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digitalmars.D.learn - Type converter from build in to user type

reply "js.mdnq" <js_adddot+mdng gmail.com> writes:
I have a struct I am trying convert from int's to the type. Since 
I can't add a opCast overload to an int I don't know how to do 
it. My opCast convertors in my class do not work for the 
assignment operator:


class myType
{
    opCast, opAssign
}

mytype = 3;

Error that we can't convert an int to myType. While it's very 
easy to go from a myType to whatever by writing a cast for it(in 
the class), how do I go from an int to myType(in the class)?

For example, I can do

mytype = mytype.opCast(3);
or mytype.opAssign(3);

but these are too verbose to be useful.
Nov 29 2012
next sibling parent "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On Friday, November 30, 2012 03:59:05 js.mdnq wrote:

 I have a struct I am trying convert from int's to the type. Since
 I can't add a opCast overload to an int I don't know how to do
 it. My opCast convertors in my class do not work for the
 assignment operator:
 
 
 class myType
 {
 opCast, opAssign
 }
 
 mytype = 3;
 
 Error that we can't convert an int to myType. While it's very
 easy to go from a myType to whatever by writing a cast for it(in
 the class), how do I go from an int to myType(in the class)?
 
 For example, I can do
 
 mytype = mytype.opCast(3);
 or mytype.opAssign(3);
 
 but these are too verbose to be useful.


You can put a constructor on your type which takes an int and then use 
std.conv.to instead of casting. It's generally preferred to use std.conv.to 
anyway, since it's generally safer (since it allows fewer (no?) unsafe casts 
and checks conversions in some cases - e.g. whether the value in the int being 
cast to ubyte will fit in a ubyte). It will also use your opCast if you define 
one, so you can then use std.conv.to in both directions.

- Jonathan M Davis
Nov 29 2012
prev sibling parent reply "jerro" <a a.com> writes:
On Friday, 30 November 2012 at 02:59:06 UTC, js.mdnq wrote:

 I have a struct I am trying convert from int's to the type. 
 Since I can't add a opCast overload to an int I don't know how 
 to do it. My opCast convertors in my class do not work for the 
 assignment operator:


 class myType
 {
    opCast, opAssign
 }

 mytype = 3;

 Error that we can't convert an int to myType. While it's very 
 easy to go from a myType to whatever by writing a cast for 
 it(in the class), how do I go from an int to myType(in the 
 class)?

 For example, I can do

 mytype = mytype.opCast(3);
 or mytype.opAssign(3);

 but these are too verbose to be useful.


Why can't you use opAssign? This works fine:

class A
{
     void opAssign(int i)
     {
         writeln(i);
     }
}

auto a = new A();
a = 1;                 // prints 1
Nov 29 2012
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 11/29/2012 07:24 PM, jerro wrote:

 On Friday, 30 November 2012 at 02:59:06 UTC, js.mdnq wrote:

 I have a struct I am trying convert from int's to the type. Since I
 can't add a opCast overload to an int I don't know how to do it. My
 opCast convertors in my class do not work for the assignment operator:


 class myType
 {
 opCast, opAssign
 }

 mytype = 3;

 Error that we can't convert an int to myType. While it's very easy to
 go from a myType to whatever by writing a cast for it(in the class),
 how do I go from an int to myType(in the class)?

 For example, I can do

 mytype = mytype.opCast(3);
 or mytype.opAssign(3);

 but these are too verbose to be useful.


 Why can't you use opAssign? This works fine:

 class A
 {
 void opAssign(int i)
 {
 writeln(i);
 }
 }

 auto a = new A();
 a = 1; // prints 1


A complete example:

import std.conv;

class C
{
     int i;

     this(int i)
     {
         this.i = i;
     }

     C opAssign(int i)
     {
         this.i = i;
         return this;
     }

     int opCast(T : int)() const
     {
         return i;
     }
}

void main()
{
     auto c = new C(42);
     c = 43;
     auto i = to!int(c);
     assert(i == 43);
}

Ali
Nov 29 2012
parent "js.mdnq" <js_adddot+mdng gmail.com> writes:
On Friday, 30 November 2012 at 03:40:31 UTC, Ali Çehreli wrote:

 On 11/29/2012 07:24 PM, jerro wrote:

 On Friday, 30 November 2012 at 02:59:06 UTC, js.mdnq wrote:

 I have a struct I am trying convert from int's to the type. 
 Since I
 can't add a opCast overload to an int I don't know how to do 
 it. My
 opCast convertors in my class do not work for the assignment 
 operator:


 class myType
 {
 opCast, opAssign
 }

 mytype = 3;

 Error that we can't convert an int to myType. While it's very 
 easy to
 go from a myType to whatever by writing a cast for it(in the 
 class),
 how do I go from an int to myType(in the class)?

 For example, I can do

 mytype = mytype.opCast(3);
 or mytype.opAssign(3);

 but these are too verbose to be useful.


 Why can't you use opAssign? This works fine:

 class A
 {
 void opAssign(int i)
 {
 writeln(i);
 }
 }

 auto a = new A();
 a = 1; // prints 1


 A complete example:

 import std.conv;

 class C
 {
     int i;

     this(int i)
     {
         this.i = i;
     }

     C opAssign(int i)
     {
         this.i = i;
         return this;
     }

     int opCast(T : int)() const
     {
         return i;
     }
 }

 void main()
 {
     auto c = new C(42);
     c = 43;
     auto i = to!int(c);
     assert(i == 43);
 }

 Ali


Thanks, I was using a generic opCast which was causing the 
problem. It must have gotten that way from other mistakes I was 
probably making.

C!T opAssign(T i) rather than C!T opAssign(T)(T i)

It thought the D compiler would automatically deduce the second 
case but I guess it's redundant.
Nov 30 2012