www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - declaration with typedef

reply Hamad Mohammad <h.battel hotmail.com> writes:
I can't compile this code

void main()
{
    typedef int number;
    number x;
    x = 314;
    writeln(x);
}
Dec 27 2010
parent reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
That's because you're creating a new type with typedef, and writeln()
doesn't know how to work with a "number" type. Use an alias:

void main()
{
   alias int number;
   number x;
   x = 314;
   writeln(x);
}

AFAIK typedef is scheduled for deprecation in D2 (if that's what you're using).

On 12/27/10, Hamad Mohammad <h.battel hotmail.com> wrote:

 I can't compile this code

 void main()
 {
     typedef int number;
     number x;
     x = 314;
     writeln(x);
 }
Dec 27 2010
parent reply Hamad Mohammad <h.battel hotmail.com> writes:
excuse me

but why that code works

struct Human
{
    string name;
    int age;
}
void main()
{
    Human person1;
    person1.name = "aaaa";
    person1.age = 12;
    writeln(person1.age);
    typedef Human Mankind;
    Mankind person2;
    person2.name = "bbbb";
    person2.age = 14;
    writeln(person2.age);
}
Dec 27 2010
parent reply David Nadlinger <see klickverbot.at> writes:
On 12/27/10 3:44 PM, Hamad Mohammad wrote:

 excuse me

 but why that code works

[…]


If you tried to assign person2 to person1 or vice versa, the same error 
would occur.

But as Andrei already said, typedef has been deemed deprecated in D2.

David
Dec 27 2010
parent reply Hamad Mohammad <h.battel otmail.com> writes:
 If you tried to assign person2 to person1 or vice versa


how?
Dec 27 2010
parent reply Stanislav Blinov <stanislav.blinov gmail.com> writes:
On 12/27/2010 06:41 PM, Hamad Mohammad wrote:

 If you tried to assign person2 to person1 or vice versa


 how?


I don't think I got what David meant with it either. Assigning instances 
of the same type is perfectly valid as long as you do not define some 
very peculiar opAssign.

Andrej, on the other hand, made a perfect point.
A typedef is deprecated in D2. typedef in D2 differs from C/C++ one. 
What typedef does (in the old, D1 way) is introducing a new, distinct 
type. Many D constructs, especially templates, can handle only certain 
types. Those, depending on conditions, may or may not include 
user-defined types.

writeln requires that a given value is 'formattable' to string. It knows 
how to deal with numerics, strings, arrays, structs and classes. But it 
does not put out assumptions on unknown types (and typedef'd type is 
"unknown" to D's type system). There are some ways to "introduce" your 
types to certain constructs. For example, if you do
writeln(person1) in your code, you'll get "Human" on the console - this 
is a default way writeln handles structs. But if you define a method 
"toString" for your Human struct, e.g:

import std.conv;

struct Human
{
	//...
	string toString()
	{
		return text(name, ": ", age);
	}
}

, then writeln(person1) would output 'aaaa: 12' to the console.
(Mind that this 'toString' idiom may change, which is the effect of 
recent discussions about this certain topic).

Generally, if you want a "distinct" type in D2, define a struct (or a 
class if you want your type to have reference semantics). If you want a 
simple alias to existing type, use "alias" declaration (alias is an 
analog of C/C++ typedef, though the keyword itself does more than this. 
You can find out additional uses in documentaion or "The D Programming 
Language" book).

At this point, language gurus should start throwing rotten tomatoes at 
my general location, but I tried to explain the thing in the easiest way 
I could.
Dec 27 2010
next sibling parent Stanislav Blinov <stanislav.blinov gmail.com> writes:
On 12/28/2010 04:09 AM, Stanislav Blinov wrote:


 typedef in D2 differs from C/C++ one.


Scratch that. I meant "in D" (having in mind D1 actually, oh well...)
Dec 27 2010
prev sibling next sibling parent Hamad Mohammad <h.battel hotmail.com> writes:
== Quote from Stanislav Blinov (stanislav.blinov gmail.com)'s article

 On 12/27/2010 06:41 PM, Hamad Mohammad wrote:

 If you tried to assign person2 to person1 or vice versa


 how?


 I don't think I got what David meant with it either. Assigning instances
 of the same type is perfectly valid as long as you do not define some
 very peculiar opAssign.
 Andrej, on the other hand, made a perfect point.
 A typedef is deprecated in D2. typedef in D2 differs from C/C++ one.
 What typedef does (in the old, D1 way) is introducing a new, distinct
 type. Many D constructs, especially templates, can handle only certain
 types. Those, depending on conditions, may or may not include
 user-defined types.
 writeln requires that a given value is 'formattable' to string. It knows
 how to deal with numerics, strings, arrays, structs and classes. But it
 does not put out assumptions on unknown types (and typedef'd type is
 "unknown" to D's type system). There are some ways to "introduce" your
 types to certain constructs. For example, if you do
 writeln(person1) in your code, you'll get "Human" on the console - this
 is a default way writeln handles structs. But if you define a method
 "toString" for your Human struct, e.g:
 import std.conv;
 struct Human
 {
 	//...
 	string toString()
 	{
 		return text(name, ": ", age);
 	}
 }
 , then writeln(person1) would output 'aaaa: 12' to the console.
 (Mind that this 'toString' idiom may change, which is the effect of
 recent discussions about this certain topic).
 Generally, if you want a "distinct" type in D2, define a struct (or a
 class if you want your type to have reference semantics). If you want a
 simple alias to existing type, use "alias" declaration (alias is an
 analog of C/C++ typedef, though the keyword itself does more than this.
 You can find out additional uses in documentaion or "The D Programming
 Language" book).
 At this point, language gurus should start throwing rotten tomatoes at
 my general location, but I tried to explain the thing in the easiest way
 I could.


thanks
Dec 28 2010
prev sibling parent reply David Nadlinger <see klickverbot.at> writes:
On 12/28/10 2:09 AM, Stanislav Blinov wrote:

 I don't think I got what David meant with it either. Assigning instances
 of the same type is perfectly valid as long as you do not define some
 very peculiar opAssign.


The point here is that person1 and person2 are not instances of the same 
type, try compiling the following snippet:

---
struct Human {
    string name;
    int age;
}

void main() {
    Human person1;

    typedef Human Mankind;
    Mankind person2 = person1;
}
---

This might not be what Hamad wanted to know when he asked »but why that 
code works«, but I could think of no other reason why the snipped he 
posted should fail to compile.

David
Dec 28 2010
parent Stanislav Blinov <blinov loniir.ru> writes:
28.12.2010 20:42, David Nadlinger пишет:

 On 12/28/10 2:09 AM, Stanislav Blinov wrote:

 I don't think I got what David meant with it either. Assigning instances
 of the same type is perfectly valid as long as you do not define some
 very peculiar opAssign.


 The point here is that person1 and person2 are not instances of the 
 same type, try compiling the following snippet:

 ---
 struct Human {
    string name;
    int age;
 }

 void main() {
    Human person1;

    typedef Human Mankind;
    Mankind person2 = person1;
 }
 ---


Oh, right, I missed that one, sorry.


 This might not be what Hamad wanted to know when he asked »but why 
 that code works«, but I could think of no other reason why the snipped 
 he posted should fail to compile.


Yes, the original snippet should compile fine because typedef'd type 
isn't actually used in any way except for accessing fields, which of 
course have built-in types.
Dec 28 2010