How to zero a dynamic array?

Note that I'm learning on Visual C++.

As I know, we should zero a dynamic array this way:

void main()
{
    int *arr = new int[15];
    memset(arr,0,15*sizeof(*arr));
    ...
    delete [] arr;
}

But I found another way to do the same:

void main()
{
    int *a = new int[15]();
    ...
    delete [] arr;
}

As you can see, I just added two parentheses after the brackets. But it works only if I set the size of the array.

I can't find any information on this, and I don't know what purposes this is needed for.

EDIT: First code is edited and verified by VC compiler now. EDIT2: So I can't find manual about 2nd code on MSDN. There I can find this one? (No Google plz)

Solution

I can't find any information on this, and I don't know what purposes this is needed for.

This is initialization code, introduced in the language (if I remember correctly) to allow for native types to be treated like generic types, when writing templates and classes.

Here's an example:

struct S { int x; S(int a) : x(a) {} }; // structure with constructor taking an int
                                        // observe constructor syntax for int member

// example factory function (not very usefull in practice but good example)
template<typename T>
T make_instance( int x ) { return T(x); /* use old syntax for constructor call */ }

auto s = make_instance<S>(10); // create S instance with value 10
auto i = make_instance<int>(10); // create integer with value 10

In this example, the template constructs an instance of any type that supports a constructor taking an int. The second example (initializing an int) will only work if the language supports initializing an integer with the following syntax:

int x(10);

This is constructor syntax. The language also supports calling the "default constructor" for a native value (i.e. calling the constructor with no arguments, which for an int sets the value to zero). This is the case in your example (calling default implicit int "constructor").