19.8.2. Vectors and operator[]

19.8.2. Vectors and operator[]
	19.8. Overloading special operators

[ fromfile: useroperators.xml id: None ]

Many lists and array-like classes offer an interface consistent with arrays, but with additional functionality. Here is an example of a container class that has

built-in protection against out-of-range array indices
Implementation of the [] operator
a simple right-justification method for numerical output

Example 19.7. src/operators/vect1/vect1.h

[ . . . . ]
class Vect {
public:
    explicit Vect(int n = 10);
    ~Vect() {
        delete []m_P;
    }
    int& operator[](int i) {   
        assert (i >= 0 && i < m_Size);
        return m_P[i];
    }
    int  ub() const {
        return (m_Size - 1);
    }   
private:
    int*  m_P;
    int   m_Size;
};

Vect::Vect(int n) : m_Size(n) {
    assert(n > 0);
    m_P = new int[m_Size];
}
[ . . . . ]

	access m_P[i]
	upper bound

In Example 19.8, we define an array of Vect objects. This gives us something akin to a matrix-like structure, where there is one fixed dimension and one that is possibly variable, or sparse, depending on the implementation details of Vect.

Example 19.8. src/operators/vect1/vect1test.cpp

#include "vect1.h"

int main() {
    Vect a(60), b[20];

    b[1][5] = 7;
    cout << " 1 element 5 = "<< b[1][5] << endl;
    for (int i = 0; i <= a.ub(); ++i)
        a[i] = 2 * i + 1;
    for (int i = a.ub(); i >= 0; --i)
        cout << ((a[i] < 100) ? " " : "" )
        << ((a[i] < 10) ? " " : "" )
        << a[i]
        << ((i % 10) ? "  " : "\n");
    cout << endl;
    cout << "Now try to access an out-of-range index"
    << endl;
    cout << a[62] << endl;
}


Output:

OOP> g++ vect1test.cc 
OOP> a.out
1 element 5 = 7
119  117  115  113  111  109  107  105  103  101
99   97   95   93   91   89   87   85   83   81
79   77   75   73   71   69   67   65   63   61
59   57   55   53   51   49   47   45   43   41
39   37   35   33   31   29   27   25   23   21
19   17   15   13   11    9    7    5    3    1
Now try to access an out-of-range index
a.out: vect1.h:12: int &Vect::operator[] (int):
Assertion `i >= 0 && i < m_Size' failed.
Aborted
OOP>


19.8.1. Conversion Operators		19.8.3. The Function-Call operator()