The scope resolution operator :: is used to refer an identifier within the scope of a class.
Left hand side of the scope resolution operator :: should be a class type name, package name, covergroup type name, coverpoint or cross name, typedef name. The right hand side of the operator should be an identifier like a variable or method name.
Why is scope resolution operator required ?
Classes and other scopes can have same identifier names and may create a namespace collision if referred to without specifying the scope. The scope resolution operator :: uniquely identifies a member or parameter of a given class.
They are also used to access static variables and methods, parameters and local parameters of a class from outside the class. It also allows access to public and protected members of a base class from within the child class.
Examples
1. Defining extern function
class ABC;
int data;
extern virtual function void display();
endclass
// Definition of an external function using scope
// resolution operator
function void ABC::display();
$display("data = 0x%0h", data);
endfunction
module tb;
initial begin
ABC abc = new();
abc.data = 32'hface_cafe;
abc.display();
end
endmodule
ncsim> run data = 0xfacecafe ncsim: *W,RNQUIE: Simulation is complete. ncsim> exit
2. Accessing static methods and functions
class ABC;
static int data;
static function void display();
$display("data = 0x%0h", data);
endfunction
endclass
module tb;
initial begin
ABC a1, a2;
// Assign to static variable before creating
// class objects, and display using class_type and
// scope resolution operator
ABC::data = 32'hface_cafe;
ABC::display();
a1 = new();
a2 = new();
$display ("a1.data=0x%0h a2.data=0x%0h", a1.data, a2.data);
end
endmodule
ncsim> run data = 0xfacecafe a1.data=0xfacecafe a2.data=0xfacecafe ncsim: *W,RNQUIE: Simulation is complete. ncsim> exit
3. Using package
package my_pkg;
typedef enum bit {FALSE, TRUE} e_bool;
endpackage
module tb;
bit val;
initial begin
// Refer to types that have been declared
// in a package. Note that package has to
// be included in compilation but not
// necessarily "imported"
val = my_pkg::TRUE;
$display("val = 0x%0h", val);
end
endmodule
ncsim> run val = 0x1 ncsim: *W,RNQUIE: Simulation is complete. ncsim> exit
4. Avoid namespace collision
package my_pkg;
typedef enum bit {FALSE, TRUE} e_bool;
endpackage
import my_pkg::*;
module tb;
typedef enum bit {TRUE, FALSE} e_bool;
initial begin
e_bool val;
// Be explicit and say that TRUE from my_pkg
// should be assigned to val
val = my_pkg::TRUE;
$display("val = 0x%0h", val);
// TRUE from current scope will be assigned to
// val
val = TRUE;
$display("val = 0x%0h", val);
end
endmodule
ncsim> run val = 0x1 val = 0x0 ncsim: *W,RNQUIE: Simulation is complete. ncsim> exit
