Programming in D page 295 - disable default constructor

[Brother Bill]

When adding an explicit constructor such as ```this(string 
fileName)```, this is "supposed" to remove the "default" 
constructor.  Then the developer needs to add back the default 
constructor.

But in this example, variable ```a4``` constructs just fine, and 
one can't add a default constructor, which is commented out.

There is a way to force the issue, by adding ```@disable 
this();```, but that seems like brute force.

Why does the default parameterless constructor still exist after 
creating an explicit constructor ```this(string fileName)```?

Does this have something to do with the difference between 
structs and classes?
That is, that structs "always" have a default parameterless 
constructor, unless ```@disable(this);``` is added.

source/app.d  -- It compiles and runs.
```
void main() {
	auto a1 = Archive("records");
	// auto a2 = Archive("");
	// auto a3 = Archive(null);
	auto a4 = Archive();
}

struct Archive {
	string fileName;

	// @disable this();  // Disable the default constructor
	// This won't compile
	// this()
	// {
	// 	this.fileName = "default_file_name";
	// }

	this(string fileName)
	in (fileName.length > 0)
	{
		this.fileName = fileName;
	}
}

```