Changing a private value of a class in method not returning the changes to main() - c++

Question

I'm having a problem that I haven't found an answer for in a week now. I have a dynamic array class and it has a method to add string values to it. It's supposed to represent an inventory you can add items to. However, I find that the changes made in the method to the private values of the class element aren't "updated" when I try to call for a print-method for the class element "backpack" later in the main(). I think this might be a problem due to referencing issues, but I've seen this work when a class hasn't been in a different module.
My "backpack" module print and add methods:
const int INITIAL_SIZE = 5;
Inventory::Inventory():
array_(new string[INITIAL_SIZE]),
max_space_(INITIAL_SIZE),
used_space_(0) {}
void Inventory::add(string item){
if ( size() == max_space_ ) {
string* new_array = new string[2 * max_space_];
for ( int i = 0; i < size(); ++i ) {
new_array[i] = array_[i];
}
delete [] array_;
array_ = new_array;
max_space_ = 2 * max_space_;
}
array_[used_space_] = item;
++used_space_;
}
void Inventory::print() {
for ( int i = 0; i < size(); ++i ) {
cout << array_[i] << endl;
}
}
The main():
Inventory inv;
string input;
while (cout << "input> "
and getline(cin,input)){
add_to_bag(input,inv);
So the point is you reset the inventory when you give it new contents. The function add_to_bag(); is:
void add_to_bag(string input, Inventory inv){
const string WHITESPACE1_REGEX = "[[:space:]]*";
const string WHITESPACE2_REGEX = "[[:space:]]+";
const string WORD_REGEX = "[[:alpha:]_]+";
const string LINE_REGEX =
WHITESPACE1_REGEX +
WORD_REGEX +
"(" +
WHITESPACE2_REGEX +
WORD_REGEX +
")*" +
WHITESPACE1_REGEX;
regex line_reg(LINE_REGEX);
regex word_regex(WORD_REGEX);
string line = input;
if ( regex_match(line, line_reg) ) {
sregex_iterator iter(line.begin(), line.end(), word_regex);
sregex_iterator end;
while ( iter != end ) {
inv.add(iter->str());
++iter;
}
} else {
cout << "Error: unknown inventory contents." << endl;
}
}

Answer 1

Your problem is:
void add_to_bag(string input, Inventory inv);
You pass a copy of the Inventory object to add_to_bag. You modify that copy ... and then it gets thrown away. The fix is to pass by reference:
void add_to_bag(string input, Inventory &inv);
Incidentally, in real-life code, I would strongly advise the use of std::vector<std::string> rather than "roll your own". There are a number of tricky exception handling issues you have got wrong here - and unless Inventory doesn't have a destructor (implying a memory leak), or does have a correct copy constructor I would have expected you to run into "double free" issues. (Read about "The Rule of Three".)

Answer 2

A simple way to design your class would be as follows:
class Inventory {
private:
std::vector<std::string> items_;
public:
Inventory(){}
~Inventory(){}
void addItem( const std::string& item ) {
items_.push_back( item );
}
void printInventory() const {
int idx = 0;
for (; idx < items_.size(); ++idx ) {
std::cout << items_[idx] << std::endl;
}
}
void clearInventory() {
items_.clear();
}
};
And as for your problem Martin Bonner had already answered it with the modifying of the copy and the removal of it afterwards and the other issues with the memory management.