About immutable and const constructors

September 25, 2012
Posted by FreeSlave
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FreeSlave
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Hello to all. I want to discuss immutable and const constructors. For a start let's write the simple class with two constructors:

class A
{
    int x;
    this() immutable
    {
        x = 42;
    }
    this()
    {
        x = 13;
    }
}

Well, dmd passes this code, but what if we want to create some object of class A?

void main()
{
    A a = new A;
}

hello.d|177|Error: constructor hello.A.this called with argument types:|
hello.d|177|Error: cannot implicitly convert expression (new A) of type immutable(A) to hello.A|

Class A has mutable constructor, but for some reason immutable one is called and then immutable object can not be converted to type A, which is mutable.

Let's rewrite this line to this:

immutable A a = new A;

hello.d|173|Error: constructor hello.A.this called with argument types:|

So what's the problem? Yes, both A's constructors really have empty argument lists, but... ah. Let's try another way:

immutable A a = new immutable A;

hello.d|173|Error: found 'A' when expecting '('|
hello.d|173|Error: basic type expected, not ;|
hello.d|173|Error: found ';' when expecting ')'|
hello.d|174|Error: semicolon expected, not '}'|

Again not. Looks like it's not correct statement at all. So I conclude there is no way to keep simultaneously both mutable and immutable constructors with same argument list. But as we saw above, compiler passes code if we don't create objects of class A, although such permission has no sense.

Well, now let's rewrite our class using const specifier instead of immutable

class A
{
    int x;
    this() const
    {
        x = 42;
    }
    this()
    {
        x = 13;
    }
}

void main()
{
    const A a = new A;
    assert(a.x == 42);
}

Project is built without errors or warning, but we get assertion failure in runtime. Error seems obvious: new A is constructed like mutable and only then assigned to object of class const A. Maybe the following code will be correct?

const A a = new const A;

hello.d|173|Error: found 'A' when expecting '('|
hello.d|173|Error: basic type expected, not ;|
hello.d|173|Error: found ';' when expecting ')'|
hello.d|174|Error: semicolon expected, not 'assert'|

But we get same errors as with immutable specifier.

I used dmd 2.060 and Code::Blocks on Debian GNU/Linux.
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