new (C++)

In the C++ programming language, new is an operator that allows dynamic memory allocation on the heap. Except for a form called the "placement new", new attempts to allocate enough memory on the heap for the new data and, if successful, returns the address to the newly allocated memory. However if new can not allocate memory on the heap it will throw an exception of type std::bad_alloc. This removes the need to explicitly check the result of an allocation.

Syntax

The syntax for new is:

p_var = new typename;

where p_var is a previously declared pointer of type typename. typename can be any basic data type or user-defined object (enum, class, and struct included). If typename is of class type, the default constructor is called to construct the object.

To initialize a new variable created via new, use the following syntax:

p_var = new type(initializer);

where initializer is the initial value assigned to the new variable, or if type is of class type, initializer is the argument(s) to a constructor.

new can also create an array:

p_var = new type [size];

In this case, size specifies the length of one-dimensional array to create. The address of the first element is returned and stored into p_var, so

p_var[n]

gives the value of the nth element (counting from zero)

Memory allocated with new must be deallocated with delete to avoid a memory leak. Arrays allocated with new[] must be deallocated with delete[].

int *p_scalar = new int(5);
int *p_array = new int[5];

Initializers cannot be specified for arrays created with new. All elements of an array are initialized with the default constructor of the type. If the type does not have a default constructor, this is a compile-time error.

Implementation

In compilers conforming to the ISO C++ standard, if there is not enough memory for the allocation, the code throws an exception of type std::bad_alloc. All subsequent code is aborted until the error is handled in a try-catch block or the program exits abnormally. The program does not need to check the value of the pointer; if no exception was thrown, the allocation succeeded. The implemented operations are defined in the header <new>. In most C++ implementations the new operator can also be overloaded to define specific behaviors.

Releasing dynamically allocated memory

Any memory dynamically allocated with new must be released with a delete command. There are two variants: one for arrays and one for single objects.

int *p_var = new int;
int *p_array = new int[50];
 
delete[] p_array;
delete p_var;

Note that the compiler is not required to generate a diagnostic message for using the wrong delete; it cannot know in general whether a pointer is to a single element or an array of elements. Furthermore, using the inappropriate deallocator will result in undefined behavior.

Reallocating memory allocated by new[]

In contrast to C's realloc, 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. The C++ standard library provides a dynamic array that can be extended or reduced in its std::vector template.

See also

• Allocators
• Delete (C++)
• Exception handling
• malloc
• Memory pool
• Placement syntax
• Pointers
• Smart pointers

References

• IBM Documentation describing C++'s operator new
• Microsoft Visual Studio operator new documentation