Wanted to declare and initialise a static variable in a .m file.

Objective-c compiler does not like it, and raise this error message:

Initializer element is not a compile-time constant

I like the explanation here on Stackoverflow:

Objective-C is a strict superset of C. It’s illegal in C to have any executable code outside of a function (or method in Objective-C).

Leading from this answer I discovered that I should use dispatch_once for initialisation