Spruce Up Your Built-in Arrays-2 : Page 2

Step 1: Array Wrapping
The following array_wrapper class template, originally presented by Bjarne Stroustrup, wraps a built-in array in a class that has no additional data members and therefore incurs no space overhead. Unlike ordinary STL containers, array_wrapper allocates its array statically on the stack, just as built-in arrays do. Most importantly, it provides the standard interface that other STL components recognize:

template <class T, int sz> struct array_wrapper
{
//uniform typedefs used in STL algorithms and containers
 typedef T value_type;
 typedef T* iterator;
 typedef const T * const_iterator;
 typedef T& reference;
 typedef const T& const_reference;

T v[sz]; //the actual array; no other data members here
// member functions of typical STL containers
operator T* () {return v;}
reference operator[] (size_t idx) {return v[idx];}
const_reference operator[] (size_t idx) const {return v[idx];}

iterator begin() {return v;}
const_iterator begin() const {return v;}

iterator end() {return v+sz;}
const_iterator end() const {return v+sz;}

size_t size() const {return sz;}
};

This class is designed for utmost efficiency. It has neither a constructor nor a destructor. Additionally, all its member functions are inlined.

You initialize an array_wrapper as you would a built-in array. The resulting object can be used wherever a built-in array is expected:

void func(int *, int sz); // C function 
//wrap an array of 20 int's & initialize all members to 0
array_wrapper <int, 20> arr={0};  
func(arr, arr.size()); //using T* conversion operator

Let's see how advantageous STL compatibility is. The following example uses the copy() algorithm and an ostream_iterator to display the contents of arr on the screen—as if it were an STL container:

        









                                

copy(arr.begin(), arr.end(),
  ostream_iterator<int>(cout, " "));

The C++98 Standard Library doesn't define this class template. However, it will be added in the next revision round. In the meantime, you can download a similar class from Boost or roll your own.

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