Mysterious oneliner template code, any one ?

Question

I was reading this page : C++ Tip: How To Get Array Length. The writer presented a piece of code to know the size of static arrays.

template<typename T, int size>
int GetArrLength(T(&)[size]){return size;} // what does '(&)' mean ?
.
.
.
int arr[17];
int arrSize = GetArrLength(arr); // arrSize = 17

Could anyone please shed the light on this code, because I couldn't understand how it really works.

Answer 1

The function is passed a reference (&) to an array of type T, and size size.

Answer 2

Wow, that's tricky. I don't know either, but if you keep reading down into the comments on that page:

is essentially

int arr[17]; int arrSize = GetArrLength(arr);

which creates this function:

int GetArrLength(int(&)[17]){return 17;}

So & must mean reference like it always does, so it's taking a reference of the array type and the size (second item in the template) is then the size of the incoming array.

Think I'll stick with old

sizeof(x)/sizeof(x[0])

Answer 3

sizeof(x)/sizeof(x[0])

Won't catch errors if the array degrads to a pointer type, but it will still compile!

Templated version is bullet proof.

Answer 4

T(&)[size] is reference to T[size]. If you don't use reference C++ will treat T[size] as T*, and function template parameter deduction would not work.