How to access pointer data member using class object in C++?

Question

I have the following code :

class FLOAT    
{
    float *num;

public:
    FLOAT(){}
    FLOAT(float f)
    {
        num = new float(f);
    }

    FLOAT operator +(FLOAT& obj)
    {
        FLOAT temp;
        temp.num = new float;

        temp.num = *num + obj.getF();
        return temp;    
    }

    float getF(){ return *num; }
    void showF(){ cout << "num : "<< *num << endl; }
};

It is showing an error.

My question is, how do I access that float *num data member using class object?

Time to read [the help pages](http://stackoverflow.com/help), especially the sections named ["What topics can I ask about here?"](http://stackoverflow.com/help/on-topic) and ["What types of questions should I avoid asking?"](http://stackoverflow.com/help/dont-ask). Also please [take the tour](http://stackoverflow.com/tour) and [read about how to ask good questions](http://stackoverflow.com/help/how-to-ask). Lastly please learn how to create a [Minimal, Complete, and Verifiable Example](http://stackoverflow.com/help/mcve). — Some programmer dude, Aug 03 '18 at 05:50
Also please read [this question checklist](https://codeblog.jonskeet.uk/2012/11/24/stack-overflow-question-checklist/), and all of http://idownvotedbecau.se/ to learn some reasons your question might be down-voted. Finally, please [learn how to debug your programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/). — Some programmer dude, Aug 03 '18 at 05:50
And it seems to me that either you have misunderstood something basic about C++ and variables/object creation, or you come from a Java or C# background. C++ is *not* Java or C#, you don't need `new` to create objects. [Here's a list of good C++ books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list/388282#388282), if you want to learn then I suggest you get a few of them and start all over. — Some programmer dude, Aug 03 '18 at 05:51
Sidenote: [The Principle of RAII](https://en.cppreference.com/w/cpp/language/raii) suggests a destructor to clean up the allocation `num` points at. This leads to [The Rule of Three (and friends)](https://en.cppreference.com/w/cpp/language/rule_of_three). The Rule of Zero is what you *really* want to aim for in C++, and that suggests `num` should be a plain old `int` if possible. — user4581301, Aug 03 '18 at 06:01
To help people answer your question, you'll need to be more specific about the error. Please [edit] your post to incorporate the exact errors you get from your [mcve] (preferably using copy+paste to avoid transcription errors). — Toby Speight, Aug 03 '18 at 10:45

Remy Lebeau · Answer 1 · 2018-08-03T06:33:06.363

There are a lot of mistakes in your class. It is simply not setup correctly.

The class's default constructor is not allocating the float at all.
the class is not following the Rule of 3/5/0. It is missing a destructor to free the float, a copy constructor and copy assignment operator to make safe copies of the float, and in C++11 and later, it is missing a move constructor and move assignment operator to safely move the float between objects.
your operator+ is not dereferencing the pointer when assigning a new value to the float.

Try this instead:

class FLOAT
{
    float *num;

public:
    FLOAT(float f = 0) : num(new float(f)) {}

    FLOAT(const FLOAT &src) : num(new float(*(src.num))) {}

    // in C++11 and later...
    FLOAT(FLOAT &&src) : num(src.num) { src.num = nullptr; }
    // alternatively:
    // FLOAT(FLOAT &&src) : num(nullptr) { std::swap(num, src.num); }

    ~FLOAT() { delete num; }

    FLOAT& operator=(const FLOAT &rhs)
    {
        *num = *(rhs.num);
        return *this;
    }

    // in C++11 and later...
    FLOAT& operator=(FLOAT &&rhs)
    {
        std::swap(num, rhs.num);
        return *this;
    }

    FLOAT operator+(const FLOAT& rhs)
    {
        FLOAT temp;
        *(temp.num) = *num + rhs.getF();
        return temp;

        // or simply:
        // return *num + rhs.getF();
    }

    float getF() const { return *num; }
    void showF() { cout << "num : " << *num << endl;    }
};

That being said, there is no good reason to dynamically allocate the float at all (except maybe as a learning experience). Let the compiler handle the memory management for you:

class FLOAT
{
    float num;

public:
    FLOAT(float f = 0) : num(f) {}

    FLOAT(const FLOAT &src) : num(src.num) {}

    FLOAT& operator=(const FLOAT &rhs)
    {
        num = rhs.num;
        return *this;
    }

    FLOAT operator+(const FLOAT& rhs)
    {
        FLOAT temp;
        temp.num = num + rhs.getF();
        return temp;

        // or simply:
        // return num + rhs.getF();
    }

    float getF() const { return num; }
    void showF() { cout << "num : " << num << endl; }
};

Which can then be simplified a little by letting the compiler implicitly define the copy constructor and copy assignment operator for you:

class FLOAT
{
    float num;

public:
    FLOAT(float f = 0) : num(f) {}

    FLOAT operator+(const FLOAT& rhs)
    {
        FLOAT temp;
        temp.num = num + rhs.getF();
        return temp;

        // or simply:
        // return num + rhs.getF();
    }

    float getF() const { return num; }
    void showF() { cout << "num : " << num << endl; }
};

Thank you very much for the detailed explanation. I really appreciated. I would like to apologize the way i asked the question . I got my answer here *(temp.num) = *num + rhs.getF(); Thank you very much. — Vivek Vishwakarma, Aug 03 '18 at 09:39
your answer is very more complete that me. plus 1 for you... — BattleTested_закалённый в бою, Aug 03 '18 at 10:14

score 1 · Accepted Answer · answered Aug 03 '18 at 06:04

1

When you assign bellow statement :

temp.num = *num + obj.getF();

Actually you assign a float number to a float pointer!

So, use of bellow :

(*temp.num) = (*num) + obj.getF();

Instead of :

temp.num = *num + obj.getF();

answered Aug 03 '18 at 06:04

BattleTested_закалённый в бою

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Thank you Very much for identifying my problem exactly, thats what i want. i was not able to figure it out. – Vivek Vishwakarma Aug 03 '18 at 09:41
Thank you. My problem solved with your answer – Vivek Vishwakarma Aug 03 '18 at 10:08

How to access pointer data member using class object in C++?

2 Answers2