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How To Use The Realloc() Function In C++ Programs

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 a RAM (Random Access Memory) that 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 general, In programming, there are two kinds of memory allocations, Static Memory Allocation, and Dynamic Memory Allocation.

In this post, you’ll learn how to use memory in C and C++, how to reallocate something in memory, how to manage memory dynamically, what memory methods or functions are used for Dynamic Memory Management, and how to use malloc(). How do I use free()? , and how do I use realloc()? By learning how to use the realloc() Function in C++, it will help you to easily build C++ applications using the C++ IDE.

What is static memory allocation in a C++ program?

Static Memory Allocation is a memory allocation method that is defined by variable definitions when programming and it has a fixed size, can not be changed during the run-time. These variables are defined variables in our programs 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. Because:

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 to declare larger arrays than 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 new and delete operations are higher level memory management operations than these ones.

Here is the Comparison Table of new and delete methods with malloc and free methods in C++,

Memory Management FeatureUsing new and delete methodsUsing malloc and free methods
Memory allocated fromfree storeheap
Use of constructor / destructorYesNo
ReturnsFully typed pointervoid* pointer
On failurenever returns NULL, ThrowsReturns NULL
Memory size requiredCalculated by compilerMust be specified in bytes
Handling arraysHas an explicit versionRequires manual calculations
ReallocatingNot handled intuitivelySimple (no copy constructor)
Call of reverseImplementation definedNo
Low memory casesCan add a new memory allocatorNot handled by user code

What are the key memory allocation functions in C and C++ programs?

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 mostly we use malloc(), calloc(), realloc() and free().

FunctionSyntax Description
mallocvoid* malloc( size_t size );allocates a block of from memory heap and returns a pointer
callocvoid* calloc( size_t num, size_t size );allocates a block of from memory heap, initializes it to zero and returns a pointer
reallocvoid *realloc( void *ptr, size_t new_size );re-allocates the size of the allocated memory block,copies the contents to a new location
freevoid free( void* ptr );Free block of memory blk allocated from memory heap

Let’s see how we use realloc() and free() functions.

Learn to use realloc() function

realloc() function is a Dynamic Memory Allocation function that reallocates main memory. realloc attempts to shrink or expand the previously allocated block to size bytes. If size is zero, the memory block is freed and NULL is returned. The block argument points to a memory block previously obtained by calling malloc, calloc, or realloc. If block is a NULL pointer, realloc works just like malloc.

realloc function adjusts the size of the allocated block to size, copying the contents to a new location if necessary.


Learn to use malloc(), realloc() and free() functions together

The free() function is a Dynamic Memory Allocation function that frees allocated block. free() deallocates a memory block allocated by a previous call to callocmalloc, or realloc.


Here is an example of how to use the C malloc(), realloc() and free() functions

Here is a full C++ example of using the C++ realloc() function

As you see dynamically you can extend the size of allocated memory dynamically by using reallocate() function in C and C++ programming languages.

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About author

Dr. Yilmaz Yoru has 35+ years of coding with more than 30+ programming languages, mostly C++ on Windows, Android, Mac-OS, iOS, Linux, and some other operating systems. He graduated and received his MSc and PhD degrees from the Department of Mechanical Engineering of Eskisehir Osmangazi University. He is the founder and CEO of ESENJA LLC Company. His interests are Programming, Thermodynamics, Fluid Mechanics, Artificial Intelligence, 2D & 3D Designs, and high-end innovations.
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