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.
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.