Dynamic Memory Allocation with new and delete

Some notes about the usage of keywords new and delete.

Syntax:

type *p = new type;     // undefined initial value for user-defined type
type *p = new type();   // default initial value, 0 for user-defined type
type *p = new type(initializer);
type *p = new type[size];          // default constructor
type *p = new type[size]();        // also default constructor
type *p = new (nothrow) type;      // don't throw bad_alloc exception
type *p = new (nothrow) type();
type *p = new (nothrow) type(initializer);
type *p = new (nothrow) type[size];
type *p = new (nothrow) type[size]();
delete p;
delete[] p;

Notes:

1. Initializers cannot be specified for arrays created with new. All elements of an array will be initialized with the default constructors. If the type doesn't have a default constructor, this will be a compile-error.
2. The behavior when operator new fails is compiler-specific. Most compilers will throw a std::bad_alloc exception. Using nothrow as above will return 0 or NULL and continue the program when new fails. Another option is to use function set_new_handler to handle new failures. The header file <new> is needed for the use of bad_alloc, nothrow, and set_new_hanlder.
3. The statement delete NULL; is allowed and no error will be reported.
4. In order to avoid double-free problem, assign the pointer to NULL after deletion.
   

   delete p;
   p = NULL;

   or

   delete[] p;
   p = NULL;

5. It is not possible to directly reallocate memory allocated with new[ ]. To extend or reduce the size of a block, one must allocate a new block of adequate size, copy over the old memory, and delete the old block.
6. Arrays allocated with new[ ] must be deallocated with delete[ ], not delete. Since the layout of arrays allocated with new[ ] is implementation defined, and possible not compatible with new. Some implementations of new[ ] embed the number of allocated objects first into the beginning of the allocated memory chunk, and return pointer to the remaining part of the array.

Related Posts:

Array Declaration and Initialization

Object Initialization