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Alexander Kamotsky
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 Posted: Fri Nov 19, 2004 3:31 pm Post subject: char* as template parameter |
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Hello all.
My Borland C++ Builder 5 has betrayed me once again. =)
Today I've checked this code under MinGW g++ 3.4.2 and VC++.Net and it
did not compile:
template <char *E> struct A {};
char c1 = 'a';
char c2[] = "asd"; // (1)
char* c3 = "qwe"; // (2)
A<&c1> a1; // ok
A<c2> a2; // ok!
A<c3> a3; // an error? =
Well, I even tried to rewrite (2):
const char* const c3 = "qwe"; // (3)
The same result. Compilers says something like "template parameter
should be constant expression"... But I can't catch the logic of the
acception of (1) and the rejection of (3). Would somebody explain me
this issue? =) Thanks.
PS: Borland compiler is accepting (2) but not (3). %-
--
Best regards,
Alexander mailto:alexandroid (AT) p5com (DOT) com
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Victor Bazarov
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| Posted: Sat Nov 20, 2004 10:35 am Post subject: Re: char* as template parameter |
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Alexander Kamotsky wrote:
| Quote: | My Borland C++ Builder 5 has betrayed me once again. =)
Today I've checked this code under MinGW g++ 3.4.2 and VC++.Net and it
did not compile:
template <char *E> struct A {};
char c1 = 'a';
char c2[] = "asd"; // (1)
char* c3 = "qwe"; // (2)
A<&c1> a1; // ok
A<c2> a2; // ok!
A<c3> a3; // an error? =
Well, I even tried to rewrite (2):
const char* const c3 = "qwe"; // (3)
The same result. Compilers says something like "template parameter
should be constant expression"... But I can't catch the logic of the
acception of (1) and the rejection of (3). Would somebody explain me
this issue? =) Thanks.
|
'c3' is a pointer that can change its value. '&c1' can't. Neither can
'c2'. Another problem: when you declare 'c3' a const pointer to const
char, it gets _internal_ linkage. A non-type template argument that is
a pointer to an object, has to have external linkage:
extern char const* c3 = "qwe";
A<c3> a3; // should be fine.
V
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Alex Vinokur
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| Posted: Sat Nov 20, 2004 6:21 pm Post subject: Re: char* as template parameter |
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"Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote
| Quote: | Alexander Kamotsky wrote:
My Borland C++ Builder 5 has betrayed me once again. =)
Today I've checked this code under MinGW g++ 3.4.2 and VC++.Net and it
did not compile:
template <char *E> struct A {};
char c1 = 'a';
char c2[] = "asd"; // (1)
char* c3 = "qwe"; // (2)
A<&c1> a1; // ok
A<c2> a2; // ok!
A<c3> a3; // an error? =
Well, I even tried to rewrite (2):
const char* const c3 = "qwe"; // (3)
The same result. Compilers says something like "template parameter
should be constant expression"... But I can't catch the logic of the
acception of (1) and the rejection of (3). Would somebody explain me
this issue? =) Thanks.
'c3' is a pointer that can change its value. '&c1' can't. Neither can
'c2'. Another problem: when you declare 'c3' a const pointer to const
char, it gets _internal_ linkage. A non-type template argument that is
a pointer to an object, has to have external linkage:
extern char const* c3 = "qwe";
A<c3> a3; // should be fine.
[snip] |
GNU g++ 3.4 doesn't accept that.
------ foo.cpp ------
template <char const* T>
struct Foo {};
extern char const* ch = "qwe";
int main ()
{
Foo<ch> foo; // Line#8
return 0;
}
---------------------
------ Compilation ------
$ gpp --version
gpp.exe (GCC) 3.4.1
[---omitted---]
$ gpp foo.cpp
foo.cpp: In function `int main()':
foo.cpp:8: error: `ch' is not a valid template argument
foo.cpp:8: error: it must be the address of an object with external linkage
foo.cpp:8: error: invalid type in declaration before ';' token
-------------------------
Digital Mars C++ 8.40.2 doesn't accept that too.
Borland C++ 5.5.1 and Microsoft C++ 13.00.9466 accept foo1.cpp
------ foo1.cpp ------
template <char const* T>
struct Foo {};
char const* ch = "qwe"; // no extern !!!!
int main ()
{
Foo<ch> foo;
return 0;
}
----------------------
--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Thomas Mang
Guest
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| Posted: Sun Nov 21, 2004 5:02 am Post subject: Re: char* as template parameter |
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"Alex Vinokur" <alexvn (AT) big-foot (DOT) com> schrieb im Newsbeitrag
news:308q42F2qer5hU1 (AT) uni-berlin (DOT) de...
| Quote: |
"Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote in message
news:8rond.11173$Ae.979 (AT) newsread1 (DOT) dllstx09.us.to.verio.net...
Alexander Kamotsky wrote:
My Borland C++ Builder 5 has betrayed me once again. =)
Today I've checked this code under MinGW g++ 3.4.2 and VC++.Net and
it
did not compile:
template <char *E> struct A {};
char c1 = 'a';
char c2[] = "asd"; // (1)
char* c3 = "qwe"; // (2)
A<&c1> a1; // ok
A<c2> a2; // ok!
A<c3> a3; // an error? =
Well, I even tried to rewrite (2):
const char* const c3 = "qwe"; // (3)
The same result. Compilers says something like "template parameter
should be constant expression"... But I can't catch the logic of the
acception of (1) and the rejection of (3). Would somebody explain me
this issue? =) Thanks.
'c3' is a pointer that can change its value. '&c1' can't. Neither can
'c2'. Another problem: when you declare 'c3' a const pointer to const
char, it gets _internal_ linkage. A non-type template argument that is
a pointer to an object, has to have external linkage:
extern char const* c3 = "qwe";
A<c3> a3; // should be fine.
[snip]
GNU g++ 3.4 doesn't accept that.
|
And is conforming to the Standard regarding this.
Using an extern const char[] should do the trick:
template <char const* T>
struct Foo {};
extern char const ch[] = "qwe";
int main ()
{
Foo<ch> foo;
return 0;
}
Don't ask me about details. I remember this trick from "C++ Template: The
complete guide".
Thomas
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Victor Bazarov
Guest
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| Posted: Sun Nov 21, 2004 6:54 am Post subject: Re: char* as template parameter |
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"Alex Vinokur" <alexvn (AT) big-foot (DOT) com> wrote...
| Quote: |
"Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote in message
news:8rond.11173$Ae.979 (AT) newsread1 (DOT) dllstx09.us.to.verio.net...
Alexander Kamotsky wrote:
My Borland C++ Builder 5 has betrayed me once again. =)
Today I've checked this code under MinGW g++ 3.4.2 and VC++.Net and it
did not compile:
template <char *E> struct A {};
char c1 = 'a';
char c2[] = "asd"; // (1)
char* c3 = "qwe"; // (2)
A<&c1> a1; // ok
A<c2> a2; // ok!
A<c3> a3; // an error? =
Well, I even tried to rewrite (2):
const char* const c3 = "qwe"; // (3)
The same result. Compilers says something like "template parameter
should be constant expression"... But I can't catch the logic of the
acception of (1) and the rejection of (3). Would somebody explain me
this issue? =) Thanks.
'c3' is a pointer that can change its value. '&c1' can't. Neither can
'c2'. Another problem: when you declare 'c3' a const pointer to const
char, it gets _internal_ linkage. A non-type template argument that is
a pointer to an object, has to have external linkage:
extern char const* c3 = "qwe";
A<c3> a3; // should be fine.
[snip]
GNU g++ 3.4 doesn't accept that.
[..]
|
Ought to be
extern char const c3[] = "qwe";
V
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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