C++ - Class constructor creates objects, but the result of the construction is different

Question

I have a class project in C++. The goal is to sort a phonebook (txt file) when inserting in a linked list.

Here are the classes :

class Person
{
    public:
        string Name
        string Firstname;
        string Adress;
        int PostalCode;
        string Telephone;
        Person();
        ~Person();
};

class Link
{
    friend class List;
    Link *next;
    Person *pers;
    public:
        Link();
        Link(string data);
};

class List
{
    // debut is the start of the chained list
    Link *start;
    public:
        List(string data)
        {
            start = NewLinkPerson(data, NULL);
        }
};

Link::Link(string data)
{
    next = NULL;
    Person p;
    p.put_data(data);
    pers = &p;
}

Link::Link()
{
    next = NULL;
    Person p;
    pers = &p;
}

And then, the function NewLinkPerson is detailed as such :

Maillon * NewLinkPerson(string data, Maillon *ssuiv)
{
    Maillon * nouveau = new Maillon(data);
    nouveau->suiv = ssuiv;
    return nouveau;
}

The problem is that the object created is not the one I ordered the program him to create. If I output the object pers of type Person in the constructor, it will be filled with the data I asked. But when it leaves the constructor, the data is gone, and is filled with random strings from the memory.

What could be causing the issue ? I have tried many things, but none of them seem to works, all mainly returned a Segmentation Fault.

EDIT : A function I forgot to put there :

void Person::put_data(string data)
{
    int sep1 = data.find("|");
    int sep2 = data.find("|", sep1+1);
    int sep3 = data.find("|", sep2+1);
    int sep4 = data.find("|", sep3+1);
    Name = data.substr(0, sep1);
    Firstname = data.substr(sep1+1, sep2-sep1-1);
    Adress = data.substr(sep2+1, sep3-sep2-1);
    Telephone = data.substr(sep4+1, data.npos);
    string ccode = data.substr(sep3+1, sep4-sep3-1);
    PostalCode = std::stoi(ccode.c_str());
}

Edit 2 : Translated to english

"The problem is that the object created is not the one I ordered him to do." <-- what does this sentence even mean? Who is "him"? — Barry, May 25 '16 at 18:51
@NathanOliver Yes, the g++ compiler told me to put it, because the object pers is a pointer, forgot the * in the post, editing now — Thibault de Villèle, May 25 '16 at 18:51
@Barry Sorry if it was not clear. What I meant was I order the program to create a new `Maillon` element, and it is filled with the data given, but only when it is created. Once the constructor is exited, the data in the `Maillon` is just a bunch of gibberish. — Thibault de Villèle, May 25 '16 at 18:53

score 6 · Accepted Answer · edited May 23 '17 at 12:17

6

Your code has undefined behavior

Link::Link(string data)
{
    next = NULL;
    Person p;
    p.put_data(data);
    pers = &p;
}

Link::Link()
{
    next = NULL;
    Person p;
    pers = &p;
}

In both constructors in the above code you create a local Person, p, and then you have pers point to it. After the constructor ends p goes out of scope, gets destroyed and now pers points to an object that no longer exists. Any dereference of pers after that will it still points to the deleted object is undefined behavior.

To fix this just get rid of the pointer for pers. If you store it as a value then you do not need any dynamic memory allocation and you get away from pointer syntax. Link should be

class Link
{
    friend class List;
    Link *next;
    Person pers; // no pointer here
    public:
        Link();
        Link(string data);
};

And then the constructors can be defined like

Link::Link(string data)
{
    next = NULL;
    pers.put_data(data);
}

Link::Link()
{
    next = NULL;
}

If you have to have pers as a pointer then you can allocate persistent storage for it with new. If you do that then you need to make sure you delete it in the destructor of the class and that you have copy symantics. For more on this see What is The Rule of Three?

edited May 23 '17 at 12:17

Community

1
1

answered May 25 '16 at 18:55

NathanOliver

171,901
28
288
402

But if I don't create a person in the constructor object, where else could I create it, and what would be the syntax ? I tried this : `(name of a link)->pers = new Person;` but it raised me an error when compiling. – Thibault de Villèle May 25 '16 at 19:01
@ThibaultdeVillèle Use the `new` inside `Link`'s constructor: `pers` is a private member of `Link`, and cannot be accessed externally from an instance of `Link` – KABoissonneault May 25 '16 at 19:03
@KABoissonneault There is no reason to use new for `pers` when we can just get rid of it being a pointer. – NathanOliver May 25 '16 at 19:07
@NathanOliver I am incredibly greatful for your answer, unfortunately since this is a project for my programming class, `pers` being a pointer is _necessary_, and cannot be avoided. If so, should I use the answer from @KABoissonneault ? – Thibault de Villèle May 25 '16 at 19:13
@ThibaultdeVillèle Remember to `delete pers` inside `Link`'s destructor (`~Link`) if you choose to allocate `pers` dynamically – KABoissonneault May 25 '16 at 19:15
@ThibaultdeVillèle Yes you have to. I added some more to the answer – NathanOliver May 25 '16 at 19:15
@NathanOliver Thank you very much. You helped me more than I could hope for. – Thibault de Villèle May 25 '16 at 19:21
@ThibaultdeVillèle No problem. Glad to help. – NathanOliver May 25 '16 at 19:21
@KABoissonneault Thanks ! Will do. – Thibault de Villèle May 25 '16 at 19:22

C++ - Class constructor creates objects, but the result of the construction is different

1 Answers1