8.2. Constructor in System Verilog (new())
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Constructor is used to create the object of the class. This is the default function in every class with name “new()”.
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This is different from “new[]” operator. This is used to provide the size of Dynamic Array.
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A constructor’s primary role is to initialize the class properties. Once the object is created using “new()” and the variables are initialized, the class handle becomes a valid object reference that points to the allocated memory for that instance.
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If the explicit definition of the constructor is not created in the class, default definition is called which initializes the variables implicitly.
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Below is the example with implicit constructor call:
Verilog
In the above code, when “p1 = new()” is executed then variable of the Packet class gets the default value of int type i.e., “addr = 0”.
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Below is the example with explicit constructor call:
Verilog
In the above code, when “p1 = new()” is executed then variable of the Packet class gets the explicit value 32’b0 as mentioned in the new() function i.e., “addr = 0”.
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Below is the example with explicit constructor call with Arguments:
Verilog
In the above code, when “p1 = new()” is executed with the argument (value = 32’hDEADDEED) then variable of the Packet class gets the argument value 32’hDEADDEED as mentioned in the new() function i.e., “addr = def_val”.​
Industry standard example:
Verilog
The important Interview question comes from constructor:
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Can a constructor be a virtual method in SystemVerilog?
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Direct answer is “NO”. In SystemVerilog, a constructor (new()) cannot be declared as “virtual”. This is because a constructor can not be overridden polymorphically, and SystemVerilog does not allow dynamic dispatch for new().
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First, let’s understand what “virtual” means.
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virtual = “runtime decision”
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virtual method = “A virtual method allows the simulator to decide at runtime whether to call the base or derived implementation based on the actual object type.”
Verilog
A virtual method requires a valid object reference to perform runtime dispatch.
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But a constructor’s purpose is to create the object itself.
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So the sequence is:
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1.new() is called
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2.object memory is allocated and initialized
3.handle becomes a valid object reference
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4.virtual method dispatch can happen
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If new() were virtual, the simulator would need an already-created object to decide which constructor to call — which is not possible.
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Therefore, virtual constructors are illegal in SystemVerilog, and the simulator reports a compile-time error.