Invalid template argument

[Kevin Grigorenko]

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_DELIMITER "."
class TextDB
{
private:
TextDB_Version<3, TDB_DEFAULT_DELIMITER> myVersion;
};

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.

Thanks for everyone's time,
Kevin Grigorenko

[tom_usenet]

On Mon, 15 Sep 2003 12:57:44 -0400, "Kevin Grigorenko"
<kz****@psu.edu> wrote:

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_DELIMITER "."
class TextDB
{
private:
TextDB_Version<3, TDB_DEFAULT_DELIMITER> myVersion;
};

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.

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

char* TDB_DEFAULT_DELIMITER = "."; //has external linkage.

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

Tom

[White Wolf]

tom_usenet wrote:

error C2975: 'S' : invalid template argument for
'TextDB::TextDB_Version', constant expression expected

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

char* TDB_DEFAULT_DELIMITER = "."; //has external linkage.

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

--
WW aka Attila

[Gianni Mariani]

tom_usenet wrote:

On Mon, 15 Sep 2003 12:57:44 -0400, "Kevin Grigorenko"
<kz****@psu.edu> wrote:
.....

template<int N, char *S>
class TextDB_Version
{

....

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

char* TDB_DEFAULT_DELIMITER = "."; //has external linkage.

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

Tom

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]

"Kevin Grigorenko" <kz****@psu.edu> wrote in message
news:bk***********@f04n12.cac.psu.edu...

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.

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]

"Kevin Saff" <go********@kevin.saff.net> wrote in message
news:HL********@news.boeing.com...

"Kevin Grigorenko" <kz****@psu.edu> wrote in message
news:bk***********@f04n12.cac.psu.edu...

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.

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, '.'>

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]

Kevin Grigorenko wrote:

I'm sorry about being a little bit oblivious, but where are the archives?

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]

"Kevin Grigorenko" <kz****@psu.edu> wrote in message
news:bk***********@f04n12.cac.psu.edu...

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.
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=e...oup%3Dcomp.lan
g.c%252B%252B.*


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.

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]

On Mon, 15 Sep 2003 20:50:24 +0300, "White Wolf" <wo***@freemail.hu>
wrote:

tom_usenet wrote:

error C2975: 'S' : invalid template argument for
'TextDB::TextDB_Version', constant expression expected

[SNIP]

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

char* TDB_DEFAULT_DELIMITER = "."; //has external linkage.

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

Whoops, that was meant to be:

char TDB_DEFAULT_DELIMITER[] = "."; //has external linkage.

Tom