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Basics of C++
Classes of C++
C++ Programming and Object-Oriented Design
3.4 Dynamic memory
In the programs seen in previous chapters, all memory needs were determined before program execution by defining the variables needed. But there may be cases where the memory needs of a program can only be determined during runtime. For example, when the memory needed depends on user input. On these cases, programs need to dynamically allocate memory, for which the C++ language integrates the operators
Operators new and newDynamic memory is allocated using operator
newis followed by a data type specifier and, if a sequence of more than one element is required, the number of these within brackets
. It returns a pointer to the beginning of the new block of memory allocated. Its syntax is:
pointer = new type
pointer = new type [number_of_elements]
The first expression is used to allocate memory to contain one single element of type
type. The second one is used to allocate a block (an array) of elements of type
number_of_elementsis an integer value representing the amount of these. For example:
In this case, the system dynamically allocates space for five elements of type
intand returns a pointer to the first element of the sequence, which is assigned to
foo(a pointer). Therefore,
foonow points to a valid block of memory with space for five elements of type
foois a pointer, and thus, the first element pointed to by
foocan be accessed either with the expression
fooor the expression
*foo(both are equivalent). The second element can be accessed either with
*(foo+1), and so on...
There is a substantial difference between declaring a normal array and allocating dynamic memory for a block of memory using
new. The most important difference is that the size of a regular array needs to be a constant expression, and thus its size has to be determined at the moment of designing the program, before it is run, whereas the dynamic memory allocation performed by
newallows to assign memory during runtime using any variable value as size.
The dynamic memory requested by our program is allocated by the system from the memory heap. However, computer memory is a limited resource, and it can be exhausted. Therefore, there are no guarantees that all requests to allocate memory using operator
neware going to be granted by the system.
C++ provides two standard mechanisms to check if the allocation was successful:
One is by handling exceptions. Using this method, an exception of type
bad_allocis thrown when the allocation fails. Exceptions are a powerful C++ feature explained later in these tutorials. But for now, you should know that if this exception is thrown and it is not handled by a specific handler, the program execution is terminated.
This exception method is the method used by default by
new, and is the one used in a declaration like:
The other method is known as
nothrow, and what happens when it is used is that when a memory allocation fails, instead of throwing a
bad_allocexception or terminating the program, the pointer returned by
newis a null pointer, and the program continues its execution normally.
This method can be specified by using a special object called
nothrow, declared in header
<new>, as argument for
In this case, if the allocation of this block of memory fails, the failure can be detected by checking if
foois a null pointer:
nothrowmethod is likely to produce less efficient code than exceptions, since it implies explicitly checking the pointer value returned after each and every allocation. Therefore, the exception mechanism is generally preferred, at least for critical allocations. Still, most of the coming examples will use the
nothrowmechanism due to its simplicity.
Operators delete and deleteIn most cases, memory allocated dynamically is only needed during specific periods of time within a program; once it is no longer needed, it can be freed so that the memory becomes available again for other requests of dynamic memory. This is the purpose of operator
delete, whose syntax is:
The first statement releases the memory of a single element allocated using
new, and the second one releases the memory allocated for arrays of elements using new and a size in brackets (
The value passed as argument to
deleteshall be either a pointer to a memory block previously allocated with
new, or a null pointer (in the case of a null pointer,
deleteproduces no effect).
How many numbers would you like to type? 5 Enter number : 75 Enter number : 436 Enter number : 1067 Enter number : 8 Enter number : 32 You have entered: 75, 436, 1067, 8, 32,
Notice how the value within brackets in the new statement is a variable value entered by the user (
i), not a constant expression:
There always exists the possibility that the user introduces a value for
iso big that the system cannot allocate enough memory for it. For example, when I tried to give a value of 1 billion to the "How many numbers" question, my system could not allocate that much memory for the program, and I got the text message we prepared for this case (
Error: memory could not be allocated).
It is considered good practice for programs to always be able to handle failures to allocate memory, either by checking the pointer value (if
nothrow) or by catching the proper exception.
Dynamic memory in CC++ integrates the operators
deletefor allocating dynamic memory. But these were not available in the C language; instead, it used a library solution, with the functions
free, defined in the header
<stdlib.h>in C). The functions are also available in C++ and can also be used to allocate and deallocate dynamic memory.
Note, though, that the memory blocks allocated by these functions are not necessarily compatible with those returned by
new, so they should not be mixed; each one should be handled with its own set of functions or operators.