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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:

// subscripting.cpp
// compile with: /EHsc
#include <iostream>

using namespace std;
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 );
   int i;

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

   v[3] = v[9];

   for ( i = 0; i <= 10; ++i )
      cout << "Element: [" << i << "] = " << v[i] << endl;
}
Array bounds violation. Element: [0] = 0 Element: [1] = 1 Element: [2] = 2 Element: [3] = 9 Element: [4] = 4 Element: [5] = 5 Element: [6] = 6 Element: [7] = 7 Element: [8] = 8 Element: [9] = 9 Array bounds violation. Element: [10] = 10

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.

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