Creating and adding object from base class to derived class array

Question

I have an issue with creating and adding objects into an array This is my base class:

class CPerson
{
    std::string first_name;
    std::string last_name;
    int birth_year;
    char sex;


public:
    CPerson();
    CPerson(std::string, std::string,int,char);

with some extra setters and getters. Then I have this derived class:

class CData :
    public CPerson
{
    int nrOfPersons;
    CPerson *Data_array; 
public:
    CData(int);
    ~CData();
};

In the constructor I want to fill the Data_array with person objects , and I do the following:

CData::CData(int x) : nrOfPersons(x)
{
    for (int i = 0; i < nrOfPersons; i++)
        Data_array[i] = CPerson();
}

But it crashes halfway there. I made the default CPerson() constructor to cout a message "Inserted" everytime I use it. So if I call in main CData database(4) it should print "Inserted" 4 times on the screen. Instead it only prints 2 times and then it crashes.

CData shouldn't inherit from CPerson. A list of people is not a person. — Mat
– Mat, Commented May 17, 2015 at 11:42

sh03 · Accepted Answer · 2015-05-17 11:46:41Z

3

You are trying to use a pointer to CPerson as an array. That would be true if you had dynamically allocated an array at construction, but that's a lot of hassle you don't really want to deal with.

Thankfully the C++ standard created a very useful utility called std::vector, that you can use:

class CData : public CPerson
{
    std::vector<CPerson> Data_array;
    // ...
};

and then:

CData::CData(int n) 
    : Data_array(n)
    {}

And the most beautiful thing about this is that your nrOfPersons member object also magically disappears. To retrieve the size you can just call Data_array.size() and voilà.

edited May 17, 2015 at 11:46

answered May 17, 2015 at 11:41

sh03

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Comments

Sébastien Garmier · Accepted Answer · 2015-05-17 19:33:53Z

-1

You need to allocate memory for Data_array:

CData::CData(int x) : nrOfPersons(x)
{
    // allocate memory
    Data_array = new CPerson[nrOfPersons];

    // fill array
    for (int i = 0; i < nrOfPersons; i++)
        Data_array[i] = CPerson();
}

Don't forget to delete the allocated array afterwards (in a destructor for instance):

delete[] Data_array

edited May 17, 2015 at 19:33

answered May 17, 2015 at 11:52

Sébastien Garmier

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8 Comments

sh03 Over a year ago

Deleting Data_array in the destructor is not the only thing required to make this class safe.

Sébastien Garmier Over a year ago

Please tell me, which one?

sh03 Over a year ago

What do you mean "which one"?

Sébastien Garmier Over a year ago

The other things required

sh03 Over a year ago

For example operator= should also be overloaded. Second of all what if CPerson constructor throws an exception? Then you have memory leak. And so on and on.

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