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digitalmars.D - Template questions

reply Edward Diener <eddielee_no_spam_here tropicsoft.com> writes:
I am going to be asking a number of questions about D templates in 
various posts, and I hope others can help me understand D templates 
despite the documentation.

First question:

In the documentation on templates I read these lines:

"Even if template arguments are implicitly converted to the same 
template parameter type, they still refer to different instances:
struct TFoo(int x) { }
static assert(is(TFoo!(3) == TFoo!(2 + 1))); // 3 and 2+1 are both 3 of 
// type
int
static assert(!is(TFoo!(3) == TFoo!(3u))); // 3u and 3 are different
// types"

To what is the "int" on line 6 refer ?
Why is line 3 considered a D template ? It does not begin with 'template 
...'. It looks like a simple 'struct' to me with a public member. How 
then can it be instantiated as if it were a template ?

Otherwise I do understand what is being said about templates being 
instantiated to different instances.
Mar 22 2008
next sibling parent reply Jason House <jason.james.house gmail.com> writes:
Edward Diener wrote:


 I am going to be asking a number of questions about D templates in
 various posts, and I hope others can help me understand D templates
 despite the documentation.
 
 First question:
 
 In the documentation on templates I read these lines:
 
 "Even if template arguments are implicitly converted to the same
 template parameter type, they still refer to different instances:
 struct TFoo(int x) { }
 static assert(is(TFoo!(3) == TFoo!(2 + 1))); // 3 and 2+1 are both 3 of
 // type
 int
 static assert(!is(TFoo!(3) == TFoo!(3u))); // 3u and 3 are different
 // types"
 
 To what is the "int" on line 6 refer ?
 Why is line 3 considered a D template ? It does not begin with 'template
 ...'. It looks like a simple 'struct' to me with a public member. How
 then can it be instantiated as if it were a template ?
 
 Otherwise I do understand what is being said about templates being
 instantiated to different instances.


You'll help people out if you give us the URL of the page or file you're
looking at, for example http://www.digitalmars.com/d/2.0/template.html

How are you extracting line numbers?

Here's text I found online (reformatted to avoid word wrap issues)

struct TFoo(int x) { }
// 3 and 2+1 are both 3 of type int
static assert(is(TFoo!(3) == TFoo!(2 + 1)));
// 3u and 3 are different types
static assert(!is(TFoo!(3) == TFoo!(3u)));

Sadly, I can't help because this doesn't make sense to me either.  The
template parameter is of type int, so I'd expect 3 and 3u to be converted
to an int (3) to fit the specified type... and therefore be of the same
type.
Mar 22 2008
parent Edward Diener <eddielee_no_spam_here tropicsoft.com> writes:
Jason House wrote:

 Edward Diener wrote:
 

 I am going to be asking a number of questions about D templates in
 various posts, and I hope others can help me understand D templates
 despite the documentation.

 First question:

 In the documentation on templates I read these lines:

 "Even if template arguments are implicitly converted to the same
 template parameter type, they still refer to different instances:
 struct TFoo(int x) { }
 static assert(is(TFoo!(3) == TFoo!(2 + 1))); // 3 and 2+1 are both 3 of
 // type
 int
 static assert(!is(TFoo!(3) == TFoo!(3u))); // 3u and 3 are different
 // types"

 To what is the "int" on line 6 refer ?
 Why is line 3 considered a D template ? It does not begin with 'template
 ...'. It looks like a simple 'struct' to me with a public member. How
 then can it be instantiated as if it were a template ?

 Otherwise I do understand what is being said about templates being
 instantiated to different instances.


 
 You'll help people out if you give us the URL of the page or file you're
 looking at, for example http://www.digitalmars.com/d/2.0/template.html


It is straight from the pdf documentation.


 
 How are you extracting line numbers?


The first line in my example is 1.

I will assume the 'int' is a typo which no one has seen fit to correct 
or update in the PDF documentation.
Mar 22 2008
prev sibling parent reply Bill Baxter <dnewsgroup billbaxter.com> writes:
Edward Diener wrote:

 I am going to be asking a number of questions about D templates in 
 various posts, and I hope others can help me understand D templates 
 despite the documentation.
 
 First question:
 
 In the documentation on templates I read these lines:
 
 "Even if template arguments are implicitly converted to the same 
 template parameter type, they still refer to different instances:
 struct TFoo(int x) { }
 static assert(is(TFoo!(3) == TFoo!(2 + 1))); // 3 and 2+1 are both 3 of 
 // type
 int
 static assert(!is(TFoo!(3) == TFoo!(3u))); // 3u and 3 are different
 // types"
 
 To what is the "int" on line 6 refer ?
 Why is line 3 considered a D template ? It does not begin with 'template 
 ...'. It looks like a simple 'struct' to me with a public member. How 
 then can it be instantiated as if it were a template ?


struct TFoo(int x) { .. }

is shorthand in D for

template TFoo(int x) {
    struct TFoo { .. }
}

Just like how with D function templates this

void Foo(int x)() { .. }

is shorthand for this:

template Foo(int x) {
    void Foo() { .. }
}

--bb
Mar 22 2008
parent reply Edward Diener <eddielee_no_spam_here tropicsoft.com> writes:
Bill Baxter wrote:

 Edward Diener wrote:

 I am going to be asking a number of questions about D templates in 
 various posts, and I hope others can help me understand D templates 
 despite the documentation.

 First question:

 In the documentation on templates I read these lines:

 "Even if template arguments are implicitly converted to the same 
 template parameter type, they still refer to different instances:
 struct TFoo(int x) { }
 static assert(is(TFoo!(3) == TFoo!(2 + 1))); // 3 and 2+1 are both 3 
 of // type
 int
 static assert(!is(TFoo!(3) == TFoo!(3u))); // 3u and 3 are different
 // types"

 To what is the "int" on line 6 refer ?
 Why is line 3 considered a D template ? It does not begin with 
 'template ...'. It looks like a simple 'struct' to me with a public 
 member. How then can it be instantiated as if it were a template ?


 
 struct TFoo(int x) { .. }
 
 is shorthand in D for
 
 template TFoo(int x) {
    struct TFoo { .. }
 }
 
 Just like how with D function templates this
 
 void Foo(int x)() { .. }
 
 is shorthand for this:
 
 template Foo(int x) {
    void Foo() { .. }
 }


Please explain the rules for this "shorthand". I can not find them in 
the PDF documenatation.

Is the rule that this applies only if there is a single data field in 
the struct/class class, with no other data or member functions of any 
kind, or a single parameter in the function case ?

If that is the rule, how does one define a class/struct with a single 
daya parameter which is not a template, or does one just give up on such 
a type ?

If that is the rule how does one define a function which takes a single 
parameter which is not a template function, or does one just give up on 
such a function ?
Mar 22 2008
next sibling parent reply Frank Benoit <keinfarbton googlemail.com> writes:
Edward Diener schrieb:

 Please explain the rules for this "shorthand". I can not find them in 
 the PDF documenatation.


http://www.digitalmars.com/d/1.0/template.html
See Class Template and ff.
"If a template declares exactly one member, and that member is a class 
with the same name as the template:"
Mar 22 2008
parent Edward Diener <eddielee_no_spam_here tropicsoft.com> writes:
Frank Benoit wrote:

 Edward Diener schrieb:

 Please explain the rules for this "shorthand". I can not find them in 
 the PDF documenatation.


 
 http://www.digitalmars.com/d/1.0/template.html
 See Class Template and ff.
 "If a template declares exactly one member, and that member is a class 
 with the same name as the template:"


Thanks !I saw the example incorrectly. The example I gave had:

struct TFoo(int x) { }

and somehow I saw this as:

struct TFoo { int x; }

That is definitely my error.
Mar 22 2008
prev sibling parent BCS <ao pathlink.com> writes:
Reply to Edward,


 I can not find them in
 the PDF documenatation.
 


I don't know how old the .pdf is but the .html is general considered to be 
more authoritative. I don't know if it makes a difference in this case except 
that more people have them handy than the .pdf.
Mar 22 2008