Referencing and Changing global variables from classes using pointers

Question

This is a practice that we did in our robotics club a while ago. We were supposed to create to classes, one that would assign user input to a variable (cin) and another one that would print it out (cout). We needed to use pointers to acheive this, and this is what I came up with.

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

using namespace std;

class Input
{
public:
    string* textString = new string();

    Input(string uInput)
    {
        textString = &uInput;
    }

    void userInput() {
        cout << "Enter some text - ";
        cin >> *textString;
    }
private:
    // Nothing yet
};


class Output
{
public:
    string* inputText = new string();

    Output(string uInput)
    {
        inputText = &uInput;
    }

    void userOutput() {
        cout << *inputText << endl;
    }
private:
    // Nothing yet
};

int main()
{
    string userInput = "EMPTY";
    cout << &userInput << endl;

    Input i = Input(userInput);
    i.userInput();

    Output o = Output(userInput);
    o.userOutput();

    return 0;
}

However, it doesn't seem to work. When I run this in Visual Studio 2018, and enter a value, it waits a couple of seconds and prints "Press any key to continue..." and ends the program. Nothing appears in the compiler either. Could somebody with more C++ knowledge help me understand what is wrong with my code? Thanks!

There are so many wrong things here, it's hard to decide where to start. Memory leaks. Undefined behavior due to addresses to temporary objects. There's definitely a very fundamental misunderstanding here how C++ works. The best thing for you to do is to spend some time [reading a good C++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). There isn't just one problem here, but a couple of them. Oh, and there are no "global variables" here; so also there's a terminology issue. — Sam Varshavchik, Dec 07 '18 at 01:39
What is the expected outcome? That `o.inputText` should have the content entered by the user? — Kingsley, Dec 07 '18 at 02:11
To Sam - I understand that my code is poorly written, because I am new to C++. That is why I am asking this question - because I don’t know why my code doesn’t work, and internet sources haven’t helped me. — George B, Dec 07 '18 at 02:13

score 1 · Accepted Answer · answered Dec 07 '18 at 02:32

The code is not creating the i and o objects correctly. I guess you could have said Input *i = new Input(userInput); - that would work, but requires further changes too.

I changed your Input::userInput() to take a pointer to a string. This same sort of pointer-to-argument layout would work for modifying both base-types and objects.

I'm really not fond of using cin and cout, personally I'd use fgets(), and then put the value from that into your string.

#include <string>
#include <iostream>


class Input
{
    private:
    std::string textString;

    public:
    Input ( std::string uInput )
    {
        textString = uInput;
    }

    void userInput( std::string *intoString )
    {
        std::cout << "Enter some text - ";
        std::getline( std::cin, textString );
        *intoString = textString;
    }
};


class Output
{
    private:
    std::string inputText;

    public:
    Output( std::string uInput )
    {
        inputText = uInput;
    }

    void userOutput()
    {
        std::cout << inputText << std::endl;
    }
};

int main(  )
{
    std::string userInput = "EMPTY";
    std::cout << userInput << std::endl;

    Input i( userInput );
    i.userInput( &userInput );

    Output o( userInput );
    o.userOutput();

    return 0;
}

It's not clear to me exactly how the Input object should work. The local textString seems redundant, but I tried to use it anyway.

Referencing and Changing global variables from classes using pointers

1 Answers1