I have a question about accessing methods of a type used in the instantiation of a template class in C++ and would greatly appreciate any help. I have researched this and nothing I can find on the internet helps with my specific problem.

Basically, I have a template class for a simple array/bag data structure.

Then, I have a template class for a dictionary, which uses the ABag class. (enough code for a min. reproducible example is below.)

There is also a KVpair class with methods related to key-value pairs (get the key, get the value, set them, etc.)

So, my question is this: When I instantiate the BDictionary class as an ABag of so many KVpairs (such as BDictionary<int, string> myDict(20);, how can I access methods of the KVpair class for the nth element of myDict? I have a find function in BDictionary for which I need to know the key and value of any given element of myDict.

But I get the C2440 error as shown in the below code. How can I find the key and value, using the methods in KVpair class, for the nth element of myDict?

(Again, I've spent a good bit of time looking up this issue but I just can't find anything specific to my problem.)

Here is enough code for a minimal reproducible example:

template <typename E>
class ABag {
};

template <typename Key, typename E>
class KVpair {};

template <typename Key, typename E>
class BDictionary {
 public:
  bool find(const Key& k, E& rtnval) const {
    for (size_t cur_pos = 0; cur_pos < 0; cur_pos++) {
      KVpair<Key, E> temp = dictionary[cur_pos];
    }
    return false;
  }

 private:
  ABag<KVpair<Key, E>> *dictionary;
};

int main() {
  BDictionary<int, int> d;
  int rv = 0;
  d.find(0, rv);
}

Again, I need to access the keys and values of different entries in the find function. Please help!

I need to keep the pointer at ABag<KVpair<Key, E>>* dictionary. Removing the '*' causes some of the KVpair functions to throw an access violation error (the error described at Access Violation writing location 0xCDCDCDCD, but in a much different setting).

I have a question about accessing methods of a type used in the instantiation of a template class in C++ and would greatly appreciate any help. I have researched this and nothing I can find on the internet helps with my specific problem.

Basically, I have a template class for a simple array/bag data structure.

Then, I have a template class for a dictionary, which uses the ABag class. (enough code for a min. reproducible example is below.)

There is also a KVpair class with methods related to key-value pairs (get the key, get the value, set them, etc.)

So, my question is this: When I instantiate the BDictionary class as an ABag of so many KVpairs (such as BDictionary<int, string> myDict(20);, how can I access methods of the KVpair class for the nth element of myDict? I have a find function in BDictionary for which I need to know the key and value of any given element of myDict.

But I get the C2440 error as shown in the below code. How can I find the key and value, using the methods in KVpair class, for the nth element of myDict?

(Again, I've spent a good bit of time looking up this issue but I just can't find anything specific to my problem.)

Here is enough code for a minimal reproducible example:

template <typename E>
class ABag {
};

template <typename Key, typename E>
class KVpair {};

template <typename Key, typename E>
class BDictionary {
 public:
  bool find(const Key& k, E& rtnval) const {
    for (size_t cur_pos = 0; cur_pos < 0; cur_pos++) {
      KVpair<Key, E> temp = dictionary[cur_pos];
    }
    return false;
  }

 private:
  ABag<KVpair<Key, E>> *dictionary;
};

int main() {
  BDictionary<int, int> d;
  int rv = 0;
  d.find(0, rv);
}

Again, I need to access the keys and values of different entries in the find function. Please help!

I need to keep the pointer at ABag<KVpair<Key, E>>* dictionary. Removing the '*' causes some of the KVpair functions to throw an access violation error (the error described at Access Violation writing location 0xCDCDCDCD, but in a much different setting).

• c++
• class
• templates

3 Answers 3

why is dictionary[cur_pos] an ABag

because you declared it as

ABag<KVpair<Key, E>>* dictionary

and that's how pointers work.

Perhaps you meant to write

(*dictionary)[cur_pos]

and add an operator[] overload to ABag?

NB. It would be better to make dictionary a non-pointer member here anyway, which saves you the ugly (*dictionary) bit, but you still need a way to actually index the dictionary's contents.

This is a simplified code for you

template <typename E>
class ABag {
 private:
  E* data;
  int used;
};

template <typename Key, typename E>
class KVpair {};

template <typename Key, typename E>
class BDictionary {
 public:
  bool find(const Key& k, E& rtnval) const {
    for (size_t cur_pos = 0; cur_pos < 0; cur_pos++) {
      KVpair<Key, E> temp = dictionary[cur_pos];
    }
    return false;
  }

 private:
  ABag<KVpair<Key, E>> *dictionary;
};

int main() {
  BDictionary<int, int> d;
  int rv = 0;
  d.find(0, rv);
}

The error means that the type of dictionary[cur_pos] is ABag<KVpair<Key, E>> that can't be assigned to KVpair<Key, E>.

As I can see, ABag handles an array. Hence, you likely wanted to add operator[] to ABag and make ABag<KVpair<Key, E>>* dictionary a regular member:

template <typename E>
class ABag {
 public:
  E& operator[](size_t);              // <-- added
  const E& operator[](size_t) const;  // <-- added
 private:
  E* data;
  int used;
};

template <typename Key, typename E>
class KVpair {};

template <typename Key, typename E>
class BDictionary {
 public:
  bool find(const Key& k, E& rtnval) const {
    for (size_t cur_pos = 0; cur_pos < 0; cur_pos++) {
      KVpair<Key, E> temp = dictionary[cur_pos];
    }
    return false;
  }

 private:
  ABag<KVpair<Key, E>> dictionary;  // <-- not a pointer
};

int main() {
  BDictionary<int, int> d;
  int rv = 0;
  d.find(0, rv);
}

Possible implementation

  E& operator[](size_t idx) {
    if (idx >= used)
      throw std::out_of_range();
    return data[idx];
  }

https://godbolt./z/aYc6EGo67

Solved using a combination of 3CxEZiVlQ and Useless's answers:

#include <cstdlib>

template <typename E>
class ABag {
    int used;
    E* data;
    public:
    E& operator[](size_t idx) { // from  3CxEZiVlQ

        if (idx >= used)
            {exit(-1);}
        return data[idx];
    }

};

template <typename Key, typename E>
class KVpair {};

template <typename Key, typename E>
class BDictionary {
 public:
  bool find(const Key& k, E& rtnval) const {
    for (size_t cur_pos = 0; cur_pos < 0; cur_pos++) {
      KVpair<Key, E> temp = (*dictionary)[cur_pos]; // from Useless
    }
    return false;
  }

 private:
  ABag<KVpair<Key, E>> *dictionary;
};

int main() {
  BDictionary<int, int> d;
  int rv = 0;
  d.find(0, rv);
}