std::calloc(3)

NAME
std::calloc - std::calloc

Synopsis
   Defined in header <cstdlib>
   void* calloc( std::size_t num, std::size_t size );
   Allocates memory for an array of num objects of size size, initializes it to all
   bits zero (implicitly creating objects in the destination area).
   If allocation succeeds, returns a pointer to the lowest (first) byte in the
   allocated memory block that is suitably aligned for any object type.
   If size is zero, the behavior is implementation defined (null pointer may be
   returned, or some non-null pointer may be returned that may not be used to access
   storage).
   The following functions are required to be thread-safe:
     * The library versions of operator new and operator delete
     * User replacement versions of global operator new and operator
       delete
     * std::calloc, std::malloc, std::realloc (since C++11)
       , std::aligned_alloc
       (since C++17), std::free
   Calls to these functions that allocate or deallocate a particular unit
   of storage occur in a single total order, and each such deallocation
   call happens-before the next allocation (if any) in this order.

Parameters
   num - number of objects
   size - size of each object

Return value
   On success, returns the pointer to the beginning of newly allocated memory. To avoid
   a memory leak, the returned pointer must be deallocated with std::free() or
   std::realloc().
   On failure, returns a null pointer.

Notes
   Due to the alignment requirements, the number of allocated bytes is not necessarily
   equal to num * size.
   Initialization to all bits zero does not guarantee that a floating-point or a
   pointer would be initialized to 0.0 and the null pointer value, respectively
   (although that is true on all common platforms).
   Originally (in C89), support for zero size was added to accommodate code such as
 OBJ *p = calloc(0, sizeof(OBJ)); // "zero-length" placeholder
 ...
 while (1)
 {
     p = realloc(p, c * sizeof(OBJ)); // reallocations until size settles
     ... // code that may change c or break out of loop
 }

Example
// Run this code
 #include <cstdlib>
 #include <iostream>
 int main()
 {
     int* p1 = (int*)std::calloc(4, sizeof(int)); // allocate and zero out an array of 4 int
     int* p2 = (int*)std::calloc(1, sizeof(int[4])); // same, naming the array type directly
     int* p3 = (int*)std::calloc(4, sizeof *p3); // same, without repeating the type name
     if (p2)
         for (int n = 0; n < 4; ++n) // print the array
             std::cout << "p2[" << n << "] == " << p2[n] << '\n';
     std::free(p1);
     std::free(p2);
     std::free(p3);
 }

Output:
 p2[0] == 0
 p2[1] == 0
 p2[2] == 0
 p2[3] == 0

See also
   C documentation for
   calloc