• Jacob Carlborg (24/24) Sep 19 2009 Is it supposed to possible to mixin a constructor? The code below
• dsimcha (16/37) Sep 19 2009 I'm not sure exactly why this example doesn't work, but it looks like a ...
• Christopher Wright (19/23) Sep 19 2009 A template mixin introduces a new scope. This is pretty annoying in some...
• Jacob Carlborg (3/26) Sep 19 2009 The problem only seems to be when I'm having constructors both in the
• BCS (4/25) Sep 19 2009 IIRC mixins can't overload with other mixins or non mixins so if you wer...
• language_fan (4/21) Sep 19 2009 Since the constructor has no meaning outside classes, should it be
• Michel Fortin (8/12) Sep 20 2009 Personally, I'd like it very much if functions from template mixins
• Christopher Wright (3/13) Sep 20 2009 Also if you could implement a function from an interface with a template...
• Ellery Newcomer (4/8) Sep 20 2009 Checking whether a constructor is inside a class happens during one of

Jacob Carlborg <doob me.com> writes:

Is it supposed to possible to mixin a constructor? The code below 
doesn't compile. The error: is "main.d(23): Error: constructor 
main.A.this() does not match parameter types (int)
main.d(23): Error: expected 0 arguments, not 1"

template C ()
{
	this (int i)
	{
		
	}
}

class A
{
	mixin C;
	
	this ()
	{
		
	}
}

void main ()
{
	auto a = new A(3);
}

dsimcha <dsimcha yahoo.com> writes:

== Quote from Jacob Carlborg (doob me.com)'s article
 Is it supposed to possible to mixin a constructor? The code below
 doesn't compile. The error: is "main.d(23): Error: constructor
 main.A.this() does not match parameter types (int)
 main.d(23): Error: expected 0 arguments, not 1"
 template C ()
 {
 	this (int i)
 	{
 	}
 }
 class A
 {
 	mixin C;
 	this ()
 	{
 	}
 }
 void main ()
 {
 	auto a = new A(3);
 }

I'm not sure exactly why this example doesn't work, but it looks like a bug.
Please file.  If you just want to insert a bunch of boilerplate without any
parameters, you probably should try a string mixin instead.  The following does
work:

enum string C = q{
    this (int i) {

    }
};

class A {
    mixin(C);

    this () {

    }
}

void main () {
    auto a = new A(3);
}

Christopher Wright <dhasenan gmail.com> writes:

Jacob Carlborg wrote:
 Is it supposed to possible to mixin a constructor? The code below 
 doesn't compile. The error: is "main.d(23): Error: constructor 
 main.A.this() does not match parameter types (int)
 main.d(23): Error: expected 0 arguments, not 1"

A template mixin introduces a new scope. This is pretty annoying in some 
cases. More often, it is convenient. Since the constructor is not in the 
same scope as the class, it might be causing problems.

If you didn't introduce a new scope by default, it'd be easy enough to 
add it in the template or around the mixin:

template MixMeIn()
{
    int imInUrScope;
    {
       int imOuttaUrScope;
    }
}


Additionally, the difference would be easily detectable, because symbol 
collision would cause the code to fail to compile. Unless the mixin adds 
a function overload and you pass the address of the overload set 
somewhere, and now you're passing the wrong overload...which is an old 
problem for D, aggravated by D's property syntax, and unlikely to be 
fixed soon.

Jacob Carlborg <doob me.com> writes:

On 9/19/09 20:55, Christopher Wright wrote:
 Jacob Carlborg wrote:
 Is it supposed to possible to mixin a constructor? The code below
 doesn't compile. The error: is "main.d(23): Error: constructor
 main.A.this() does not match parameter types (int)
 main.d(23): Error: expected 0 arguments, not 1"

 A template mixin introduces a new scope. This is pretty annoying in some
 cases. More often, it is convenient. Since the constructor is not in the
 same scope as the class, it might be causing problems.

The problem only seems to be when I'm having constructors both in the 
template and in the class.

 If you didn't introduce a new scope by default, it'd be easy enough to
 add it in the template or around the mixin:

 template MixMeIn()
 {
 int imInUrScope;
 {
 int imOuttaUrScope;
 }
 }


 Additionally, the difference would be easily detectable, because symbol
 collision would cause the code to fail to compile. Unless the mixin adds
 a function overload and you pass the address of the overload set
 somewhere, and now you're passing the wrong overload...which is an old
 problem for D, aggravated by D's property syntax, and unlikely to be
 fixed soon.

BCS <none anon.com> writes:

Hello Jacob,

 Is it supposed to possible to mixin a constructor? The code below
 doesn't compile. The error: is "main.d(23): Error: constructor
 main.A.this() does not match parameter types (int)
 main.d(23): Error: expected 0 arguments, not 1"

IIRC mixins can't overload with other mixins or non mixins so if you were 
to drop the this() that should work. OTOH I'd be surprised if you can do 
that without breaking something else in the general case.

 template C ()
 {
 this (int i)
 {
 }
 }
 class A
 {
 mixin C;
 this ()
 {
 }
 }
 void main ()
 {
 auto a = new A(3);
 }

language_fan <foo bar.com.invalid> writes:

 template C ()
 {
 this (int i)
 {
 }
 }
 class A
 {
 mixin C;
 this ()
 {
 }
 }
 void main ()
 {
 auto a = new A(3);
 }


Since the constructor has no meaning outside classes, should it be 
interpreted as a free function if mixed in a non-class context? I really 
wonder how this could be valid code. Does the grammar even support the 
3rd line?

Michel Fortin <michel.fortin michelf.com> writes:

On 2009-09-19 21:17:36 -0400, language_fan <foo bar.com.invalid> said:

 Since the constructor has no meaning outside classes, should it be
 interpreted as a free function if mixed in a non-class context? I really
 wonder how this could be valid code. Does the grammar even support the
 3rd line?

Personally, I'd like it very much if functions from template mixins 
could overload with functions from outside the mixin. It'd allow me to 
replace string mixins with template mixins in quite a few places.

-- 
Michel Fortin
michel.fortin michelf.com
http://michelf.com/

Christopher Wright <dhasenan gmail.com> writes:

Michel Fortin wrote:
 On 2009-09-19 21:17:36 -0400, language_fan <foo bar.com.invalid> said:
 
 Since the constructor has no meaning outside classes, should it be
 interpreted as a free function if mixed in a non-class context? I really
 wonder how this could be valid code. Does the grammar even support the
 3rd line?

 
 Personally, I'd like it very much if functions from template mixins 
 could overload with functions from outside the mixin. It'd allow me to 
 replace string mixins with template mixins in quite a few places.

Also if you could implement a function from an interface with a template 
mixin.

Ellery Newcomer <ellery-newcomer utulsa.edu> writes:

language_fan wrote:
 Since the constructor has no meaning outside classes, should it be 
 interpreted as a free function if mixed in a non-class context? I really 
 wonder how this could be valid code. Does the grammar even support the 
 3rd line?

Checking whether a constructor is inside a class happens during one of
the semantic passes. The parser makes no distinction between
class/interface/template/struct/union bodies.