In computers, all data and operations during runtime are stored in the memory of our computers, IoTs, or in other microdevices. This memory is generally RAM (Random Access Memory). RAM allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory.
In this post, you’ll learn how to use memory in C and C++, how to allocate something in memory and manage memory dynamically, what memory methods or functions are used for Dynamic Memory Management, how to use free(), and how to use calloc().
By learning how to use two types of memory allocations in programming: static memory allocation and dynamic memory allocation, as well as how to compile C++ in Windows. It will help you to easily build C++ applications.
Table of Contents
What does dynamic memory allocation mean?
Static Memory Allocation is a memory allocation method that is defined by variable definitions when programming. It has a fixed size and cannot be changed at run-time. These variables are items like constants, strings, pointers, arrays, and structures. When a program is compiled, the compiler allocates part of the memory to store data. This is called Static Memory Allocation or Compile-time Memory. There are limitations in such static memory allocation to use these kinds of variables.
What kind of limitations are there on C/C++ static memory allocations?
These allocations are done in memory exclusively allocated to a program. We can’t increase the size of static allocations to handle more new elements. Thus, this may result in declaring larger arrays than really required which means a waste of memory usage. If we used less data than expected, Static Memory Allocations don’t allow us to reduce array size to save memory. It is hard to create advanced dynamic data structures which can be deleted, reallocated variables, linked lists, trees, and other data, which are essential in most real-life programming situations.
What does dynamic memory allocation mean?
Dynamic Memory Allocation is a memory allocation method in which the memory is allocated during the execution of a program (at run-time). Dynamic Memory Management functions/methods involve the use of pointers and standard library functions. Sometimes we use pointers to point to the address of blocks of memory which is allocated dynamically. So we can easily access or operate on those dynamic memory allocations.
Note that malloc, calloc, realloc functions comes from C language included in the <alloc.h> and it can be used with C++ included in the <cstdlib> library. These functions might be very dangerous in Modern C++ thus using the “new” and “delete” operations are recommended for higher level memory management operations than malloc, calloc and realloc.
Here is the Comparison Table of new and delete methods with malloc and free methods in C++,
| Memory Management Feature | Using new and delete methods | Using malloc and free methods |
|---|---|---|
| Memory allocated from | free store | heap |
| Use of constructor / destructor | Yes | No |
| Returns | Fully typed pointer | void* pointer |
| On failure | never returns NULL, Throws | Returns NULL |
| Memory size required | Calculated by compiler | Must be specified in bytes |
| Handling arrays | Has an explicit version | Requires manual calculations |
| Reallocating | Not handled intuitively | Simple (no copy constructor) |
| Call of reverse | Implementation defined | No |
| Low memory cases | Can add a new memory allocator | Not handled by user code |
| Overridable | Yes | No |
The C++ language is a great programming language with its ancestor C programming language. C programming language has both Static Memory Allocation and Dynamic Memory Allocation methods. Most used Dynamic Memory Allocation functions are defined in header <stdlib.h> and <cstdlib> libraries, mostly we use malloc(), calloc(), realloc() and free().
| Function | Syntax | Description |
| malloc | void* malloc( size_t size ); | allocates a block of from memory heap and returns a pointer |
| calloc | void* calloc( size_t num, size_t size ); | allocates a block of from memory heap, initializes it to zero and returns a pointer |
| realloc | void* realloc( void *ptr, size_t new_size ); | re-allocates the size of the allocated memory block,, copies the contents to a new location |
| free | void free( void* ptr ); | Free block of memory blk allocated from memory heap |
Let’s see how we use calloc() and free() functions.
How do you use the calloc() function?
calloc() function is a Dynamic Memory Allocation function that allocates memory for an array of elements and initializes all bytes in the allocated storage to zero. calloc provides access to the C memory heap. The heap is available for dynamic allocation of variable-sized blocks of memory. Many data structures, such as trees and lists, naturally employ heap memory allocation. calloc allocates a block of size nitems * size. The block is initialized to 0. calloc returns a pointer to the newly allocated block. If not enough space exists for the new block or if nitems or size is 0, calloc returns NULL.
In the large data models, all the space beyond the program stack to the end of available memory is available for the heap.
Syntax:
For example we can allocate char array as below,
How do you use calloc() and free() functions?
free() function is a Dynamic Memory Allocation function that frees allocated block. free() deallocates a memory block allocated by a previous call to calloc, malloc, or realloc.
Syntax:
Here is a full C and C++ example about calloc() function
What about a full C++ example of using the calloc() function?
Note that calloc is slower than malloc because of time needed to fill memory allocation zero. Do not use if you don’t need to zero the allocation.
Where can I read more about C / C++ memory management?
You can click the following dynamic search link to view other articles on this blog about C and C++ memory management: https://learncplusplus.org/?s=memory+allocation
Did you know you can download a free trial of C+ Builder? Go to this link to try it out now.