Novice Question: Function Template Specialization

[CoolPint]

While I was testing my understanding of Functioin Template features by
playing with simple function templates, I got into a problem which I
cannot understand. I would be very grateful if someone offered me a
kind explanation. TIA

Function template and specialization below works fine:

template <typename T>
T maximum (T a, T b)
{
cout << "Template Called: ";
return a < b ? b: a;
}

#include <cstring>
template<>
const char * maximum<const char *>(const char * a, const char * b)
{
cout << "Normal called: ";
return strcmp(a,b) < 0 ? b: a;
}

BUT if I change the template to accept const reference to T like
below:
template <typename T>
const T &maximum (const T & a, const T & b)
{
cout << "Template Called: ";
return a < b ? b: a;
}

#include <cstring>
template<>
const char * maximum<const char *>(const char * a, const char * b)
{
cout << "Normal called: ";
return strcmp(a,b) < 0 ? b: a;
}

The compiler (g++ v3.2) generates the following error message.

tt.cpp:25: template-id `maximum<const char*>' for `const char*
maximum(const
char*, const char*)' does not match any template declaration

Can anyone explain what the problem is?

P.S I am trying to learn the features by playing around. So I would
really appreciate an explanation as to what is causing the problem
rather than "You should overload the template with normal function",
etc types of replies. Thank you very much in advance.

[Rolf Magnus]

Re: Novice Question: Function Template Specialization

CoolPint wrote:
[color=blue]
> While I was testing my understanding of Functioin Template features by
> playing with simple function templates, I got into a problem which I
> cannot understand. I would be very grateful if someone offered me a
> kind explanation. TIA
>
> Function template and specialization below works fine:
>
> template <typename T>
> T maximum (T a, T b)
> {
> cout << "Template Called: ";
> return a < b ? b: a;
> }
>
> #include <cstring>
> template<>
> const char * maximum<const char *>(const char * a, const char * b)
> {
> cout << "Normal called: ";
> return strcmp(a,b) < 0 ? b: a;
> }
>
> BUT if I change the template to accept const reference to T like
> below:
> template <typename T>
> const T &maximum (const T & a, const T & b)
> {
> cout << "Template Called: ";
> return a < b ? b: a;
> }
>
> #include <cstring>
> template<>
> const char * maximum<const char *>(const char * a, const char * b)
> {
> cout << "Normal called: ";
> return strcmp(a,b) < 0 ? b: a;
> }
>
> The compiler (g++ v3.2) generates the following error message.
>
> tt.cpp:25: template-id `maximum<const char*>' for `const char*
> maximum(const
> char*, const char*)' does not match any template declaration
>
> Can anyone explain what the problem is?[/color]

Your specialization doesn't fit the template. The template has
references to const as parameters and return type, but the
specialization uses a non-const non-reference type. Try:

#include <cstring>
template<>
const char * const & maximum<const char *>(const char * const & a,
const char * const & b)
{
cout << "Normal called: ";
return strcmp(a,b) < 0 ? b: a;
}

[tom_usenet]

Re: Novice Question: Function Template Specialization

On 19 Nov 2003 07:44:49 -0800, coolpint@yahoo. co.uk (CoolPint) wrote:
[color=blue]
>While I was testing my understanding of Functioin Template features by
>playing with simple function templates, I got into a problem which I
>cannot understand. I would be very grateful if someone offered me a
>kind explanation. TIA[/color]
[snip][color=blue]
>BUT if I change the template to accept const reference to T like
>below:
>template <typename T>
>const T &maximum (const T & a, const T & b)
>{
> cout << "Template Called: ";
> return a < b ? b: a;
>}
>
>#include <cstring>
>template<>
>const char * maximum<const char *>(const char * a, const char * b)
>{
> cout << "Normal called: ";
> return strcmp(a,b) < 0 ? b: a;
>}
>
>The compiler (g++ v3.2) generates the following error message.
>
>tt.cpp:25: template-id `maximum<const char*>' for `const char*
>maximum(cons t
> char*, const char*)' does not match any template declaration
>
>Can anyone explain what the problem is?[/color]

A specialization has to have the same arguments as the source
template, otherwise it isn't a specialization of that template.
Substituting T=const char* gives:

#include <cstring>
template<>
inline const char*& maximum<const char*>(const char*& a, const char*&
b)
{
cout << "Normal called: ";
return strcmp(a,b) < 0 ? b: a;
}

If you inline it, the reference parameters shouldn't add any overhead
(and you can define it in a header).

Tom

[Gianni Mariani]

Re: Novice Question: Function Template Specialization

CoolPint wrote:[color=blue]
> While I was testing my understanding of Functioin Template features by
> playing with simple function templates, I got into a problem which I
> cannot understand. I would be very grateful if someone offered me a
> kind explanation. TIA
>
> Function template and specialization below works fine:
>
> template <typename T>
> T maximum (T a, T b)
> {
> cout << "Template Called: ";
> return a < b ? b: a;
> }
>
> #include <cstring>
> template<>
> const char * maximum<const char *>(const char * a, const char * b)
> {
> cout << "Normal called: ";
> return strcmp(a,b) < 0 ? b: a;
> }
>
> BUT if I change the template to accept const reference to T like
> below:
> template <typename T>
> const T &maximum (const T & a, const T & b)[/color]

This template is for const references !
[color=blue]
> {
> cout << "Template Called: ";
> return a < b ? b: a;
> }
>
> #include <cstring>
> template<>
> const char * maximum<const char *>(const char * a, const char * b)[/color]

This has no references !

Try:

const char * &maximum<con st char *>(const char * &a, const char * &b)
[color=blue]
> {
> cout << "Normal called: ";
> return strcmp(a,b) < 0 ? b: a;
> }
>
> The compiler (g++ v3.2) generates the following error message.
>
> tt.cpp:25: template-id `maximum<const char*>' for `const char*
> maximum(const
> char*, const char*)' does not match any template declaration
>
> Can anyone explain what the problem is?[/color]

See comments above.

[tom_usenet]

Re: Novice Question: Function Template Specialization

On Wed, 19 Nov 2003 16:07:16 +0000, tom_usenet
<tom_usenet@hot mail.com> wrote:
[color=blue]
>A specialization has to have the same arguments as the source
>template, otherwise it isn't a specialization of that template.
>Substituting T=const char* gives:
>
>#include <cstring>
>template<>
>inline const char*& maximum<const char*>(const char*& a, const char*&
>b)[/color]

Doh! As Rolf says, it doesn't give that at all, but:

template<>
inline const char* const& maximum<const char*>(const char* const& a,
const char* const& b)

Tom

[CoolPint]

Re: Novice Question: Function Template Specialization: Still having the problem

> Try:[color=blue]
>
> const char * &maximum<con st char *>(const char * &a, const char * &b)
>[color=green]
> > {
> > cout << "Normal called: ";
> > return strcmp(a,b) < 0 ? b: a;
> > }
> >[/color][/color]

Thank you for answering my question. All the replies suggested the
same solution (to have the parameters to be const reference to make
them same as
the original template function).

I think I tried doing that before posting the question. Anyhow, I
tried again after your replies but unfortunately the compiler still
complains!

tt.cpp:26: template-id `maximum<const char*>' for `const char*&
maximum(const
char*&, const char*&)' does not match any template declaration

Is it my compiler or am I still missing something? I would appreciate
further answer/explanation on this problem. TIA

[Gianni Mariani]

Re: Novice Question: Function Template Specialization: Still havingthe problem

CoolPint wrote:[color=blue][color=green]
>>Try:
>>
>>const char * &maximum<con st char *>(const char * &a, const char * &b)
>>
>>[color=darkred]
>>>{
>>> cout << "Normal called: ";
>>> return strcmp(a,b) < 0 ? b: a;
>>>}
>>>[/color][/color]
>
>
> Thank you for answering my question. All the replies suggested the
> same solution (to have the parameters to be const reference to make
> them same as
> the original template function).
>
> I think I tried doing that before posting the question. Anyhow, I
> tried again after your replies but unfortunately the compiler still
> complains!
>
> tt.cpp:26: template-id `maximum<const char*>' for `const char*&
> maximum(const
> char*&, const char*&)' does not match any template declaration
>
> Is it my compiler or am I still missing something? I would appreciate
> further answer/explanation on this problem. TIA[/color]

Tom's second reply got it right.

i.e.
template <typename T>
const T & maximum (const T & a, const T & b);

template<>
const char * const & maximum<const char *>(const char * const & a, const
char * const & b)
{
....
}

[CoolPint]

Re: Novice Question: Function Template Specialization: Still having the problem

Gianni Mariani <gi2nospam@mari ani.ws> wrote in message news:<bpj8eg$ll t@dispatch.conc entric.net>...[color=blue]
> Tom's second reply got it right.
>
> i.e.
> template <typename T>
> const T & maximum (const T & a, const T & b);
>
> template<>
> const char * const & maximum<const char *>(const char * const & a, const
> char * const & b)
> {
> ...
> }[/color]

Thank you for that. I tried various combinations and I missed his
second reply. It works. Thank you all those who spent time to answer
my silly question.

Now I see that specialization has to specialize the very same format
off "function parameter" of the ogiginal function template.