Go4Expert - Memory Management In C and C + +

Original Non English Version here.

For simple applications, it's enough just to rely on automatic memory management through local variables. But once the data become larger, it is no longer imperative to request memory from the heap and manage.

Content

A function families

In principle, belong to the memory always two parts. First, an area of memory are requested, and second, the memory used at the end of the work be returned. For this reason, memory management functions normally exist in pairs, sharing the work.

When working with C + + objects is yet to fill the requested memory with sensible starting values or the data of the object at the end of the work to clean up again. For this purpose, there are constructors and destructors that are called automatically to suit the use or management must be handled manually.

Over time, various function pairs were developed, which can be requested and memory freed. It is particularly important to the different methods of memory management not to interfere with each other. In the simplest case, it is "just" a memory leak (if the sharing function "just forget" to call the Objektdestruktor or returns too little heap memory), in the worst case, can not be predicted what will make the program (for example, it can happen that the release function tries to interpret something in her memory structure unknown).

For this reason, libraries, heap areas and request the user to pass, always provide a compatible release function, which is suitable to its own working memory - or in the documentation at least set the method of the memory they use.

1.1 malloc () and free ()

This family dates back to C and has been for reasons of compatibility for C + +: Accepted

malloc (s)
allocates a memory area with (at least) s bytes in size and returns a pointer to the beginning of this area. The content of this field is undefined.
calloc (n, s)
enough allocates memory for n objects of size s and initialize it with zeros
realloc (p, s)
the amount of memory changes, the p shows on s bytes and returns the new address of the memory (realloc (NULL, s) "is equivalent to 'malloc (s)" and "realloc (p, 0)" corresponds "free (p)")
free (p)
there by the malloc () etc. requested memory range again free (p must be the NULL pointer, in which case nothing happens)

These functions do not care about the contents of the managed memory areas and therefore should only be used with POD (plain old data) types.

Note: The function realloc () can also single-handedly take over the entire memory of a program by its parameters are set appropriately:

Code:

char* str = realloc (NULL, 100);/ / memory allocation 

... 

realloc (str, 0);/ / Release memory

1.2 New and delete

For C + +, a new mechanism for memory management, which provides a better approach to the class. Unlike malloc () / free () the memory areas are regarded as objects of arbitrary classes, and now initialized with an appropriate constructor.

new T (...)
enough reserved memory for an object of type T, and calls its constructor according to the specified parameter list (the parameter list can be removed also, then the default constructor uses)
delete p;
calls the destructor of the object pointed to by p, and are then free the memory

Another advantage of new and delete is the fact that they can be overridden by the user. This requires either a global (for every new call) or static member function (for their own class, and all children) "operator new ()" or "operator delete ()", or the free memory can request. These operators work with unstructured data fields - the call to constructors and destructors are independently:

Code:

void* operator new(size_t s) 

( 

  void* p; 

  / / Reserve memory for at least s bytes 

  return p; 

) 



void operator delete(void* p) 

( 

  / / Give free hand p 

)

Both functions can obtain additional parameters of any type to implement the so-called "placement new". These parameters are by calling "new (values) T (ctor parameter) passed. The most common use cases of this are the specification of a destination address (an object to initialize in available memory) and the installation of debugging information (the MFC-operator DEBUG_NEW passes "as file name and program line to be able to track memory leaks).

Note: "operator new" and "operator delete" operate with bare areas of memory. The constructor and destructor of the object involved is called when operator new has come back before or operator delete is called.

Caution: Do not overcharge the global operators only if you are absolutely sure what you want to achieve. The global version of operator new and operator delete takes really ANY memory request within the program - and in contrast to klasseninternen operators can not here from the predefined memory access operators.

Another typical application for operator new and operator delete is to use a pool manager: in order to save computing time, the memory of precaution required in larger blocks, replaced and forwarded as needed to the program. From the program of shared memory is not returned to the system, but also for future requirements repealed.

1.3 New [] and delete []

In parallel with new and delete, playing with various objects, there are versions to manage whole object array:

new T [n]
enough reserved memory for n objects of type T, and calls for an object whose default constructor
Note: It is not possible to specify parameters for the constructor invoked
delete [] p;
calls for all objects in the array after p destructor on the memory and are then free to
Just like new / delete can be adapted to these functions by "be void * operator new [] (size_t s);" or "void operator delete [] (void * p); overwritten.

Note: new [] remember (just like malloc ()) internally the size of the allocated memory. However, there is no portable way to access this information.

Note: Despite the name similarity is not guaranteed that new / delete and new [] / delete [] are compatible with each other. So try NIE, the two mix with each other.

1.4 Other families

Many libraries define your own function pairs in order to request and release memory, eg VirtualAlloc () / manage Virtual Free () from the WinAPI, the memory in the "shared memory". All such families function in common is that they consist of at least two functions - one for requesting memory, one for release. Analogous to the malloc () family may be added other features that can reorganize the managed memory.

Because different memory strategies are not necessarily compatible with each other, you should always use the correct release function if you need more memory not yours. Therefore, it is usually a good idea to have a program on a strategy and set within this largely unsustainable. If that is not generally possible, should know where he is to his memory relate to each case are set for each pointer used.

Even in the standard library yet another function family is defined, which can be used to manage temporary storage:

get_temporary_buffer <T> (num)
allocates a temporary buffer for a maximum of num objects of type T and is a couple with its address and available disk space back to the
return_temporary_buffer (p)
have requested a temporary buffer back to the system

These temporary buffer can be used within a function to provide large amounts of additional memory (eg, merge sort requires an auxiliary memory for its data). Make sure that the returned memory region may be less than requested.

Allocators

The containers of the STL default to new [] and delete [] to know the memory request for their elements. However, as it should be possible to switch to a different memory management, this behavior had been moved into so-called allocators - these can be interchanged, to use a different memory behavior. In addition, each container class as the last template parameter an indication of the allocator class. The default value for this is std:: allocator <T> - this allocates memory using new [] and returns it with delete [] free again.

An allocator has to be used with the STL, provide some basic methods and definitions. If you write an allocator for its own purposes, the requirements may be lower.

2.1 management

The most important are the Allokatorklasse course, the functions that can manage the memory:

allocate (n, h = 0)
Reserved uninitialized memory for n elements and returns it (eg by: operator new () or malloc ()). The optionally specified pointer h can be used to influence the storage strategy.
construct (p, val)
Initializes the object pointed to by p, as a copy of val (usually via placement new)
destroy (p)
Destroy the object pointed to by p (usually by direct destructor call)
deallocate (p, n)
Is an area behind free p consisting of n elements (the parameter can be useful if the size of memory used can not be determined from the pointer itself) - The objects must first be destroyed

In addition, the allocator max_size the helper methods () that can specify the maximum size of a memory block (in elements) and address (ref) can be defined by the object at a given address.

Custom allocators are generally allocate the methods (), deallocate () and max_size () override to implement the storage strategy. construct () and destroy () are likely to differ only in the rarest cases, by the standard version:

Code:

template<typename T> 

class Alloc 

( 

  ... 

public: 

  void construct (pointer p, const T & val) 

  ( New((void*) p) T val ();) 

  void destroy (pointer p) 

  (P-> ~ T ();) 

  ... 

);

2.2 rebind <>

In order to offer the ability to store various objects that contains a substructure Allokatorklasse called "Rebind. This is used as the associative container classes, which receive as an allocator template parameter <T>, but need an internal TreeNode allocator <<T>>.

This structure is pretty simple and serves only to "packaging" of the alternate type:

Code:

template<typename T> 

class Alloc 

( 

  ... 

public: 

  template<tyename U> 

  struct rebind 

  ( typedef Alloc <U> other;); 

  ... 

);

In this way, one can easily allocator for any data obtained by inspecting the structure:

Code:

template<typename T,typename A = std:: allocator <T>> 

class mycont 

( 

public: 

  //... 

  typedef A allocator_type; 

  typedef typename A:: rebind <T*>:: other ptr_allocator,/ / Pointer to compatible allocator 

  //... 

);

2.3 Comparisons

Different allocators can be compared with the operators == and! =. These features will be released equality that two allocators are interchangeable - is requested by a memory on b again.

For example, building allocators, to delete the new /, are interchangeable, as is required for the delete-object call no tax. In contrast pool allocators are limited interchangeable because each allocator manages its own list of reserve memory.

Note: STL compatible allocators have to be interchangeable. reduce most of the comparison operators to a "return true reason," or return false; ".

2.4 Type definitions

Allocators also define some basic types used by the connected container classes continue:

value_type (T)
the underlying value type
size_type (size_t) and difference_type (ptrdiff_t)
two integer types for specifying memory sizes (unsigned) or "points-difference (signed)
pointer (T *) and const_pointer (const T *)
Pointer to variable or constant objects
reference (T &) and const_reference (const T &)
Reference to variable or constant objects

Note: STL compatible allocators is the award of these basic types defined (according to the above information).

Auxiliary functions for memory management

In addition to the standard library allocators defines some utility functions that can deal with non-uninitialized memory. These serve as a logical extension of memset () and memcpy ():

uninitialized_fill (beg, end, val)
initializes the field (in the sense STL) [beg, end [with the corresponding number of copies of 'val'
(Uninitialized_fill_n beg, n, val)
initializes the array behind 'beg' with n copies of 'val'
uninitialized_copy (beg, end, nbeg)
initializes the array behind 'nbeg' with copies of the range [beg, end [

These functions can be also referred to in section 1.3 restriction bypass that new [] can only call the default constructor:

Code:

T * data =::operator new[] (n *sizeof(T)); 

unitialized_fill_n (data, n, T (4711)); 

... 

delete[] data;

In an emergency, it is also possible to fill a new placement of uninitialized memory area with values:

Code:

* p myType = (myType *) malloc (sizeof(myType)); 

new(p) myType (params); 

... 

p-> ~ myType ();/ / direct call destructor 

free (p);

(Note: should normally take place this construction rather directly to new / delete be used)

3.1 Rohspeicher iterators

Full Name:

Code:

template<typename OutIt,typename T> 

class raw_storage_iterator;

This Hilfsiteratoren can be used to an uninitialized memory as a target STL algorithm indicated. They translate into calls on assignments of the copy constructor placement new.

The template parameter of this class are, first, an output iterator that is used as the basis for the iterator arithmetic (usually enough for this one 'T *') and second, the data type used.

3.2 Functions of the C library

Unlike C + + interprets the C library typically anything in the memory areas, which it manages. Logically, obtained from malloc () a bunch of bytes with no internal structure. Then it depends on the programmer, as he fills the memory and interpreted.

To process this naked memory areas can be either the malloc () pointer obtained in the "right" type casting and processing of hand or some ready-made functions of the C library (memcpy (), memset (), etc.), or (with restrictions ) the string processing functions (strcpy (), strcat () to take, etc.) to help.

Note: Try to be better not to interpret bare memory than C + + objects. Most methods expect an existing basic structure and lead to undefined behavior if they get junk data set before.

Smart Pointer

The management of heap memory (no matter how it was obtained) on bare hands always suffer the same problem - no one can verify when and by whom the memory must be freed. Forget the last owner of the release shows a memory leak, because no one knows more about the existence of the memory block. If the memory released too early, you get access violations and undefined behavior if other users want to access the shared memory.

A solution to this problem is to make the pointer itself to instruct the memory management, instead of bare hands by so-called "smart pointer" is used. These differ in principle three different approaches to control the access and control:

Reference counting (boost:: shared_ptr, boost:: shared_array)
The referenced object counts how many pointers point to it. And only when the last pointer from the object, separates the memory is freed.
Exclusive access (boost:: scoped_ptr, boost:: scoped_array)
The address may not be transferred. The owner of the pointer and puts him in the end is also responsible to release him again.
Ownership transfer (std:: auto_ptr)
There is always just a pointer that points to a memory (owner). When an assignment is the possession and passed the former owner loses access to the memory.

The operation and characteristics of the different smart pointer classes was some time ago in the article "Smart pointer" peter's declared.

Note: Normal smart pointer classes to a specific memory function (usually new / delete and new [] / delete []) defined and therefore expect that their managed storage "was created correctly."

The above smart pointer classes (except auto_ptr to find) into the Boost Library or TR1 extension of the standard. The auto_ptr class is part of the C + + Standard Library.

STL Container

Whom the whole burden of memory management is too complicated, it can also make it easier - and use the various classes of the STL containers. All the container take care of their own so to provide enough memory and to manage, so that the user no longer needs to worry about where their data is housed.

The usable container classes differ mainly from the perspective of memory in it, the strategy that they require memory and organize:

vector <> and basic_string <>
use a contiguous area of memory that grows as needed
deque <>
usually used a two-layered structure with multiple memory blocks hung together logically to be extended if needed or released
list <>
uses a linked list, where each element has a pointer to predecessor and successor
set <>, multiset <>, map <> and multimap <>
use a (balanced) binary tree of individually requested node into which they sort their elements

Note: This will of course depend on the type of allocator, where the containers maintain their memory.