从单独的类C ++访问类的成员时遇到麻烦

Question

I am having a lot of trouble with a relatively simple task. I have two header files, input.h and grains.h , both of which have classes defined within them. I have included all header and source files for this project below.

My problem is that when input->from_file(fname) is executed, the value printed to the screen is correct, let's say it is 4 . Then when it moves on the grains->get_pars() , the value printed to the screen is garbage, usually ~605937280 . I know this type of garbage value is indicative of the variable not being set, but I don't understand how it is not being set.

My goal is to have input.cpp read some file for some parameters, which are important to grains.cpp , and pass them along. I thought that derived classes would do the trick, but something is not working right. Any hints on what I have done wrong would be greatly appreciated. Also, any suggestions to achieve this goal aside from the one I have presented are very welcome, thanks. Note, the code as shown compiles just fine.

//input.h
#ifndef Input_H
#define Input_H
#include <iostream>
#include <fstream>
#include <sstream>
#include <cstdlib>
#include <cstring>

class Input {
  protected:
    int in_grains;
  public:
    void from_file(std::string);
};
#endif

//grains.h
#ifndef Grains_H
#define Grains_H
#include "input.h"

class Grains : protected Input {
  protected:
    int grains;
  public:
    void get_pars(void);
};
#endif


//input.cpp
#include "input.h"

void Input::from_file(std::string infile)
{ 
    std::ifstream input(infile.c_str());
    std::istringstream iss;
    std::string line, keyword;

    char arg1[50], arg2[50], arg3[50];

    while(std::getline(input,line)) {
        iss.clear();
        iss.str(line);
        iss >> keyword >> arg1 >> arg2 >> arg3;
        if ((keyword == "GRAINS") || (keyword == "Grains") || (keyword == "grains")) {
            this->in_grains = atoi(arg1);
        }
    }
    fprintf(stdout,"%i\n",in_grains );
}


//grains.cpp
#include "grains.h"

void Grains::get_pars(void)
{ 
    this->grains = in_grains;
    fprintf(stdout,"%i\n",grains );
}


//main.cpp
#include "input.h"
#include "grains.h"

int main(int nargs, char *argv[])
{ 
    Input obj1;
    Input *input = &obj1;

    Grains obj2;
    Grains *grains = &obj2;
    std::string fname = argv[1];

    input->from_file(fname.c_str());
    grains->get_pars();

    return 0;

}

Answer 1

I am guessing that when you execute:

input->from_file(fname.c_str());
grains->get_pars();

you are expecting the in_grains from input to be available as grains->grains . input and grains are two different objects. in_grains has not been set on the object grains points to.

Perhaps you meant to use:

int main(int nargs, char *argv[])
{ 
    Grains obj;
    Input *input = &obj;
    Grains *grains = &obj;
    std::string fname = argv[1];

    input->from_file(fname.c_str());
    grains->get_pars();

    return 0;    
}

However, to use that, you have to make Input a public base class of Grain , not a protected base class.

class Grains : public Input {