UVM1.1的例子中uart_config类在uart_tx_agent中为什么不需要实例化

下面是uart_config的类声明，
class uart_config extends uvm_object;
//UART topology parameters
uvm_active_passive_enumis_tx_active = UVM_ACTIVE;
uvm_active_passive_enumis_rx_active = UVM_PASSIVE;
// UART device parameters
rand bit [7:0]baud_rate_gen;// Baud Rate Generator Register
rand bit [7:0]baud_rate_div;// Baud Rate Divider Register
// Line Control Register Fields
rand bit [1:0]char_length; // Character length (ua_lcr[1:0])
rand bitnbstop;// Number stop bits (ua_lcr[2])
rand bitparity_en ;// Parity Enable(ua_lcr[3])
rand bit [1:0]parity_mode;// Parity Mode(ua_lcr[5:4])
int unsigned chrl;
int unsigned stop_bt;
// Control Register Fields
rand bitcts_en ;
rand bitrts_en ;
// Calculated in post_randomize() so not randomized
byte unsigned char_len_val;// (8, 7 or 6)
byte unsigned stop_bit_val;// (1, 1.5 or 2)
// These default constraints can be overriden
// Constrain configuration based on UVC/RTL capabilities
//constraint c_num_stop_bits{ nbstopinside {[0:1]};}
constraint c_bgen{ baud_rate_gen== 1;}
constraint c_brgr{ baud_rate_div== 0;}
constraint c_rts_en{ rts_en== 0;}
constraint c_cts_en{ cts_en== 0;}
// These declarations implement the create() and get_type_name()
// as well as enable automation of the tx_frame's fields
`uvm_object_utils_begin(uart_config)
`uvm_field_enum(uvm_active_passive_enum, is_tx_active, UVM_DEFAULT)
`uvm_field_enum(uvm_active_passive_enum, is_rx_active, UVM_DEFAULT)
`uvm_field_int(baud_rate_gen, UVM_DEFAULT + UVM_DEC)
`uvm_field_int(baud_rate_div, UVM_DEFAULT + UVM_DEC)
`uvm_field_int(char_length, UVM_DEFAULT)
`uvm_field_int(nbstop, UVM_DEFAULT )
`uvm_field_int(parity_en, UVM_DEFAULT)
`uvm_field_int(parity_mode, UVM_DEFAULT)
`uvm_object_utils_end
下面是uart_tx_agent中的声明句柄，但是没有实例化，
class uart_tx_agent extends uvm_agent;
// Active/Passive Agent configuration
//uvm_active_passive_enum is_active = UVM_ACTIVE;
// Configuration
uart_config cfg;
。
function void uart_tx_agent::build_phase(uvm_phase phase);
super.build_phase(phase);
// Configure
if (cfg == null) begin
if (！uvm_config_db#(uart_config)::get(this, "", "cfg", cfg))
`uvm_warning("NOCONFIG", "Config not set for Tx agent, using default is_active field")
end

求大神指点

顶一下

一般agent 的wrapper已经通过config_db set下去了，只要factory注册过还是什么automation过，在build-phase阶段直接就可以get到。这里只要一个handle，一get就相当于分配了实际空间了。

还是不太理解，我自己想想，谢谢啦

在env或base test等其它地方create了

这里的只是一个指向其它实例的指针，所有不需要实例化，跟c++的指针概念是一致的