function methods in series of calls

fmsc.cpp

#include <iostream>
#include "Sample_c.h"

int main()
{
    Sample_c cl1("kryssel");

    cl1.f1('a').display().f2('b').display().f3('c').display().f4('d'); /// this would call first left most to rightmost functions
                                                                      /// how? because f1 returns *this which is cl1 and other functions too and returns a reference which is that object 
                                                                      /// not a copy of cl1 which is annoying


    return 0;
}

Sample_c.cpp

#include <iostream>
#include <string>
#include "Sample_c.h"

Sample_c::Sample_c (std::string dat) :
                    data_member1 (dat) { }

Sample_c &Sample_c::f1 (char c1)
{
    this->data_member1 = c1;
    return *this;
}

Sample_c &Sample_c::f2 (char c2)
{
    this->data_member1 = c2;
    return *this;
}

Sample_c &Sample_c::f3 (char c3)
{
    this->data_member1 = c3;
    return *this;
}

Sample_c &Sample_c::f4 (char c4)
{
    this->data_member1 = c4;
    return *this;
}

Sample_c &Sample_c::display ()
{
    std::cout << this->data_member1 << std::endl;
    return *this;
}

Sample_c.h

#ifndef SAMPLE_C_H
#define SAMPLE_C_H

#include <iostream>
#include <string>

class Sample_c
{
    public:
    Sample_c () = default;
    Sample_c (std::string);

    Sample_c &f1 (char);
    Sample_c &f2 (char);
    Sample_c &f3 (char);
    Sample_c &f4 (char);
    Sample_c &display ();


    private:
    std::string data_member1 = "sample";
};
#endif // SAMPLE_C_H