New (C )

In the C++ programming language, and are a pair of language constructs that perform dynamic memory allocation, object construction and object destruction.

Overview

Except for a form called the "placement new", the operator denotes a request for memory allocation on a process's heap. If sufficient memory is available, initialises the memory, calling object constructors if necessary, and returns the address to the newly allocated and initialised memory. A request, in its simplest form, looks as follows:

p = new T;

where is a previously declared pointer of type (or some other type to which a pointer can be assigned, such as a superclass of ). The default constructor for , if any, is called to construct a instance in the allocated memory buffer.

If not enough memory is available in the free store for an object of type , the request indicates failure by throwing an exception of type . This removes the need to explicitly check the result of an allocation.

The deallocation counterpart of is , which first calls the destructor (if any) on its argument and then returns the memory allocated by back to the free store. Every call to must be matched by a call to ; failure to do so causes memory leaks.

syntax has several variants that allow finer control over memory allocation and object construction. A function call-like syntax is used to call a different constructor than the default one and pass it arguments, e.g.,

p = new T(argument);

calls a single-argument constructor instead of the default constructor when initializing the newly allocated buffer.

A different variant allocates and initialises arrays of objects rather than single objects:
p = new T

This requests a memory buffer from the free store that is large enough to hold a contiguous array of objects of type , contiguously, and calls the default constructor on each element of the array.

Memory allocated with the must be deallocated with the operator, rather than . Using the inappropriate form results in undefined behavior. C++ compilers are not required to generate a diagnostic message for using the wrong form.

The C++11 standard specifies an additional syntax,

p = new T ;

that initializes each to .

Error handling

If cannot find sufficient memory to service an allocation request, it can report its error in three distinct ways. Firstly, the ISO C++ standard allows programs to register a custom function called a with the C++ runtime; if it does, then this function is called whenever encounters an error. The may attempt to make more memory available, or terminate the program if it can't.

If no is installed, instead throws an exception of type . Thus, the program does not need to check the value of the returned pointer, as is the habit in C; if no exception was thrown, the allocation succeeded.

The third method of error handling is provided by the variant form , which specifies that no exception should be thrown; instead, a null pointer is returned to signal an allocation error.

Overloading

The operator can be overloaded so that specific types (classes) use custom memory allocation algorithms for their instances. For example, the following is a variant of the singleton pattern where the first call allocates an instance and all subsequent calls return this same instance:

#include
#include

class Singleton ;

This feature was available from early on in C++'s history, although the specific overloading mechanism changed. It was added to the language because object-oriented C++ programs tended to allocate many small objects with , which internally used the C allocator (see ); that, however, was optimized for the fewer and larger allocations performed by typical C programs. Stroustrup reported that in early applications, the C function was "the most common performance bottleneck in real systems", with programs spending up to 50% of their time in this function.

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Relation to malloc and free

Since standard C++ subsumes the C standard library, the C dynamic memory allocation routines , , and are also available to C++ programmers. The use of these routines is discouraged for most uses, since they do not perform object initialization and destruction. and were, in fact, introduced in the first version of C++ (then called "C with Classes") to avoid the necessity of manual object initialization.

In contrast to the C routines, which allow growing or shrinking an allocated array with , it is not possible to change the size of a memory buffer allocated by . The C++ standard library instead provides a dynamic array (collection) that can be extended or reduced in its template class.

The C++ standard does not specify any relation between / and the C memory allocation routines, but and are typically implemented as wrappers around and . Mixing the two families of operations, e.g., 'ing 'ly allocated memory or 'ing 'd memory, causes undefined behavior and in practice can lead to various catastrophic results such as failure to release locks and thus deadlock. Section 4.4, Common C++ Memory Management Errors.

See also

* Allocator (C++)
* Exception handling
* Memory pool
* Pointer (computer programming)
* Resource Acquisition Is Initialization (RAII)
* Smart pointers

References

{{Programming languages

C++
Articles with example C++ code