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digitalmars.D.learn - Defining constant values in struct

reply "tcak" <tcak gmail.com> writes:
As far as I known, when I define a string with enum and it is 
used at different parts of code, that string is repeated again 
and again in executable file instead of passing a pointer to 
string. So, using enum with string doesn't seem like a good idea.

Hence, I defined string as const to make it belong to struct 
itself instead of instances, but it comes with 'need `this` for 
...' error. This indicates that the string doesn't belong to 
struct itself actually.

Because there is nothing like namespace in D, I used a sub-struct.

[code]
struct TableSchema{
	const string TABLE = "users";

	struct FieldTypes{
		const string ID = "BIGINT";
	}

	const string CREATESQL = "... id " ~ FieldTypes.ID ~ "...";
}
[/code]


But compiler doesn't allow me to access FieldTypes.ID. It says 
that it needs `this`. I tried with `static shared`, used `class` 
instead of `struct` etc. But couldn't have come up with a nice 
solution.
Jun 16 2015
next sibling parent reply "jklp" <jklp nowhere.fr> writes:
On Tuesday, 16 June 2015 at 21:17:37 UTC, tcak wrote:

 As far as I known, when I define a string with enum and it is 
 used at different parts of code, that string is repeated again 
 and again in executable file instead of passing a pointer to 
 string. So, using enum with string doesn't seem like a good 
 idea.

 Hence, I defined string as const to make it belong to struct 
 itself instead of instances, but it comes with 'need `this` for 
 ...' error. This indicates that the string doesn't belong to 
 struct itself actually.

 Because there is nothing like namespace in D, I used a 
 sub-struct.

 [code]
 struct TableSchema{
 	const string TABLE = "users";

 	struct FieldTypes{
 		const string ID = "BIGINT";
 	}

 	const string CREATESQL = "... id " ~ FieldTypes.ID ~ "...";
 }
 [/code]


 But compiler doesn't allow me to access FieldTypes.ID. It says 
 that it needs `this`. I tried with `static shared`, used 
 `class` instead of `struct` etc. But couldn't have come up with 
 a nice solution.


Do i miss a detail in your requirement ?

---
struct TableSchema{
	const string TABLE = "users";

	struct FieldTypes{
		static const string ID = "BIGINT";
	}

	const string CREATESQL = "... id " ~ FieldTypes.ID ~ "...";
}
---

because this works.
Jun 16 2015
parent "tcak" <tcak gmail.com> writes:
On Tuesday, 16 June 2015 at 21:38:22 UTC, jklp wrote:

 On Tuesday, 16 June 2015 at 21:17:37 UTC, tcak wrote:

 [...]


 Do i miss a detail in your requirement ?

 ---
 struct TableSchema{
 	const string TABLE = "users";

 	struct FieldTypes{
 		static const string ID = "BIGINT";
 	}

 	const string CREATESQL = "... id " ~ FieldTypes.ID ~ "...";
 }
 ---

 because this works.


Hmm, I never defined it as `static const` before. Tried with 
`static shared` only. Yes, it works like this.
Jun 16 2015
prev sibling next sibling parent "anonymous" <anonymous example.com> writes:
On Tuesday, 16 June 2015 at 21:17:37 UTC, tcak wrote:

 As far as I known, when I define a string with enum and it is 
 used at different parts of code, that string is repeated again 
 and again in executable file instead of passing a pointer to 
 string. So, using enum with string doesn't seem like a good 
 idea.


I'm not sure how true that is. For example, this prints the same 
address twice:
----
import std.stdio;
enum e = "foo";
void main()
{
     auto a = e;
     auto b = e;
     writeln(a.ptr);
     writeln(b.ptr);
}
----

In contrast, it prints two different addresses when e is defined 
as `[1, 2, 3]` instead.

[...]

 [code]
 struct TableSchema{
 	const string TABLE = "users";

 	struct FieldTypes{
 		const string ID = "BIGINT";
 	}

 	const string CREATESQL = "... id " ~ FieldTypes.ID ~ "...";
 }
 [/code]


 But compiler doesn't allow me to access FieldTypes.ID. It says 
 that it needs `this`. I tried with `static shared`, used 
 `class` instead of `struct` etc. But couldn't have come up with 
 a nice solution.


`static immutable` is where it's at.
Jun 16 2015
prev sibling next sibling parent "Alex Parrill" <initrd.gz gmail.com> writes:
On Tuesday, 16 June 2015 at 21:17:37 UTC, tcak wrote:

 As far as I known, when I define a string with enum and it is 
 used at different parts of code, that string is repeated again 
 and again in executable file instead of passing a pointer to 
 string. So, using enum with string doesn't seem like a good 
 idea.


String literals are merged by the compiler, much like in C and 
Java.

import std.stdio;
enum mystring = "hello world!";

void main() {
	string str1 = mystring;
	string str2 = mystring;
	
	writeln(str1.ptr == str2.ptr); // prints true
}
Jun 16 2015
prev sibling parent reply Mike Parker <aldacron gmail.com> writes:
On 6/17/2015 6:17 AM, tcak wrote:

 As far as I known, when I define a string with enum and it is used at
 different parts of code, that string is repeated again and again in
 executable file instead of passing a pointer to string. So, using enum
 with string doesn't seem like a good idea.


The string is not repeated again and again in the *executable*. The 
compiler is smart enough to recognize the same string is used in 
multiple places. It will be placed in the data segment only once. It's 
no different than using a string literal.
Jun 16 2015
parent ketmar <ketmar ketmar.no-ip.org> writes:
On Wed, 17 Jun 2015 09:49:56 +0900, Mike Parker wrote:


 On 6/17/2015 6:17 AM, tcak wrote:

 As far as I known, when I define a string with enum and it is used at
 different parts of code, that string is repeated again and again in
 executable file instead of passing a pointer to string. So, using enum
 with string doesn't seem like a good idea.


 The string is not repeated again and again in the *executable*. The
 compiler is smart enough to recognize the same string is used in
 multiple places. It will be placed in the data segment only once. It's
 no different than using a string literal.


but only to some extent[1].

[1] https://issues.dlang.org/show_bug.cgi?id=3D14459=
Jun 17 2015