;
    abundance = initialAbundance ;
    massnumber = initialMassnumber ;
} ;

double Material::getDensity( double New_Nuclei_produced, double Exhausted_Nuclei ) const
{
    double density ;

    // I'm looking for a function that modifies the density for each reaction that occurs

    if (New_Nuclei_produced > 1 | Exhausted_Nuclei > 1)
    {

        density = (Number_of_targets(mass, massnumber, abundance) + New_Nuclei_produced - Exhausted_Nuclei) / volume;

        if (density == 0)
        {
            std::cout << "Exhausted material" << std::endl;
            exit(1);
        }
    }

    if (New_Nuclei_produced < 1 && Exhausted_Nuclei < 1)
    {
        density = Number_of_targets(mass, massnumber, abundance) / volume;
    }

    return density;
} ;

Can someone please help me understanding what I’m doing wrong?

I tried to rearrange the definition of my class in various ways. The strange thing is that when I was working with my class in a simple file called just Material.cpp without the definition of the class in a file and it’s detail in another, everything worked nice and sound.