I've come came across this snippet in the article PVS-Studio vs Chromium
template <typename T, size_t N>
char (&ArraySizeHelper(T (&array)[N]))[N];
#define arraysize(array) (sizeof(ArraySizeHelper(array)))
I've seen other templates to do the same thing, like Use templates to get an array's size and end address.
I understand those, but I've been having difficulty with this one.
The function template is named ArraySizeHelper, for a function that takes one argument, a reference to a T [N], and returns a reference to a char [N].
The macro passes your object (let's say it's X obj[M]) as the argument. The compiler infers that T == X and N == M. So it declares a function with a return type of char (&)[M]. The macro then wraps this return value with sizeof, so it's really doing sizeof(char [M]), which is M.
If you give it a non-array type (e.g. a T *), then the template parameter inference will fail.
As @Alf points out below, the advantage of this hybrid template-macro system over the alternative template-only approach is that this gives you a compile-time constant.