How does this "size of array" template function work?

Question

Can someone explain how this code works? I know that the purpose of this code is to get the length of an array, but I don't know how this code works:

template<typename T, int size>
int GetArrLength(T(&)[size]){return size;}

Answer 1

First let's dissect the parameter, T(&)[size]. Read declarations from inside out, right to left, parenthesis group first: It's an unnamed parameter that is a reference to an array of size size of type T.

That is, it accepts a reference to any array, where the type and size of the array are template parameters.

If we call it as such:

int a[10];
GetArrLength(a);

The compiler will try to deduce the template parameters. For the parameter type to match what you're passing, T must be int and size must be 10 (making the parameter a reference to an array of 10 ints).

You then return that size, giving you the number of elements in an array.

---

There are two "problems" with this code. Firstly, sizes cannot be negative, so it doesn't make sense to use a signed type as the template parameter and return type. Rather, an unsigned type should be used; best would be std::size_t:

template<typename T, std::size_t Size>
std::size_t GetArrLength(T(&)[Size]) { return size; }

The second is that the result of this function is not a constant-expression, even though an array's size is. While that's fine in most situations, it would be better if we could get a constant-expression from it. That's where you end up with this solution:

template <std::size_t N>
struct type_of_size
{
    typedef char type[N];
};

template <typename T, std::size_t Size>
typename type_of_size<Size>::type& sizeof_array_helper(T(&)[Size]);

#define sizeof_array(pArray) sizeof(sizeof_array_helper(pArray))

This is used as such:

int a[10];
const std::size_t n = sizeof_array(a); // constant-expression!

It works by three things: the first is the same idea as above, that template parameters will be filled out giving you the array's size.

The second part is using that information to make a type with a specific size, hence the type_of_size helper. That part isn't strictly necessary, but I think it makes the code easier to read. A char[N] has a size equal to N, always, hence we can abuse that to "store" the size of the array...in the size of a type itself!

The third part is getting that size with sizeof. It doesn't actually evaluate anything, so we don't need a definition for the function. It simply says "If you were to do this...the size would be...". And the size is our "stored" size, in the char array.