how to use exceptions and pointers in a vector class

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

Why are you trying to implement own vector when there is `std::vector` already? — LihO, Oct 13 '13 at 17:25
Please don't post *all* of your code, only the parts relevant to the problem at hand. In this case the code throwing the exceptions and the code catching them. Also *what* problems with the exceptions do you have? What happens when you throw an exception? What did you expect to happen? — Some programmer dude, Oct 13 '13 at 17:27
You need to read about [initialization lists](http://stackoverflow.com/q/1711990/140719), _[The Rule of Three](http://stackoverflow.com/q/4172722/140719)_, `const` [member function](http://stackoverflow.com/q/4059932/140719) and a lot more. Also, your `clear()` function violates the class' invariants: `maxsize` != no of allocated `int` objects, `vsize` != no of used ones, `array` pointing to nowhere. (There's more, but that's where I stopped.) I very much hope this homework isn't due tomorrow, because you have a lot of [good reading](http://stackoverflow.com/q/388242/140719) to do. — sbi, Oct 13 '13 at 17:40

score 1 · Accepted Answer · edited May 23 '17 at 12:05

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?

Michał Góral · Answer 2 · 2013-10-13T17:41:56.463

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;
}

how to use exceptions and pointers in a vector class

2 Answers2