protected类属性或方法具有local成员的所有特征，除此之外的是，protected类属性或方法对扩展类是可见的。
也就是说，protected类属性或方法可以被类自身和扩展类访问，依然不允许在类的外部访问。
声明一个protected类属性或方法，需要通过关键字“protected”。
class packet;
protected int addr;
protected function void disp(input int data);
$display(“From packet”);
$display(” data = %h”, data);
$display(” addr = %h”, addr);
endfunction
endclass
class eth_packet extends packet;
function set_addr;
addr = ‘hf;
//protected property ‘addr’ visible to extended class
endfunction
function void eth_disp;
super.disp(‘hf);
//protected method ‘disp’ visible to extended class
endfunction
function void disp(input int data);
//OK to override ‘protected’ method in extended class
$display(“From eth_packet”);
$display(” data = %h”, data);
endfunction
endclass
module class_TOP( );
initial begin
packet p1;
eth_packet e1;
e1 = new( );
p1 = new( );
//p1.disp(20); //COMPILE ERROR
//protected methods ‘disp’ not visible outside the class
e1.eth_disp;
e1.disp(‘hffff);
end
endmodule
仿真log：
From packet
data = 000000ff
addr = 00000000
From eth_packet
data = 0000ffff
V C S S i m u l a t i o n R e p o r t
在上面的例子中，我们声明了一个类“packet”，其中包含protected成员“addr”和一个protected方法“disp” 。
在扩展类“eth_packet”中，我们直接访问成员“addr”以及调用父类的“disp”方法都不会发生编译错误。
在module中直接调用“p1.disp(20);”就会发生编译错误：
Error-[SV-ICMA] Illegal class method access
testbench.sv, 39
Protected method ‘disp’ of class ‘packet’ is not visible to scope
‘class_TOP’.
Please make sure that the above method is called only from its own class
properties as it is declared as protected.
审核编辑：汤梓红