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To overload the [ ] subscripting operator involves these steps: 1 The subscripting operator must be a non-static member of the class for which it is defined It cannot be a non-member function Therefore, add operator[ ]( ) as a member to the class on which it will be operating 2 Inside operator[ ]( ), perform the desired action, which usually involves accessing some object through an index 3 On completion, have operator[ ]( ) return the object (or a reference to the object) based on the index
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The [ ] is a binary operator for the purposes of overloading, and it must be overloaded by a non-static member function It has this general form: ret-type operator[ ]( indx-type idx) { // Access the item speci ed by idx } The subscript is passed in idx, which is often an int, but can be any type For example, in an associative container, idx could be a key The function can return any type, but it will usually be the type of element being obtained
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When the [ ] is evaluated, the object being subscripted must be an instance of the class for which the subscripting operator is defined This instance is passed via this The object within the [ ] is passed in idx For example, given an object called obj, the expression
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translates into this call to the operator[ ]( ) function:
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In this case, 5 is passed to the idx parameter A pointer to obj, the object that generated the call, is passed via this You can design the operator[ ]( ) function in such a way that the [ ] can be used on both the left and right sides of an assignment statement To do this, simply specify the return value of operator[ ]( ) as a reference After doing this, the following expressions are valid:
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x = obj[4]; obj[5] = 9;
Overloading the [ ] operator provides a means of implementing safe array indexing in C++ This is one of its main uses and one of its principal advantages As you know, in C++, it is possible to overrun (or underrun) an array boundary at runtime However, if you create a class that contains the array and allow access to that array only through the overloaded [ ] subscripting operator, then you can intercept an out-of-range index The following example illustrates this
Example
The following program shows how to overload the subscripting operator by using it to create a "safe array" that prevents boundary errors It defines a generic class called safe_array, which encapsulates an array The type of the array is specified by a template type parameter called T The length of the array is specified by a non-type template parameter called len The array encapsulated by safe_array is called ar The length of the array is stored in a variable called length Both are private members of safe_array The array elements are accessed only through the overloaded operator[ ]( ) It first confirms that an array access is within bounds If it is, operator[ ]( ) then returns a reference to the element The length of the array can be obtained by calling the method getlen( )
// // // // // // // Overload [] to create a generic safe array type The operator[]() function checks for array boundary errors so that an overrun or underrun is prevented Notice that this example uses a non-type template parameter to specify the size of the array
#include <iostream> #include <cstdlib> using namespace std;
7:
Potpourri
// Here, T specifies the type of the array and the non-type // parameter len specifies the length of the array template <class T, int len> class safe_array { // The array ar is declared to be of type T and of length len // The array is private Access is allowed only through operator[]() // In this way, boundary errors can be prevented T ar[len]; int length; public: // Create a safe_array of type T with a length of len safe_array(); // Overload the subscripting operator so that it accesses // the elements in ar T &operator[](int i); // Return the length of the array int getlen() { return length; } }; // Create a safe_array of type T with a length of len // The len variable is a non-type template parameter template <class T, int len> safe_array<T, len>::safe_array() { // Initialize the array elements to their default value for(int i=0; i < len; ++i) ar[i] = T(); length = len; } // Return a reference to the element at the specified index // Provide range checking to prevent boundary errors template <class T, int len> T &safe_array<T, len>::operator[](int i) { if(i < 0 || i > len-1) { // Take appropriate action here This is just // a placeholder response cout << "\nIndex value of " << i << " is out-of-bounds\n"; exit(1); } return ar[i]; } // This is a simple class used to demonstrate an array of objects // Notice that the default constructor gives x the value -1 class myclass { public: int x; myclass(int i) { x = i; }; myclass() { x = -1; } }; int main() {
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