Assignment operator overload / retrieve function - c++

Question

I'm trying to retrieve an item from a BST for an assignment in class. Some of the provided code I am not allowed to alter. The instructor has created a retrieve function in the driver (which I can't change) that looks like this
static void retrieveItem(char *name)
{
Data const *data;
cout << ">>> retrieve " << name << endl << endl;
if (database->retrieve(name, data))
cout << *data << endl;
else
cout << "not found" << endl;
cout << endl;
}
The function it calls in the BST class looks looks like this (so far). I cannot change the arguments to the function call.
bool BST::retrieve(const char *key, Data const *& data) const
{
int rIndex = 0;
while (rIndex <= capacity)
{
if (strcmp(items[rIndex].data.getName(), key) == 0)
{
data = items[rIndex].data;
return true;
}
else if (strcmp(items[rIndex].data.getName(), key) < 0)
rIndex = (rIndex * 2) + 1;
else if (strcmp(items[rIndex].data.getName(), key) > 0)
rIndex = (rIndex * 2) + 2;
}
return false;
}
There is an array of structs called items that looks like this
struct Item
{
Data data; // the data instance must be specified this way, NOT as a pointer
bool isEmpty = true;
int loc = 0;
};
Item *items;
Finally I have the following assignment overload and copy constructors implemented for the data class (can't change the source file on this one)
Data::Data(const Data& source)
{
strcpy(this->name, source.name);
}
Data& Data::operator=(const Data& data2)
{
strcpy(this->name, data2.name);
return *this;
}
Please correct me if I'm wrong, but it seems like the goal of his retrieve function in the driver is to search for a data object using a key(name), and then copy it into the data argument sent to function. Unfortunately, the line
data = items[rIndex].data;
in my retrieve function doesn't work with a . or an ->]
I'm 90% sure the . is the correct way to access that, but I'm given the error "no suitable conversion type from 'Data' to 'const Data *' exists"
How else could I achieve this without using the assignment overload operator, or my implementation of the overload wrong?

Answer 1

bool BST::retrieve(const char *key, Data const *& data) const
The second argument is a reference to a pointer to const Data, so you must set it to a pointer to items[rIndex].data, not to its value.
Consider the following
void foo(int & out)
{
out = 42;
}
When it is called like so
// ...
int x = 0;
foo(x);
std::cout << x;
42 will be printed, because a reference to x was passed into the function. Your situation is a little different - you are passed a reference to a pointer, so that the caller can retrieve a pointer to the data in a similar way to the above, for example:
int x; // global
// ...
void foo(int *& out)
{
x = 42;
out = &x; // set out to pointer to x
}
int main()
{
int * ptr = nullptr;
foo(ptr); // foo will set ptr to the pointer to x
std::cout << *ptr; // prints 42
}
Again, 42 will be printed. Note the use of the unary ref, &, and deref, *, operators to get a pointer to x and dereference ptr to extract its value.
As an aside, it's not possible to tell how broken your implementation of Data::operator= is without seeing more of the class. As it stands it is broken because it is undefined behaviour to use strcpy on overlapping regions of memory - which will happen if someone tries to assign an object instance to itself (see strcpy). For cases other than self-assignment, it is only correct if the destination of the assignment always has enough space for the string in the source. Otherwise you will write to unallocated memory. Also, if there are any additional member variables, they will need copying too.