What does “new int(100)” do? [duplicate]

Answer 1

It allocates one object of type int and initialized it to value 100.

A lot of people doesn't know that you can pass an initializer to new, there's a particular idiom that should be made more widely known so as to avoid using memset:

new int[100]();

This will allocate an array of int and zero-initialize its elements.

Also, you shouldn't be using an array version of new. Ever. There's std::vector for that purpose.

Answer 2

Wikipedia new(C++) quote:

int *p_scalar = new int(5); //allocates an integer, set to 5. (same syntax as constructors)
int *p_array = new int[5];  //allocates an array of 5 adjacent integers. (undefined values)

UPDATE

In the current Wikipedia article new and delete (C++) the example is removed.

Additionally here's the less intuitive but fully reliable C++ reference for new and new[].

Answer 3

The first one creates a single new integer, initializes it to the value 100 and returns a pointer to it.

In C/C++ there is no difference between a pointer to an array and a pointer to a single value (a pointer to an array is in fact just a pointer to its first element). So this is a valid way to create an array with one element.

Answer 4

The first line allocates a single int and initializes it to 100. Think of the int(100) as a constructor call.

Since this is a scalar allocation, trying to access arr[1] or to free the memory using delete[] would lead to undefined behaviour.