How to make a template class a data member of another class? - c++

Question

So what I'm trying to do is write a class with a template class as its data member. This is part of my homework.
So far this is what I have:
letter_frequencies.h
#include "ltr_freq_pair.h"
#include "darray.h"
class letter_frequencies
{
private:
darray<letterFrequencyStruct> array;
public:
letter_frequencies();
void outputFrequencies();
};
letter_frequencies.cpp
#include <iostream>
#include "letter_frequencies.h"
letter_frequencies::letter_frequencies()
{}
void letter_frequencies::outputFrequencies()
{
for(int index = 0; index < 26; index ++)
{
std::cout << "\n" << array[index].letter;
std::cout << " " << array[index].frequency;
}
}
Everything compiles, but once I run the program it has this error:
a.out: darray.cpp:44: T& darray<T>::operator[](unsigned int) [with T = letterFrequencyStruct]: Assertion 'index < used' failed.
Abort (core dumped)
The code snippet I'm trying to use for this class is:
//Check if Class Works
letter_frequencies array;
array.outputFrequencies();
Right now, the darray works just fine and the program does exactly what I need it to do, but one of the requirements is to have a class named letter_frequency with a data member darray<structTypename> name, which is why I'm fussing with this in the first place.
Also, how would I make the class call the explicit constructor for the darray? I want to set the size to 26.
darray.h
#ifndef DARRAY_H
#define DARRAY_H
#include <iostream>
template <typename T>
class darray
{
public:
//Constructors
darray();
darray(const darray&);
explicit darray(unsigned);
//Destructor
~darray();
//Constant Members
unsigned size() const {return used;}
unsigned capacity() const {return cap;}
//Modification Members
T& operator [] (unsigned);
void pop_back();
void push_back(const T&);
const T* find(const T&) const;
void resize(unsigned);
//Operators
T& operator = (const darray&);
bool operator == (const darray&) const;
//Friends
template <typename X>
friend std::ostream& operator << (std::ostream& output, const darray<X>& array);
private:
T* arr;
unsigned used, cap;
};
template <typename X>
std::ostream& operator << (std::ostream& output, const darray<X>& array)
{
for(int index = 0; index < array.size; index++)
{
output << index << array[index];
}
return output;
}
#include "darray.cpp"
#endif

Answer 1

As I mentioned before, you'd have to make letter_frequencies constructor like this:
letter_frequencies::letter_frequencies() : array(26) {
}
(so you initialize the array with the necessary number of elements).
BUT: I don't think you should make your own home-grown array when std::vector would be better...