Array creation int *array = new int[sizeof(int)*n]

Question

Can you explain what is happening with this statement particularly inside the parentheses?

int *array = new int[sizeof(int) * n];

Answer 1

That is a complete C++ statement and parentheses () of sizeof operator contain the type int and sizeof(type) gives the size of the enclosing type in bytes:

int *array = new int[ sizeof(int) * n ];
                            ^^^^^

I believe you meant the expression inside square brackets [] of new expression ie sizeof(int) * n .

Semantically, the expression might be wrong.

If you allocate 10 integers with new operator then sizeof(int) is taken care of automatically. You just have to provide the number of elements of the type you're allocating for.

For example:

int  n = 10;                // n can be an integer entered by the user
int* a = new int[n];        // allocate an array of 10 integers

The array elements would be default-initialized in this case and the pointer to the first element of the array is returned.

And, when you do it like this (suppose you're on a 64-bit machine ie sizeof(int) is 8):

int  n = 10;
int  s = sizeof(int) * n;   // 8 x 10 = 80
int* a = new int[ s ];      // allocate an array of 80 integers

This is what I earlier referred to as semantically wrong because the intention might have been to allocate 10 integers, not 80.

The expression sizeof(int) * n is required for malloc() where you have to provide the exact number of bytes you want to allocate so you need to provide the exact number of elements of the type. The malloc() function does not initialize the allocated bytes and it returns the pointer to the first element also.

For example:

int  n = 10;
int  s = sizeof(int) * n;
int* a = (int*) malloc( s );

You have to initialize the memory allocated by malloc() . For more information on this, refer to the above links of the new and malloc() .

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Aside, you are allocating memory so you are responsible to release it when you're done. So, ideally, new and new[] would follow delete and delete[] respectively. And, malloc() would be followed by free() .

Refer to smart pointers eg std::unique_pointer and std::shared_ptr along with std::make_unique and std::make_shared for RAII -based automatic memory management.