String error when expanding a dynamic array - c++

Question

I'm having a problem with trying to append an extra element to a dynamic array. I know I can use vectors, but for academic purposes, I have to use arrays.
#include <iostream>
using namespace std;
template <class T>
class MyList
{
protected:
T* elems;
int itsSize;
public:
MyList(int itsSize):itsSize(itsSize)
{
elems = new T[itsSize];
}
~MyList()
{
delete [] elems;
}
int getSize()
{
return itsSize;
}
virtual void addElem(){}
virtual void getElem(){}
};
template <class T>
class MyStack : public MyList<T>
{
int counter;
public:
MyStack(int size):MyList<T>::MyList(size){counter=0;}
void addElem()
{
T* tmp = new T[counter+1];
for (int i = 0; i<counter+1; i++)
tmp[i] = MyList<T>::elems[i];
counter++;
delete [] MyList<T>::elems;
MyList<T>::elems = tmp;
cin >> MyList<T>::elems[counter-1];
if(cin.fail())
{
cin.clear();
string line;
getline(cin, line);
throw "Wrong String Input--> will enter 0";
}
}
void getElem()
{
for(int i=counter-1; i>=0; i--)
{
cout << "Element-->" << MyList<T>::elems[i] << endl;
}
}
};
int main()
{
int storeChoice;
cout << "Would you like to store integers, strings or rectangles (1,2, or 3)?" << endl;
cin >> storeChoice;
if(storeChoice==1)
{
MyStack<int> numList(1);
cout << "Enter num:";
numList.addElem();
bool choiceAddLoop = true;
while(choiceAddLoop == true)
{
try
{
char choiceAdd;
cout << "Would you like to enter another elem?(y/n)" << endl;
cin >> choiceAdd;
if(choiceAdd=='y')
{
try
{
cout << "Enter num:";
numList.addElem();
}
catch(const char* wrongInput)
{
cout << wrongInput << endl;
}
}
else if(choiceAdd=='n')
{
choiceAddLoop=false;
break;
}
else
throw "Invalid Input.";
}
catch(const char* invalidChoice)
{
cout << invalidChoice;
}
}
cout << endl << "All Elements" << endl;
numList.getElem();
}
else if(storeChoice==2)
{
MyStack<string> stringList(1);
cout << "Enter string:";
stringList.addElem();
bool choiceAddLoop = true;
while(choiceAddLoop == true)
{
try
{
char choiceAdd;
cout << "Would you like to enter another elem?(y/n)" << endl;
cin >> choiceAdd;
if(choiceAdd=='y')
{
cout << "Enter string:";
stringList.addElem();
}
else if(choiceAdd=='n')
{
choiceAddLoop=false;
break;
}
else
throw "Invalid Input.";
}
catch(const char* invalidChoice)
{
cout << invalidChoice;
}
}
cout << endl << "All Elements" << endl;
stringList.getElem();
}
}
When I select the first choice ( integer ) the code will work:
Would you like to store integers, strings or rectangles (1,2, or 3)?
1
Enter num:22
Would you like to enter another elem?(y/n)
y
Enter num:3
Would you like to enter another elem?(y/n)
n
All Elements
Element-->3
Element-->22
Process returned 0 (0x0) execution time : 5.162 s
Press any key to continue.
A problem occurs in the second choice (string):
Would you like to store integers, strings or rectangles (1,2, or 3)?
2
Enter string:hello
Would you like to enter another elem?(y/n)
y
Enter string:terminate called after throwing an instance of 'std::length_error'
what(): basic_string::_M_create
Process returned 3 (0x3) execution time : 6.761 s
Press any key to continue.
It's the same code for both, why does it work only with integers?

Answer 1

The error happens when you call MyStack<T>::addElem(). When you copy the elements from the old array to the new array your loop should stop at counter, not counter+1. Correct version:
for (int i = 0; i<counter; i++)
tmp[i] = MyList<T>::elems[i];
Accessing MyList<T>::elems[counter], which is out of bounds, results in undefined behavior. For the int-case you were just lucky that the data currently stored there could be correctly interpreted as integer, with std::string you had less luck. So the random data which you wanted to interpret as std::string caused the error you got.
Live demo