Errors when initializing class with an array as a member variable in C++

Question

I'm just starting to learn C++ and can't figure out what I'm doing wrong.

I'm trying to build a class "Holder" that holds an array of up to 100 of another class "Obj". I have the following two classes:

class Obj {
public:
    Obj(int k, char v): key(k), val(v) {};
private:
    int key;
    char val;

   friend class Holder;
};


class Holder {
private:
    enum {MAX_SIZE = 100};
    Obj p[MAX_SIZE];
    int pSize = 0;
public:
    Holder();
    ~Holder();
//...
};

When initializing the class Holder from main(), as follows...

int main() {
    Holder test;

    return 0;
}

I'm receiving these errors after running the program:

undefined reference to "Holder::Holder()" and undefined reference to "Holder::~Holder()"

I can't tell if I'm correctly using an array as a class member variable in "Holder"? Or if I'm missing something in the constructor?

You are showing the declarations of your constructor and destructor for `Holder` but where are the definitions? — super, Mar 27 '18 at 04:53
Is it your wish that the compiler will generate the c'tor and d'tor for `Holder`? — StoryTeller - Unslander Monica, Mar 27 '18 at 04:55
similar question: https://stackoverflow.com/questions/15712821/c-error-undefined-reference-to-classfunction — Eziz Durdyyev, Mar 27 '18 at 04:57
constructor and destructor need to be implemented for `Holder` for this program to compile. Either leave them to `=default;` or give some definition — Wander3r, Mar 27 '18 at 05:00
If you think there is a problem with the array, remove it and see. — juanchopanza, Mar 27 '18 at 05:37
Possible duplicate of [C++ error 'Undefined reference to Class::Function()'](https://stackoverflow.com/questions/15712821/c-error-undefined-reference-to-classfunction) — chtz, Mar 27 '18 at 07:41

score 0 · Answer 1 · answered Mar 27 '18 at 05:11

0

Try this code snippet. Since constructor of Holder will require auto initialization of an Obj array, a default constructor is used :

class Obj {
public:
    Obj() :key(0),val(0){};   //Additional - default constructor
    Obj(int k, char v): key(k), val(v) {};
private:
    int key;
    char val;

   friend class Holder;
};


class Holder {
private:
    enum {MAX_SIZE = 100};
    Obj p[MAX_SIZE];
    int pSize = 0;
public:
    Holder(){}   // constructor definition 
    ~Holder(){}   // destructor definition 
//...
};

int main()
{
    Holder test;
}

answered Mar 27 '18 at 05:11

seccpur

4,996
2
13
21

Removing the user defined constructor and destructor would be a better idea. That at least lets the user zero-initialize using value initialization. – juanchopanza Mar 27 '18 at 06:14
Also the error has nothing to do with the data members. – juanchopanza Mar 27 '18 at 06:17
This got rid of the error! Thanks a lot for your reply, seccpur! – sato_jessica Apr 06 '18 at 01:42

score -1 · Answer 2 · answered Mar 27 '18 at 07:27

-1

Consider using std::array (or std::vector) and perhaps also std::pair instead of Obj.

std::array<std::pair<int, char>, 100> p;  // Will automatically init with zeroes
// Example of using it:
p[50] = std::make_pair(1234, 'a');
std::cout << "key:" << p[50].first << ", val:" << p[50].second << std::endl;

answered Mar 27 '18 at 07:27

Tronic

1,248
12
16

Someone downvoted -- maybe you could also comment why this is not good? – Tronic Mar 28 '18 at 04:57

Errors when initializing class with an array as a member variable in C++

2 Answers2