Does `new unsigned int` Initialize Memory to Zero in C ?

Operator new initializes memory to zero

In the following code snippet:

#include <iostream>

int main(){
  unsigned int* wsk2 = new unsigned int(5);
  std::cout << "wsk2: " << wsk2 << " " << *wsk2 << std::endl;
  delete wsk2;
  wsk2 = new unsigned int;
  std::cout << "wsk2: " << wsk2 << " " << *wsk2 << std::endl;
  return 0;
}

The expected result is that the memory is not initialized to zero, but the output is:

wsk2: 0x928e008 5
wsk2: 0x928e008 0

It seems that the operator new is initializing the memory to zero, but it is actually not.

How it works:

There are two versions of the operator new:

wsk = new unsigned int;      // default initialized (ie nothing happens)
wsk = new unsigned int();    // zero    initialized (ie set to 0)

The default initialization does not initialize the memory, while the zero initialization sets the memory to zero.

It also works for arrays:

wsa = new unsigned int[5];   // default initialized (ie nothing happens)
wsa = new unsigned int[5](); // zero    initialized (ie all elements set to 0)

To confirm that the memory is actually zeroed out, we can use placement new with a known piece of memory:

#include <new>
#include <iostream>


int main()
{
    unsigned int   wsa[5] = {1,2,3,4,5};

    // Use placement new (to use a know piece of memory).
    // In the way described above.
    // 
    unsigned int*    wsp = new (wsa) unsigned int[5]();

    std::cout << wsa[0] << "\n";   // If these are zero then it worked as described.
    std::cout << wsa[1] << "\n";   // If they contain the numbers 1 - 5 then it failed.
    std::cout << wsa[2] << "\n";
    std::cout << wsa[3] << "\n";
    std::cout << wsa[4] << "\n";
}

The output of this code is:

0
0
0
0
0

Which confirms that the memory is indeed zeroed out by the zero-initialization version of the operator new.

The above is the detailed content of Does `new unsigned int` Initialize Memory to Zero in C ?. For more information, please follow other related articles on the PHP Chinese website!