What does "new int(100)" do?

Possible Duplicate:
is this a variable or function

I mistakenly used something like:

int *arr = new int(100);

and it passes compile, but I knew this is wrong. It should be

int *arr = new int[100];

What does the compiler think it is when I wrote the wrong one?

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.

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[].

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.