Join our newsletter for the latest updates.

C++ malloc()

In this tutorial, we will learn about the C++ malloc() function with the help of examples.

The malloc() function in C++ allocates a block of uninitialized memory to a pointer. It is defined in the cstdlib header file.

Example

#include <iostream>
#include <cstdlib>
using namespace std;

int main() {

  // allocate memory of int size to an int pointer
  int* ptr = (int*) malloc(sizeof(int));

  // assign the value 5 to allocated memory
  *ptr = 5;

  cout << *ptr;

  return 0;
}

// Output: 5

malloc() Syntax

The syntax of malloc() is:

malloc(size_t size);

Here, size is the size of the memory (in bytes) that we want to allocate.


malloc() Parameters

The malloc() function takes the following parameter:

  • size - an unsigned integral value (casted to size_t) which represents the memory block in bytes

malloc() Return Value

The malloc() function returns:

  • a void pointer to the uninitialized memory block allocated by the function
  • null pointer if allocation fails

Note: If the size is zero, the value returned depends on the implementation of the library. It may or may not be a null pointer.


malloc() Prototype

The prototype of malloc() as defined in the cstdlib header file is:

void* malloc(size_t size);

Since the return type is void*, we can type cast it to most other primitive types without issues.


Example 1: C++ malloc()

#include <iostream>
#include <cstdlib>
using namespace std;

int main() {

// allocate 5 int memory blocks int* ptr = (int*) malloc(5 * sizeof(int));
// check if memory has been allocated successfully if (!ptr) { cout << "Memory Allocation Failed"; exit(1); } cout << "Initializing values..." << endl << endl; for (int i = 0; i < 5; i++) { ptr[i] = i * 2 + 1; } cout << "Initialized values" << endl; // print the values in allocated memories for (int i = 0; i < 5; i++) { // ptr[i] and *(ptr+i) can be used interchangeably cout << *(ptr + i) << endl; } // deallocate memory free(ptr); return 0; }

Output

Initializing values...

Initialized values
1
3
5
7
9

Here, we have used malloc() to allocate 5 blocks of int memory to the ptr pointer. Thus, ptr now acts as an array.

int* ptr = (int*) malloc(5 * sizeof(int));

Notice that we have type casted the void pointer returned by malloc() to int*.

We then check if the allocation was successful or not using an if statement. If it was not successful, we exit the program.

if (!ptr) {
  cout << "Memory Allocation Failed";
  exit(1);
}

Then, we have used a for loop to initialize the allocated memory blocks with integer values, and another for loop to print those values.

Finally, we have deallocated the memory using the free() function.

free(ptr);

Example 2: C++ malloc() with Size Zero

#include <iostream>
#include <cstdlib>
using namespace std;

int main() {

// allocate memory of size 0 int *ptr = (int*) malloc(0);
if (ptr==NULL) { cout << "Null pointer"; } else { cout << "Address = " << ptr; } // deallocate memory free(ptr); return 0; }

Output

Address = 0x371530

Here, we have used malloc() to allocate int memory of size 0 to the ptr pointer.

int* ptr = (int*) malloc(0);

Using an if statement, we then check whether malloc() returned a null pointer or if it returned an address.

Finally, we deallocated the memory using free().

Did you find this article helpful?