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digitalmars.D.learn - Creating a template mixin for explicit casts.

reply Sjoerd Nijboer <sjoerdnijboer gmail.com> writes:
Given the following code
`struct Foo(T)
	if(isNumeric!T)
{
	T t;
	.. other code
}

struct Bar(T)
	if(isNumeric!T)
{
	T t;
	.. other code
}

Foo!float foo_float;

Foo!double foo_double;

Bar!float bar_float;
`

I want to make a template mixin that is able to cast one of these 
generic structs to the other explicitly. I have a bunch of these 
structs and therefore I thought it would make sense to do it by 
template mixin. I just don't know what language or library 
features I need to use and how to apply them.

`
fooDouble = cast (Foo!double) foo_float;		// case [1] if 
possible, done by
							// an implicit cast. Else by
							//explicit.

fooFloat = cast (Foo!float) foo_double;			// case [2]

barFloat = cast (Bar!float) foo_float;			// case [3]
`

How would I do this in D?
May 17 2018
next sibling parent reply Paul Backus <snarwin gmail.com> writes:
On Thursday, 17 May 2018 at 15:25:37 UTC, Sjoerd Nijboer wrote:

 I want to make a template mixin that is able to cast one of 
 these generic structs to the other explicitly. I have a bunch 
 of these structs and therefore I thought it would make sense to 
 do it by template mixin. I just don't know what language or 
 library features I need to use and how to apply them.


It sounds like you want to overload opCast:

https://dlang.org/spec/operatoroverloading.html#cast
May 17 2018
parent reply Sjoerd Nijboer <sjoerdnijboer gmail.com> writes:
On Thursday, 17 May 2018 at 16:27:48 UTC, Paul Backus wrote:

 On Thursday, 17 May 2018 at 15:25:37 UTC, Sjoerd Nijboer wrote:

 I want to make a template mixin that is able to cast one of 
 these generic structs to the other explicitly. I have a bunch 
 of these structs and therefore I thought it would make sense 
 to do it by template mixin. I just don't know what language or 
 library features I need to use and how to apply them.


 It sounds like you want to overload opCast:

 https://dlang.org/spec/operatoroverloading.html#cast


Something like:
`
Foo!T opCast(U)(Foo!U old)
	if (isImplicitlyConvertible!(T, U))
	{
		return Foo!T(old.t);
	}

Foo!T opCast(UTemplate, U)(UTemplate!U old)
	if (isImplicitlyConvertible!(T, U) && isNumerical!U)
	{
		return Foo!T(old.t);
	}

`
But then how do you put this into a mixin template so I can write 
something like
`
struct Foo(T)
	if(isNumerical(T))
{
	mixin castingRules(typeof(this) T);
}
`
May 17 2018
parent Sjoerd Nijboer <sjoerdnijboer gmail.com> writes:
On Thursday, 17 May 2018 at 20:38:13 UTC, Sjoerd Nijboer wrote:

 But then how do you put this into a mixin template so I can ...
 	mixin castingRules(typeof(this) T);


I guess I can refine my question to "How do you let a mixin 
template detect the template name it is instantiated with and 
return its type whitout the template arguments.
May 17 2018
prev sibling parent Meta <jared771 gmail.com> writes:
On Thursday, 17 May 2018 at 15:25:37 UTC, Sjoerd Nijboer wrote:

 Given the following code
 `struct Foo(T)
 	if(isNumeric!T)
 {
 	T t;
 	.. other code
 }

 struct Bar(T)
 	if(isNumeric!T)
 {
 	T t;
 	.. other code
 }

 Foo!float foo_float;

 Foo!double foo_double;

 Bar!float bar_float;
 `

 I want to make a template mixin that is able to cast one of 
 these generic structs to the other explicitly. I have a bunch 
 of these structs and therefore I thought it would make sense to 
 do it by template mixin. I just don't know what language or 
 library features I need to use and how to apply them.

 `
 fooDouble = cast (Foo!double) foo_float;		// case [1] if 
 possible, done by
 							// an implicit cast. Else by
 							//explicit.

 fooFloat = cast (Foo!float) foo_double;			// case [2]

 barFloat = cast (Bar!float) foo_float;			// case [3]
 `

 How would I do this in D?


Before you write anything yourself, check whether 
std.conv.castFrom does what you need:

https://dlang.org/phobos/std_conv.html#castFrom
May 17 2018