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Novice Question: Function Template Specialization

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.
Jul 22 '05 #1
7 4837
CoolPint wrote:
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?


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;
}

Jul 22 '05 #2
On 19 Nov 2003 07:44:49 -0800, co******@yahoo.co.uk (CoolPint) wrote:
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 [snip]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?


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
Jul 22 '05 #3
CoolPint wrote:
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)
This template is for const references !
{
cout << "Template Called: ";
return a < b ? b: a;
}

#include <cstring>
template<>
const char * maximum<const char *>(const char * a, const char * b)
This has no references !

Try:

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?


See comments above.

Jul 22 '05 #4
On Wed, 19 Nov 2003 16:07:16 +0000, tom_usenet
<to********@hotmail.com> wrote:
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)


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
Jul 22 '05 #5
> Try:

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


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
Jul 22 '05 #6
CoolPint wrote:
Try:

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

{
cout << "Normal called: ";
return strcmp(a,b) < 0 ? b: a;
}

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


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)
{
....
}

Jul 22 '05 #7
Gianni Mariani <gi*******@mariani.ws> wrote in message news:<bp********@dispatch.concentric.net>...
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)
{
...
}


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.
Jul 22 '05 #8

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