Does `new` in C Initialize Memory to Zero?

Operator new Initializes Memory to Zero

In C , the new operator is commonly used for dynamic memory allocation. It allocates a block of memory of the specified size and returns a pointer to the allocated memory. However, there is a common misconception that new does not initialize the allocated memory with zeroes. This article clarifies how new initializes memory, addressing the apparent discrepancies.

Default and Zero Initialization

When using new to create an object of a primitive type (e.g., int, unsigned int), there are two variations:

• wsk = new unsigned int; - Default initialization: No initialization is performed, and the memory remains uninitialized.
• wsk = new unsigned int(); - Zero initialization: The memory is set to zero.

The same applies to arrays:

• wsa = new unsigned int[5]; - Default initialization
• wsa = new unsigned int[5](); - Zero initialization

Zero Initialization Clarification

While new generally initializes primitives with indeterminate values, the exception arises when using zero initialization with the empty parentheses syntax (new type();). This initializes the memory with zeros. The C 11 standard supports this behavior:

[C++11: 5.3.4/15]: A new-expression that creates an object of type T initializes that object as follows: [...] If the new-initializer is interpreted according to the initialization rules of 8.5 for direct-initialization.

Placement New

Placement new allows allocating memory in a specific location. Utilizing this technique with zero initialization, it becomes evident that the memory is indeed set to zero:

//... (memory allocation and initialization code)
wsa[0] = 1;  // Modify the memory location
wsa[1] = 2;  // Modify the memory location
// ... (more modifications)
//...

If the memory contained the original values (1, 2, ...), then zero initialization would have failed. However, the output confirms that the allocated memory has been set to zero.

Therefore, while new typically does not initialize primitive memory with zeroes, using zero initialization (new type();) initializes the memory to zero. This holds true for both direct memory allocation and placement new.

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