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sample samp2(samp);
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Both forms result in the copy constructor being used to create a copy of samp
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Bonus Example: A Safe Array that Uses Dynamic Allocation
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The preceding example clearly showed when a copy constructor is and is not called However, it did not illustrate the type of situation in which one is needed This example does It demonstrates the necessity of the copy constructor by developing another implementation of a "safe array," which is an array that prevents boundary overruns and underruns The approach used here relies on dynamically allocated memory to hold the underlying array As you will see, this technique requires an explicit copy constructor to avoid problems Before we begin, it is useful to contrast this approach with the one shown in Overload the Subscripting Operator [ ] earlier in this chapter In that recipe, the example created an array type called safe_array that encapsulated a static array that actually held the elements Thus, each safe_array was backed by a full-length static array As a result, if a very large safe array was needed, the resulting safe_array object would also be very large because it would encapsulate the entire array The version developed here uses a different approach Called dyn_safe_array, it dynamically allocates memory for the array and stores only a pointer to that memory This has the advantage of making the safe-array objects smaller much smaller in some cases This makes them more efficient when they are passed to functions, for example Of course, it takes a bit more work to implement a safe array that uses dynamic memory, because both a copy constructor and an overloaded assignment operator are needed Like safe_array shown earlier, dyn_safe_array overloads the subscripting operator [ ] to allow normal, array-like subscripting to access the elements in the array The dyn_safe_array class is generic, which means that it can be used to create any type of array The number of elements in the array is passed as a non-type argument in its template specification Its constructor then allocates sufficient memory to hold the array of the desired size and type A pointer to this memory is stored in aptr The destructor for dyn_safe_array automatically frees this memory when an object goes out of scope Otherwise, because the [ ] is overloaded, a dyn_safe_array can be used just like a normal array When one dyn_safe_array is used to initialize another, the copy constructor is called It creates a copy of the original object by first allocating memory for the array and then copying elements from the original array into the newly allocated memory This way, each object's aptr points to its own array Without the copy constructor, an identical copy of a dyn_safe_array would be made, which would result in two objects having aptrs that pointed to the same memory Among other potential troubles, this would result in an attempt to free the same memory more than once when the objects go out of scope The copy constructor prevents this The same type of problem that the copy constructor prevents can also occur when one dyn_safe_array object is assigned to another To avoid this problem, the assignment operator is also overloaded so that the contents of the array are copied, but the dynamically allocated memory used by each object remains separate
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One last point: The copy constructor and the overloaded assignment operator display a message each time they are called This is simply for illustration Normally, neither would generate any output
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// // // // // // // // // // // // // // // A generic safe-array class that prevents array boundary errors It uses the subscripting operator to access the array elements This version differs from the approach used in the recipe: Overload the Subscripting Operator [] because it allocates memory for the array dynamically rather than statically An explicit copy constructor is implemented so that a copy of a safe array object uses its own allocated memory Therefore, the original object and the copy DO NOT point to the same memory The assignment operator is also overloaded for the same reason In both cases, the contents of the array are copied so that both the original array and the copy contain the same values
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#include <iostream> #include <new> #include <cstdlib> using namespace std; // A generic safe-array class that dynamically allocates memory // for the array The length of the array is passed as a non-type // argument in the template specification template <class T, int len> class dyn_safe_array { T *aptr; // pointer to the memory that holds the array int length; // number of elements in the array public: // The dyn_safe_array constructor dyn_safe_array(); // The dyn_safe_array copy constructor dyn_safe_array(const dyn_safe_array &obj); // Release the allocated memory when a dyn_safe_array object // goes out of scope ~dyn_safe_array() { delete [] aptr; } // Overload assignment dyn_safe_array &operator=(const dyn_safe_array<T,len> &rh_op); // Use the subscripting operator to access elements in // the safe array T &operator[](int i);
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