How to reserve memory for a slice in a struct

[Namal]

Hello,

I am new to D. According to that power of 2 rule I want to
reserve 2 sized chunks to my array, But how do I do that in a
struct? Say:

struct Stack(int){

int a[];


}


Where do I put that a.reserve(2);

Because right after the declaration of a I get an error

Error: unexpected ( in declarator
Error: basic type expected, not 2
Error: found '2' when expecting ')'
Error: no identifier for declarator a.reserve(int)
Error: semicolon expected following function declaration
Error: Declaration expected, not ')'