Struct Union behavior

[Voitech via Digitalmars-d-learn]

Hello, i am new to D language and trying to learn it by coding.
I compile my programs on Xubuntu 14.04 with DMD64 D Compiler 
v2.069.2.
So i have a struct/union which contains two fields representing 
real and string values:

public union Element
{
	private real _value;
	private string _rawValue;

	@property{
		real value(){
			return _value;
		}
		void value(real value){
			_value=value;
		}

	}
	@property {
		ref string rawValue(){
			return _rawValue;
		}
		void rawValue(ref string value){
			_rawValue=value;
		}

	}

	
	public Element opBinary(string op)(Element other){
		Element newElement;
		real value=mixin("this.value "~op~"other.value");
		newElement.value=value;
		return newElement;
	}

	public bool opEquals(Element other){
		if(this.rawValue == null && other.rawValue == null){
			return this.opEquals(other.rawValue);
		}
		return this.opEquals(other.value);
	}
	public bool opEquals(real value){
		return this.value==value;
	}
	public bool opEquals(string rawValue){
		return this.rawValue==rawValue;
	}

	unittest{
		Element e1 = {4},e2 ={5};
		writeln("e1 is at address: ",&e1);
		writeln("e2 is at address: ",&e2);
		writeln("e1.value is at address: ",&e1._value);
		writeln("e2.value is at address: ",&e2._value);
		assert(e1+e2==9);
	}

	unittest{
		Element s1={_rawValue:"+"},s2,s3,s4;
		writeln("s1 is at address: ",&s1);
		writeln("s2 is at address: ",&s2);
		writeln("s3 is at address: ",&s3);
		writeln("s1.value is at address: ",&s1._value);
		writeln("s2.value is at address: ",&s2._value);
		writeln("s3.value is at address: ",&s3._value);
		writeln("s4.value is at address: ",&s4._value);
		writeln("s1.rawValue ",s1._rawValue);
		writeln("s1.value: ",s1._value);
		writeln("s2.value: ",s2._value);
		writeln("s3.value: ",s3._value);
		writeln("s4.value: ",s4._value );
		assert(s1!=s2);
	}
}

The unit test results are:

e1 is at address: 7FFCEFBA7510
e2 is at address: 7FFCEFBA7520
e1.value is at address: 7FFCEFBA7510
e2.value is at address: 7FFCEFBA7520
s1 is at address: 7FFCEFBA7500
s2 is at address: 7FFCEFBA7510
s3 is at address: 7FFCEFBA7520
s1.value is at address: 7FFCEFBA7500
s2.value is at address: 7FFCEFBA7510
s3.value is at address: 7FFCEFBA7520
s4.value is at address: 7FFCEFBA7530
s1.rawValue +
s1.value: 0.125
s2.value: 4
s3.value: 5
s4.value: 9

Can anyone tell me why s2,s3,s4 have initialized _value field?  
Shouldn't it be collected by GC when first unit test (with e1,e2) 
finishes ? If not how to handle this behavior, to use union 
without preinitialized fields.