c++ - How does this Array Size Template Work?

Question

I came across this snippet

template <typename T, size_t N>  
char (&ArraySizeHelper(T (&array)[N]))[N];  
#define arraysize(array) (sizeof(ArraySizeHelper(array))) 

in this article http://software.intel.com/en-us/articles/pvs-studio-vs-chromium/

I've seen other templates to do the same thing, like this one

Use templates to get an array's size and end address

and I understand those, but I've been having difficulty with this one.

Any help would be appreciated.

Answer 1

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.