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digitalmars.D.learn - Implicit conversion from array of class to array of interface

reply "Phil Deets" <pjdeets2 gmail.com> writes:
(D 2.033) I have a need to do something like this code:

interface I {}
class C : I {}
class D : I {}

void f(I[]) {}
void f(bool) {}

void g(T)(T param) {
	f(param);
}

int main()
{
	bool b;
	C[] c;
	D[] d;
	g(b);
	g(c);
	g(d);
	return 0;
}

This results in the output:

temp.d(9): Error: function temp.f (I[] _param_0) is not callable using  
argument types (C[])
temp.d(9): Error: cannot implicitly convert expression (param) of type C[]  
to bool
temp.d(18): Error: template instance temp.g!(C[]) error instantiating

As you can see, I can't cast C[] to I[] when I call f because T can be  
bool too. My workaround for now is:

interface I {}
class C : I {}
class D : I {}

void f(I[]) {}
void f(C[] param) {
	f(cast(I[])param);
}
void f(D[] param) {
	f(cast(I[])param);
}
void f(bool) {}

void g(T)(T param) {
	f(param);
}

int main()
{
	bool b;
	C[] c;
	D[] d;
	g(b);
	g(c);
	g(d);
	return 0;
}

Is there a better way to deal with this? Is this behavior a design bug?

Phil Deets

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Dec 13 2009
next sibling parent reply "Phil Deets" <pjdeets2 gmail.com> writes:
On Sun, 13 Dec 2009 03:22:31 -0500, Phil Deets <pjdeets2 gmail.com> wrote:


 (D 2.033) I have a need to do something like this code:

 interface I {}
 class C : I {}
 class D : I {}

 void f(I[]) {}
 void f(bool) {}

 void g(T)(T param) {
 	f(param);
 }

 int main()
 {
 	bool b;
 	C[] c;
 	D[] d;
 	g(b);
 	g(c);
 	g(d);
 	return 0;
 }

 This results in the output:

 temp.d(9): Error: function temp.f (I[] _param_0) is not callable using  
 argument types (C[])
 temp.d(9): Error: cannot implicitly convert expression (param) of type  
 C[] to bool
 temp.d(18): Error: template instance temp.g!(C[]) error instantiating

 As you can see, I can't cast C[] to I[] when I call f because T can be  
 bool too. My workaround for now is:

 interface I {}
 class C : I {}
 class D : I {}

 void f(I[]) {}
 void f(C[] param) {
 	f(cast(I[])param);
 }
 void f(D[] param) {
 	f(cast(I[])param);
 }
 void f(bool) {}

 void g(T)(T param) {
 	f(param);
 }

 int main()
 {
 	bool b;
 	C[] c;
 	D[] d;
 	g(b);
 	g(c);
 	g(d);
 	return 0;
 }

 Is there a better way to deal with this? Is this behavior a design bug?

 Phil Deets


I found another workaround which doesn't require a bunch of extra  
overloads. I'll probably update it to use that template someone wrote in  
that thread about static duck-typing.

//interface I {void h();}
class C /*: I*/ {void h() {}}
class D /*: I*/ {void h() {}}

void f(T)(T param)
{
	static if (__traits(compiles, param[0].h()))
	{
		// do I[] overload here
	}
	else
	{
		pragma(msg, "Unsupported type for f.")
		static assert(0);
	}
}
void f(T:bool)(T param) {}

void g(T)(T param) {
	f(param);
}

int main()
{
	bool b;
	C[] c;
	D[] d;
	g(b);
	g(c);
	g(d);
	return 0;
}

By the way, how do I move the bool specialization into the general  
function?

void f(T)(T param)
{
	static if (__traits(compiles, param[0].h()))
	{
		// do I[] overload here
	}
	else static if (T is bool) // how do I do this?
	{
	}
	else
	{
		pragma(msg, "Unsupported type for f.")
		static assert(0);
	}
}
Dec 13 2009
parent "Phil Deets" <pjdeets2 gmail.com> writes:
On Sun, 13 Dec 2009 04:08:19 -0500, Phil Deets <pjdeets2 gmail.com> wrote:


 I found another workaround which doesn't require a bunch of extra  
 overloads. I'll probably update it to use that template someone wrote in  
 that thread about static duck-typing.


I looked up this post. It was: "Re: static interface" by Simen kjaeraas at  
Thursday, November 19, 2009 6:47:08 AM.


 By the way, how do I move the bool specialization into the general  
 function?

 void f(T)(T param)
 {
 	static if (__traits(compiles, param[0].h()))
 	{
 		// do I[] overload here
 	}
 	else static if (T is bool) // how do I do this?
 	{
 	}
 	else
 	{
 		pragma(msg, "Unsupported type for f.")
 		static assert(0);
 	}
 }


I figured it out; it's:

else static if (is(T == bool))

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Dec 13 2009
prev sibling parent reply Frank Benoit <keinfarbton googlemail.com> writes:
casting an array of class references to an array of interface references
(or vice versa) will not work at runtime. Your program will crash.

This is because if the invisible pointer correction that is done if you
cast a single class ref to an interface ref.


C c = new C;
I i = c;

writefln( "c=%d i=%i", cast(uint)cast(void*)c, cast(uint)cast(void*)i);

This shows, the numeric values are not equal.
At the assignment were an imlicit cast is taking place, dmd inserts a
pointer adjustement. If you cast an array, nothing physically is done to
the array content. In fact you need to run a loop over the array and
cast each member on it own.
Dec 13 2009
parent "Phil Deets" <pjdeets2 gmail.com> writes:
On Sun, 13 Dec 2009 04:16:19 -0500, Frank Benoit  
<keinfarbton googlemail.com> wrote:


 casting an array of class references to an array of interface references
 (or vice versa) will not work at runtime. Your program will crash.

 This is because if the invisible pointer correction that is done if you
 cast a single class ref to an interface ref.


 C c = new C;
 I i = c;

 writefln( "c=%d i=%i", cast(uint)cast(void*)c, cast(uint)cast(void*)i);

 This shows, the numeric values are not equal.
 At the assignment were an imlicit cast is taking place, dmd inserts a
 pointer adjustement. If you cast an array, nothing physically is done to
 the array content. In fact you need to run a loop over the array and
 cast each member on it own.


Thanks for informing me, my newer workaround avoids this problem.

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Dec 13 2009