how to use exceptions and pointers in a vector class - c++

Question

I have this vector class, and I was provided with a driver to test the class. Most of it seems to work fine but I think there is something wrong with the exceptions part (which I haven't quite fully understood)
Here is the code for the class .cpp file
int myVector::at(int i)
{
if(i<vsize)
return array[i];
throw 10;
}
and here is the driver code
#include "myVector.h"
#include <iostream>
using namespace std;
int main()
{
// Create a default vector (cap = 2)
myVector sam;
// push some data into sam
cout << "\nPushing three values into sam";
sam.push_back(21);
sam.push_back(31);
sam.push_back(41);
cout << "\nThe values in sam are: ";
// test for out of bounds condition here
for (int i = 0; i < sam.size( ) + 1; i++)
{
try
{
cout << sam.at(i) << " ";
}
catch(int badIndex)
{
cout << "\nOut of bounds at index " << badIndex << endl;
}
}
cout << "\n--------------\n";
// clear sam and display its size and capacity
sam.clear( );
cout << "\nsam has been cleared.";
cout << "\nSam's size is now " << sam.size( );
cout << "\nSam's capacity is now " << sam.capacity( ) << endl;
cout << "---------------\n";
// Push 12 values into the vector - it should grow
cout << "\nPush 12 values into sam.";
for (int i = 0; i < 12; i++)
sam.push_back(i);
cout << "\nSam's size is now " << sam.size( );
cout << "\nSam's capcacity is now " << sam.capacity( ) << endl;
cout << "---------------\n";
cout << "\nTest to see if contents are correct...";
// display the values in the vector
for (int i = 0; i < sam.size( ); i++)
{
cout << sam.at(i) << " ";
}
cout << "\n--------------\n";
cout << "\n\nTest Complete...";
cout << endl;
system("PAUSE");
return 0;
}
Any help is appreciated. Thanks

Answer 1

The driver that you have provided:
try {
cout << sam.at(i) << " ";
}
catch(int badIndex) {
cout << "\nOut of bounds at index " << badIndex << endl;
}
expects that int will be thrown (a bit weird design, but well... this is the code that will use your class...). Your implementation of at() might look like this:
int& myVector::at(int i) throw(int) {
if (i < vsize)
return array[i];
throw i;
}
just try to follow one simple rule: throw by value, catch by reference.
Also note that you have a pointer:
private:
int* array;
which points to dynamically allocated memory allocated in constructor and copy constructor and freed in destructor :
myVector::myVector(int i)
{
...
array = new int[maxsize];
}
myVector::myVector(const myVector& v)//copy constructor
{
...
array =new int[maxsize];
}
myVector::~myVector()
{
delete[] array;
}
But how about the assignment operator ? See What is The Rule of Three?

Answer 2

Your stop condition of for loop ends it one element after the last one (i.e. you cannot access 4th element of sam vector because there are only three elements).
std::vector::at throws std::out_of_range exception in such situation (see: http://en.cppreference.com/w/cpp/container/vector/at), not int one. So you should change your exception handling part to something like this:
#include <exception>
try
{
cout << sam.at(i) << " ";
}
catch(std::out_of_range exc)
{
cout << "\nOut of bounds at index " << exc.what() << endl;
}