Can someone explain this template code that gives me the size of an array?

template<typename T, size_t n>
size_t array_size(const T (&)[n])
{
    return n;
}

int a[] = {1, 2, 3};
int *ap = a; // a pointer, size is lost
int (&ar)[3] = a; // a reference to the array, size is not lost

template<typename T, size_t n>
size_t array_size(const T (&)[n]) {
    return n;
}

size_t array_size(const int (&)[3]) {
    return 3;
}

size_t size_of_a = array_size(a);

template<typename T, size_t n>
char (& array_size(const T (&)[n]) )[n];

size_t size_of_a = sizeof(array_size(a));

int havingSameSize[sizeof(array_size(a))];

// Note that you don't need to name the array, since you don't
// actually reference the parameter at all.
size_t array_size(const int (&)[1])
{
    return 1;
}

size_t array_size(const int (&)[2])
{
    return 2;
}

size_t array_size(const int (&)[3])
{
    return 3;
}
// etc...

int a[2];
array_size(a);

template <int n>
size_t array_size(const int (&)[n])
{
    return n;
}

template <typename T, int n>
size_t array_size(const T (&)[n])
{
    return n;
}

const int &a[1];

const int (&a)[1];

size_t array_size(const int (&)[1])

char a[10];
array_size(a);    // returns 10