Subscripting

The subscript operator ([ ]), like the function-call operator, is considered a binary operator. The subscript operator must be a nonstatic member function that takes a single argument. This argument can be of any type and designates the desired array subscript.

The following example demonstrates how to create a vector of type int that implements bounds checking:

#include <iostream.h>

class IntVector
{
public:
  IntVector( int cElements );
  ~IntVector() { delete _iElements; }
  int& operator[]( int nSubscript );
private:
  int *_iElements;
  int _iUpperBound;
};

// Construct an IntVector.
IntVector::IntVector( int cElements )
{
  _iElements = new int[cElements];
  _iUpperBound = cElements;
}

// Subscript operator for IntVector.
int& IntVector::operator[]( int nSubscript )
{
  static int iErr = -1;

  if( nSubscript >= 0 && nSubscript < _iUpperBound )
    return _iElements[nSubscript];
  else
  {
    clog << "Array bounds violation." << endl;
    return iErr;
  }
}

// Test the IntVector class.
int main()
{
  IntVector v( 10 );

  for( int i = 0; i <= 10; ++i )
    v[i] = i;

  v[3] = v[9];

  for( i = 0; i <= 10; ++i )
    cout << "Element: [" << i << "] = " << v[i]
                << endl;

  return v[0];
}

When i reaches 10 in the preceding program, operator[] detects that an out-of-bounds subscript is being used and issues an error message.

Note that the function operator[] returns a reference type. This causes it to be an l-value, allowing you to use subscripted expressions on either side of assignment operators.