Subscripting

The latest version of this topic can be found at 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.

Example

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  

Comments

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.

See Also

Operator Overloading