www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - What Does Mean?

reply Ron Tarrant <rontarrant gmail.com> writes:
This is frustrating and makes me feel like a complete newb. 
Worse, it's impossible to search for. Ever try Googling a single 
character?

The D documentation also doesn't seem to explain the meaning of 
this or any other token. Sure, most of them are obvious, but this 
one eludes me. All I can find is this: 
https://dlang.org/spec/lex.html#tokens

Would someone please tell me what an at sign ( ) means when it's 
used like this:

bool isLeaf()  property
{
    return children.length == 0;
}

In fact, I have no idea what   means anywhere other than in an 
email address. Is this a common thing in contemporary languages?

And while I'm asking, does an underscore have special meaning 
when used either at the beginning or end of a variable name?

How about a double underscore?

I know underscores are sort of left over from C, but do these 
characters have special meaning in D or are they simply a 
convention... like T in a template definition?
Apr 08 2019
next sibling parent Alex <sascha.orlov gmail.com> writes:
On Monday, 8 April 2019 at 11:58:49 UTC, Ron Tarrant wrote:

 This is frustrating and makes me feel like a complete newb. 
 Worse, it's impossible to search for. Ever try Googling a 
 single character?

 The D documentation also doesn't seem to explain the meaning of 
 this or any other token. Sure, most of them are obvious, but 
 this one eludes me. All I can find is this: 
 https://dlang.org/spec/lex.html#tokens

 Would someone please tell me what an at sign ( ) means when 
 it's used like this:

 bool isLeaf()  property
 {
    return children.length == 0;
 }

 In fact, I have no idea what   means anywhere other than in an 
 email address. Is this a common thing in contemporary languages?

 And while I'm asking, does an underscore have special meaning 
 when used either at the beginning or end of a variable name?

 How about a double underscore?

 I know underscores are sort of left over from C, but do these 
 characters have special meaning in D or are they simply a 
 convention... like T in a template definition?


Some of the attributes in D go with an " ".

https://forum.dlang.org/post/fiwfcsqmjsndcjixipgz forum.dlang.org
https://wiki.dlang.org/Language_Designs_Explained
https://dlang.org/spec/attribute.html
Apr 08 2019
prev sibling next sibling parent Kagamin <spam here.lot> writes:
https://dlang.org/spec/function.html#property-functions
Apr 08 2019
prev sibling next sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Monday, 8 April 2019 at 11:58:49 UTC, Ron Tarrant wrote:

 Would someone please tell me what an at sign ( ) means when 
 it's used like this:

 bool isLeaf()  property



In that case, it means nothing. We just defined the word to be 
` property`, with the   included. So it is just part of the name. 
Same with  safe,  trusted,  system,  nogc, and a few others built 
into the language.

It was arbitrary decided to put the   in the words so it wouldn't 
conflict with existing variable names like `int safe;`


In some other contexts though, it just indicates to the parser 
that it is a user-defined attribute instead of continuing the 
declaration with a name. In these, it is a prefix to another user 
value. For example:

struct MyCustomAttribute {} // perfectly ordinary struct

 MyCustomAttribute() void func() {}

In that function, the   before the name MyCustomAttribute tells 
the compiler that it is just an annotation instead of a return 
value or the defined name of the new function.


So, two purposes for the  : it is just part of some built-in 
keywords, and it can indicate you are using a user-define d name 
as an attribute/annotation instead of as a return value, etc.


 And while I'm asking, does an underscore have special meaning 
 when used either at the beginning or end of a variable name?


Nothing special there, you are allowed to use the _ anywhere in a 
name. Some people use it as a convention on private members, 
unused members, or keyword conflicts (like `body_` so the 
compiler doesn't see it as the keyword `body`).

But it is just a user convention, it doesn't mean anything 
special to the compiler... except:


 How about a double underscore?


A leading double underscore is somewhat special. Those are 
allowed in any name, but those are reserved for compiler 
implementation details.

So like `__ctor` is the special name the compiler uses for 
constructors. You can call it (though since this is an internal 
compiler implementation detail you are choosing to exploit, it is 
liable to break at any time) or define stuff with these names to 
do various hacks.

You should avoid __names though since the compiler reserves them 
for its own use.
Apr 08 2019
next sibling parent Ron Tarrant <rontarrant gmail.com> writes:
Well, that was quick!

Thanks Adam, Kagamin, and Alex.
Apr 08 2019
prev sibling parent reply wjoe <fake example.com> writes:
On Monday, 8 April 2019 at 12:16:13 UTC, Adam D. Ruppe wrote:

 On Monday, 8 April 2019 at 11:58:49 UTC, Ron Tarrant wrote:

 And while I'm asking, does an underscore have special meaning 
 when used either at the beginning or end of a variable name?


 Nothing special there, you are allowed to use the _ anywhere in 
 a name. Some people use it as a convention on private members, 
 unused members, or keyword conflicts (like `body_` so the 
 compiler doesn't see it as the keyword `body`).


It's not entirely related but another use of the underscore is to 
make integers more readable. E.g.:

int x = 1_000_000_000_000;
Apr 12 2019
parent Ron Tarrant <rontarrant gmail.com> writes:
On Friday, 12 April 2019 at 10:01:40 UTC, wjoe wrote:


 It's not entirely related but another use of the underscore is 
 to make integers more readable. E.g.:

 int x = 1_000_000_000_000;


And, I suspect, to make numbers easier to translate between 
English Canadian and French Canadian:

Canadian: $1,200,333.45
Canadien: $1.200.333,45
Apr 12 2019
prev sibling parent reply XavierAP <n3minis-git yahoo.es> writes:
On Monday, 8 April 2019 at 11:58:49 UTC, Ron Tarrant wrote:

 And while I'm asking, does an underscore have special meaning 
 when used either at the beginning or end of a variable name?


In D,   is used as Adam has explained as a prefix indicating 
attributes (either user-defined ones or, confusingly enough, some 
of the standard ones).

The only other example of language using  , in an almost but not 

allows you to define names that would collide with reserved 
words, for example string  class = "menu"; Of course you should 
never do this unless you absolutely need for interop.

Underscore prefixes are used in some languages by pure user 
convention mainly for private members (fields), to avoid name 
clashing. For example in D you could have a public property 
length and a private member _length.

Python takes this a step further. Since it supports classes but 
no public/private visibility at all, users and IDEs have convened 
to use (one or two) underscore prefixes to signal members that 
aren't meant to be accessed publicly from outside the class, even 
if there's nothing stopping you (besides auto code completion not 
showing them).

For C and C++ the convention (recognized by the standards) is 
different: names prefixed by any number of underscores are all 
reserved; basically because the global namespace is so badly 
polluted already. In this case I've seen some other annoying 
conventions, for example private member variables being prefixed 
with m_
Apr 08 2019
next sibling parent reply JN <666total wp.pl> writes:
On Monday, 8 April 2019 at 14:19:04 UTC, XavierAP wrote:

 The only other example of language using  , in an almost but 

 that allows you to define names that would collide with 
 reserved words, for example string  class = "menu"; Of course 
 you should never do this unless you absolutely need for interop.


Java uses   for annotations too. Pascal uses   for "address of", 
like & in D.
Apr 08 2019
parent Ron Tarrant <rontarrant gmail.com> writes:
On Monday, 8 April 2019 at 14:27:11 UTC, JN wrote:


 Java uses   for annotations too. Pascal uses   for "address 
 of", like & in D.


Just one of the many reasons I balked at Java... many MANY 
reasons.

Thanks, JN.
Apr 08 2019
prev sibling parent Ron Tarrant <rontarrant gmail.com> writes:
On Monday, 8 April 2019 at 14:19:04 UTC, XavierAP wrote:

 On Monday, 8 April 2019 at 11:58:49 UTC, Ron Tarrant wrote:

 And while I'm asking, does an underscore have special meaning 
 when used either at the beginning or end of a variable name?


 In D,   is used as Adam has explained as a prefix indicating 
 attributes (either user-defined ones or, confusingly enough, 
 some of the standard ones).


Yup. Confusion. Yup. :)


 Of course you should never do this unless you absolutely need 
 for interop.


No fear of that. I was just trying to understand the example I 
cited.


 For example in D you could have a public property length and a 
 private member _length.


This seems like a good practice.


 In this case I've seen some other annoying conventions, for 
 example private member variables being prefixed with m_


The things I missed by not getting my degree... or by not keeping 
my nose to the grindstone for the last 30-odd years. :)

Thanks, Xavier.
Apr 08 2019