Error: "Invalid use of member 'filename' in static member function"

Question

I'm very new to c++ and I'm creating a Task List for a project, and I keep getting this error on both static functions within "TaskIO.cpp" when I try to run the code. The goal of the save_tasks() function is to write the task data to a separate file. The goal of the load_tasks() function is to read off tasks within the same file.

//Filename: TaskIO.cpp
#include "TaskIO.hpp"
#include <string>
#include <fstream>
#include <vector>
#include <limits>


using namespace std;

//void save_tasks(const std::vector<Task>& tasks);
//vector<Task> load_task();

TaskIO::TaskIO(string filename) :
TaskReader(filename),
Task_Writer(filename)
{}

TaskReader::TaskReader(string filename_param){
    filename = filename_param;
}

vector<Task> TaskReader::load_task(){
vector<Task> tasks;
Task task;
ifstream input_file(filename); //first error
if(input_file){
    while (input_file >> task.task_name >> task.task_complete){
        tasks.push_back(task);
    }
    input_file.close();
}
    return tasks;
}

Task_Writer::Task_Writer(string filename_param){
    filename = filename_param;
}

void Task_Writer::save_tasks(const vector<Task>& tasks){
    ofstream output_file(filename); //second error
    for(Task task : tasks){
        output_file << task.task_name << '\t' << task.task_complete << '\n';
    }
    output_file.close();
}

//Filename: taskIO.hpp
#ifndef TaskIO_hpp
#define TaskIO_hpp

#include <string>
#include <vector>
#include <string>
#include <limits>
#include "Task.hpp"

using namespace std;

class Task_Writer {
private:
    string filename;
public:
    Task_Writer(string filename = "");
    static void save_tasks(const std::vector<Task>& tasks);
};

class TaskReader {
private:
    string filename;
public:
    TaskReader(string filename = "tasks.txt");
    static vector<Task> load_task();
};

class TaskIO : public TaskReader, public Task_Writer
{
public:
    TaskIO(string filename = "tasks.txt");
};

#endif /* TaskIO_hpp */

Answer 1

static member function can only access class's static member variable.

here static function save_tasks() load_task() are trying to access not static member filename.