Invalid template argument

**tom_usenet** · Jul 19 '05, 05:34 PM

Re: Invalid template argument

On Mon, 15 Sep 2003 12:57:44 -0400, "Kevin Grigorenko"
<kzg110@psu.edu > wrote:
[color=blue]
>Hello all,
>
>Okay, I've got a templated class that that takes an int and a char *, but
>when I try to instantiate an object of that template, VS.NET complains with:
>
>error C2975: 'S' : invalid template argument for 'TextDB::TextDB _Version',
>constant expression expected
>
>template<int N, char *S>
>class TextDB_Version
>{
> [...]
>};
>
>#define TDB_DEFAULT_DEL IMITER "."
>class TextDB
>{
>private:
> TextDB_Version< 3, TDB_DEFAULT_DEL IMITER> myVersion;
>};
>
>I can't understand what's wrong with this. It seems to me from
>documentatio n that I should be able to use char *'s as literal template
>arguments, what am I missing here? I tried taking out the #define and put
>in just a string there, TextDB_Version< 3, "."> myVersion; and that had the
>same error obviously.[/color]

Literals don't have external linkage, but template parameters must. So
you need:

char* TDB_DEFAULT_DEL IMITER = "."; //has external linkage.

class TextDB
{
private:
TextDB_Version< 3, TDB_DEFAULT_DEL IMITER> myVersion;
};

Tom

**White Wolf** · Jul 19 '05, 05:34 PM

Re: Invalid template argument

tom_usenet wrote:[color=blue][color=green]
>> error C2975: 'S' : invalid template argument for
>> 'TextDB::TextDB _Version', constant expression expected[/color][/color]
[SNIP][color=blue]
> Literals don't have external linkage, but template parameters must. So
> you need:
>
> char* TDB_DEFAULT_DEL IMITER = "."; //has external linkage.[/color]

Is that really a compile time constant (required for template
instantiation)?

--
WW aka Attila

**Gianni Mariani** · Jul 19 '05, 05:34 PM

Re: Invalid template argument

tom_usenet wrote:[color=blue]
> On Mon, 15 Sep 2003 12:57:44 -0400, "Kevin Grigorenko"
> <kzg110@psu.edu > wrote:[/color]

.....
[color=blue][color=green]
>>
>>template<in t N, char *S>
>>class TextDB_Version
>>{[/color][/color]
....
[color=blue]
>
> Literals don't have external linkage, but template parameters must. So
> you need:
>
> char* TDB_DEFAULT_DEL IMITER = "."; //has external linkage.
>
> class TextDB
> {
> private:
> TextDB_Version< 3, TDB_DEFAULT_DEL IMITER> myVersion;
> };
>
> Tom[/color]

The alternative is to use a single char as a delimiter:

template<int N, char D>

like:

TextDB_Version< 3, '.'>

if you only have single character delimiters.

**Kevin Saff** · Jul 19 '05, 05:34 PM

Re: Invalid template argument

"Kevin Grigorenko" <kzg110@psu.edu > wrote in message
news:bk4r2d$11a g$1@f04n12.cac. psu.edu...[color=blue]
> Hello all,
>
> SNIP
>
> I can't understand what's wrong with this. It seems to me from
> documentation that I should be able to use char *'s as literal template
> arguments, what am I missing here? I tried taking out the #define and put
> in just a string there, TextDB_Version< 3, "."> myVersion; and that had the
> same error obviously.[/color]

This is addressed many times in the archives (but not in the FAQ yet, it
appears). I'm sure you can search google groups for the long answer. The
short answer is char* is fine for template arguments, but you just can't
create templates of the form TemplateClass<" text">. In this particular
case, why don't you just declare your template as:

template<int N, char S>
class TextDB_Version
{
[...]
};

and then you can instantiate with

TextDB_Version< 3, '.'>

**Kevin Grigorenko** · Jul 19 '05, 05:34 PM

Re: Invalid template argument

"Kevin Saff" <google.com@kev in.saff.net> wrote in message
news:HL9psw.EtL @news.boeing.co m...[color=blue]
> "Kevin Grigorenko" <kzg110@psu.edu > wrote in message
> news:bk4r2d$11a g$1@f04n12.cac. psu.edu...[color=green]
> > Hello all,
> >
> > SNIP
> >
> > I can't understand what's wrong with this. It seems to me from
> > documentation that I should be able to use char *'s as literal template
> > arguments, what am I missing here? I tried taking out the #define and[/color][/color]
put[color=blue][color=green]
> > in just a string there, TextDB_Version< 3, "."> myVersion; and that had[/color][/color]
the[color=blue][color=green]
> > same error obviously.[/color]
>
> This is addressed many times in the archives (but not in the FAQ yet, it
> appears). I'm sure you can search google groups for the long answer. The
> short answer is char* is fine for template arguments, but you just can't
> create templates of the form TemplateClass<" text">. In this particular
> case, why don't you just declare your template as:
>
> template<int N, char S>
> class TextDB_Version
> {
> [...]
> };
>
> and then you can instantiate with
>
> TextDB_Version< 3, '.'>
>
>[/color]

I'm sorry about being a little bit oblivious, but where are the archives? I
will use this suggestion however and just use one character.

The interesting thing is that in the description of the error, VS.NET gives
this as an example of how to fix it:

// C2975.cpp
template <char *P>
class x
{
char * f()
{
return P;
}
};

x<"abc"> *p = 0; // C2975 addr of object with internal linkage

Is this standardized? Does it have something to do with "internal linkage,"
which i must say i'm also oblivious about.

Thanks again,
Kevin Grigorenko

**Kevin Goodsell** · Jul 19 '05, 05:34 PM

Re: Invalid template argument

Kevin Grigorenko wrote:[color=blue]
>
> I'm sorry about being a little bit oblivious, but where are the archives?[/color]

http://groups.google.com

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

**Kevin Saff** · Jul 19 '05, 05:35 PM

Re: Invalid template argument

"Kevin Grigorenko" <kzg110@psu.edu > wrote in message
news:bk52la$1e1 e$1@f04n12.cac. psu.edu...[color=blue]
> I'm sorry about being a little bit oblivious, but where are the archives?[/color]
I[color=blue]
> will use this suggestion however and just use one character.[/color]

Sorry, here's a link:

http://groups.google.com/groups?q=comp.lang.c%2B%2B

and here are some posts you may be interested in:

http://groups.google.com/groups?hl=en&q=template+char*&meta=group%3Dcomp.lan

g.c%252B%252B.*

[color=blue]
>
> The interesting thing is that in the description of the error, VS.NET[/color]
gives[color=blue]
> this as an example of how to fix it:
>
>

> // C2975.cpp
> template <char *P>
> class x
> {
> char * f()
> {
> return P;
> }
> };
>
> x<"abc"> *p = 0; // C2975 addr of object with internal linkage
>

>
> Is this standardized? Does it have something to do with "internal[/color]
linkage,"[color=blue]
> which i must say i'm also oblivious about.[/color]

That looks like just an example of the error, rather than a fix. They're
constructing a null pointer to a x<"abc"> object, but "abc" has internal
linkage so isn't eligible for use as a template parameter.

Internal linkage means the variable name can't be accessed in external
compilation units. For one thing, note that x<"abc"> would at least
represent a different class in a different compilation unit, because the
address of the string will be different. This means at best the template
class would have internal linkage as well. Still it seems like there could
be use for such a thing; apparently the full reason template parameters
cannot have internal linkage has something to do with how template names are
required to be mangled.

**tom_usenet** · Jul 19 '05, 05:36 PM

Re: Invalid template argument

On Mon, 15 Sep 2003 20:50:24 +0300, "White Wolf" <wolof@freemail .hu>
wrote:
[color=blue]
>tom_usenet wrote:[color=green][color=darkred]
>>> error C2975: 'S' : invalid template argument for
>>> 'TextDB::TextDB _Version', constant expression expected[/color][/color]
>[SNIP][color=green]
>> Literals don't have external linkage, but template parameters must. So
>> you need:
>>
>> char* TDB_DEFAULT_DEL IMITER = "."; //has external linkage.[/color]
>
>Is that really a compile time constant (required for template
>instantiation) ?[/color]

Whoops, that was meant to be:

char TDB_DEFAULT_DEL IMITER[] = "."; //has external linkage.

Tom

Invalid template argument

Invalid template argument

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