How to fix binary operator - c++

Question

Error C2676 binary '[': 'collection::item' does not define this operator or a conversion to a type acceptable to the predefined operator
I read few posts and Microsoft VS site, but cannot understand how to fix the problem
template<typename T1, typename T2>
class collection {
private:
class item {
public:
T1 item; T2 key;
};
unsigned int top;
item array = new item[top];
public:
collection& operator[](unsigned int i) {
return array[i];
}
collection(int top) {
this->top = top;
}
void coutarr() {
for (int i = 0; array[i] != 0; i++) {
cout << array[i].item << endl;
}
}
void extendarray() {
item x = new item[top*2];
for (int i = 0; i < top; i++) {
x[i] = array[i];
}
delete []array;
swap(array, x);
top = top*2;
}
void addvar(int i, T1 item, T2 key) {
array[i].item = item; array[i].key = key; //Here 2 Errors
}
};
If someone can explain what shall I do, I'd be very grateful. Thanks.

Answer 1

Error C2676 binary '[': 'collection::item' does not define this operator or a conversion to a type acceptable to the predefined operator
this is because
item array = new item[top];
must be
item * array = new item[top];
but you also have to move the initialization in the constructor when top has a value
Out of that is was better that the constructor collection(int top) get an unsigned value being collection(unsigned top) or better collection(size_t top)
Copy contructor, operator= ... are missing while an attribute is a pointer
What about to check the validity of i in collection& operator[](unsigned int i) and void addvar(int i, T1 item, T2 key) to produce an exception when it is invalid ? addvar is also a strange name because it is a set, a add means the size is increased. You cannot access to an element on a const instance, add const collection& operator[](unsigned int i) const
In void extendarray() to double the size will not do a lot if the initial size is 0 ;-)

Answer 2

Just a guess: top is unititialized. That makes array an array with zero length at best (which is really bad) and invokes undefined behavior at worst.

Answer 3

It seems you mean at least something like the following
template<typename T1, typename T2>
class collection {
private:
class item {
public:
T1 item; T2 key;
};
unsigned int top;
item *array;
^^^^^^^^^^^^^
public:
auto & operator[](unsigned int i) {
^^^^
return array[i];
}
collection(unsigned int top) : top( top ), array( new item[top]{} )
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
{
}
~collection() { delete []array; }
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
collection( const collection & ) = delete;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
collection & operator =( const collection & ) = delete;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
void coutarr() {
for (int i = 0; array[i] != 0; i++) {
cout << array[i].item << endl;
}
}
void extendarray() {
item x = new item[top*2];
for (int i = 0; i < top; i++) {
x[i] = array[i];
}
delete []array;
swap(array, x);
top = top*2;
}
void addvar(int i, const T1 &item, const T2 &key) {
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
array[i].item = item; array[i].key = key; //Here 2 Errors
}
};
Of course you can futher develop the code.