473,382 Members | 1,476 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes to post your question to a community of 473,382 software developers and data experts.

repost: template function specialization & basic question...

FIRST POST

Hi All: I'm trying to write a simple specialization before moving on to
something a bit more complex - always a good idea in my case, at least. :o)

I'm trying to adapt the example from Stroustrup, 3rd ed., The C++
Programming Language p. 344

I'm using MSDev 6.0 in case that's an issue. Here's the source...

/* begin snippet */
/* specializations.hpp */

#include <functional>
using namespace std;

class foo
{
public:
int m_foo;
foo():m_foo(0){}
};

template<> bool less<foo>(const foo& afoo, const foo& bfoo)
{
return afoo.m_foo < bfoo.m_foo;
}

/* specializationstest.cpp */
#include "specializations.hpp"

int main()
{
foo f,g;
g.m_foo = 8;

bool done = less<foo>(f,g);

return done;
}

Compiling...
specializationstest.cpp

error C2935: 'less<class foo>' : template-class-id redefined as a global
function

error C2912: explicit specialization; 'bool __cdecl less<foo>(const class
foo &,const class foo &)' is not a function template
see declaration of 'less<class foo>'

error C2661: 'less<class foo>::less<class foo>' : no overloaded function
takes 2 parameters
Error executing cl.exe.

specializations.exe - 3 error(s), 0 warning(s)
/* end snippet */
Any help would be appreciated.

I'm trying to do this with the intent of specializing an iterator operator*
and & in the stuff I'm working on right now.

i.e.
something like

modified by OP:
had been glibly using foo as the template args for the lists
in the example code below - was 3 am, gimme a break :o)
this makes more sense

template<typename T>
class foo<T>
{
public:
list<T> foolist;
list<T*>p_foolist;
list<int> somestupidlist;
typedef list<T*>::iterator iterator;
}

template<> T& foo::iterator::operator*(){/*...*/}

where dereferencing the iterator will return an object reference, not a
pointer to the object which is what the list contains.
so, any suggestions here would be welcome as well...

And thanks to all those who replied to my prior questions...

regards,
L.

SECOND POST

Hi folks: Can anyone tell me what the difference is between the two
functions below? In particular why does the function that returns a
reference give me warnings about returning the address of a temporary
variable, whereas the prior function compiles without any errors or warnings

template<typename foo>
class fooset<foo>
{
...
list<foo> fooList;
typedef list<foo>::iterator iterator;
...

public:

iterator begin()
{
return fooList.begin();
}

/*
iterator& begin()
{
return fooList.begin();
}
*/
iterator
};

thanks,
and regards,
L.
Jul 22 '05 #1
6 1749
"NKOBAYE027" <NK********@Rogers.Com> wrote in
news:Y9*********************@twister01.bloor.is.ne t.cable.rogers.com:
Hi All: I'm trying to write a simple specialization before moving on
to something a bit more complex - always a good idea in my case, at
least. :o)

I'm trying to adapt the example from Stroustrup, 3rd ed., The C++
Programming Language p. 344

I'm using MSDev 6.0 in case that's an issue. Here's the source...

(...)
template<> bool less<foo>(const foo& afoo, const foo& bfoo)
{
return afoo.m_foo < bfoo.m_foo;
}


1. std::less is not a function, but a functor. It provides an overloaded
operator(), so it can be 'invoked' like a function.

2. Function templates cannot be specialized.

3. Why don't you write an operator < overload for your foo class?

Cheers,
bartek
Jul 22 '05 #2
> 1. std::less is not a function, but a functor. It provides an overloaded
operator(), so it can be 'invoked' like a function.

2. Function templates cannot be specialized.
Function templates can be specialized. They cannot be
_partially_specialized_. (OTOH, MSVC6 doesn't support partial template
specialization anyway.)

3. Why don't you write an operator < overload for your foo class?

Cheers,
bartek

Jul 22 '05 #3
"Adam H. Peterson" <ah**@email.byu.edu> wrote in
news:c1***********@acs2.byu.edu:
Function templates can be specialized. They cannot be
_partially_specialized_. (OTOH, MSVC6 doesn't support partial
template specialization anyway.)


Oops, of course right. Sorry for the confusion.

Cheers,
bartek
Jul 22 '05 #4
Thanks bartek.

The reason I don't overload the < is because I'm not aiming to do that. This
was a first attempt at specializing a function - and I failed miserably. :o)
I see now that I was missing the original definition of the template
function 'less' provided in the text (on the same page, no less...npi). So,
I guess, what I should do is simply ask if specializing a list<class
T*>::iterator is possible such that dereferencing the iterator will return
the object pointed to by T* instead of T*. If so how?

Sorry for the confusion - it's all mine!

I've since tried the specialized version and it works when I appropriately
define it and the original, but would still greatly appreciate any input on
the iterator issue mentioned above.

regards,
L.
Jul 22 '05 #5
NKOBAYE027 wrote:
Thanks bartek.

The reason I don't overload the < is because I'm not aiming to do that. This
was a first attempt at specializing a function - and I failed miserably. :o)
I see now that I was missing the original definition of the template
function 'less' provided in the text (on the same page, no less...npi). So,
I guess, what I should do is simply ask if specializing a list<class
T*>::iterator is possible such that dereferencing the iterator will return
the object pointed to by T* instead of T*. If so how?


I don't think you can do this with template specialization. In my
experience (and I think in all cases) when you specialize a function or
class, you can't alter the signatures of the functions -- what
parameters they take, their constness, their return type.

It looks like you're trying to create something akin to a polymorphic
container. You can probably search on that -- there's been lots of
discussion, research, etc. on that topic.

Personally, I would start to solve such a problem by using composition
with a standard list<>. Something like:
#include <list>

template<typename T>
class ptr_list {
public:
class iterator;
// Yada yada
private:
std::list<T*> data;
};

template<typename T>
class ptr_list<T>::iterator {
public:
T &operator*() const {return **it;}
// Yada yada
private:
std::list<T*>::iterator it;
};
It would take some work to get the full iterator interface working, but
you may not need all of that depending on your usage.

Adam H. Peterson
Jul 22 '05 #6
Thanks, Adam - that was exactly what I was looking for. Your pointer about
specializations not being able to change function signatures is what I
needed. So, I do need to reinvent the wheel, so-to-speak, for the iterator
class. I can see no other way than the one you mentioned - and I'd already
considered that but was hoping for an easier way out...Ugh...c'est la vie,
non?

regards,
L.

"Adam H. Peterson" <ah**@email.byu.edu> wrote in message
news:c1***********@acs2.byu.edu...
NKOBAYE027 wrote:
Thanks bartek.

The reason I don't overload the < is because I'm not aiming to do that. This was a first attempt at specializing a function - and I failed miserably. :o) I see now that I was missing the original definition of the template
function 'less' provided in the text (on the same page, no less...npi). So, I guess, what I should do is simply ask if specializing a list<class
T*>::iterator is possible such that dereferencing the iterator will return the object pointed to by T* instead of T*. If so how?


I don't think you can do this with template specialization. In my
experience (and I think in all cases) when you specialize a function or
class, you can't alter the signatures of the functions -- what
parameters they take, their constness, their return type.

It looks like you're trying to create something akin to a polymorphic
container. You can probably search on that -- there's been lots of
discussion, research, etc. on that topic.

Personally, I would start to solve such a problem by using composition
with a standard list<>. Something like:
#include <list>

template<typename T>
class ptr_list {
public:
class iterator;
// Yada yada
private:
std::list<T*> data;
};

template<typename T>
class ptr_list<T>::iterator {
public:
T &operator*() const {return **it;}
// Yada yada
private:
std::list<T*>::iterator it;
};
It would take some work to get the full iterator interface working, but
you may not need all of that depending on your usage.

Adam H. Peterson

Jul 22 '05 #7

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

17
by: Paul MG | last post by:
Hi Template partial specialization always seems like a fairly straightforward concept - until I try to do it :). I am trying to implement the input sequence type (from Stroustrup section...
2
by: SainTiss | last post by:
Hi, If you've got a template class with lots of methods, and then you've got a type which works with the template, except for one method... What you need to do there is specialize the...
6
by: Dave | last post by:
Hello all, Consider this function template definition: template<typename T> void foo(T) {} If foo is never called, this template will never be instantiated. Now consider this explicit...
7
by: Siemel Naran | last post by:
Hi. I have a function template <class InputIter, class OutputIter> void f(InputIter begin, InputIter end, OutputIter result); With c of type char* and cc of type const char*, the code...
1
by: Alfonso Morra | last post by:
if I have a class template declared as ff: (BTW is this a partial specialization? - I think it is) template <typename T1, myenum_1 e1=OK, my_enum_2=NONE> class A { public: A(); virtual...
9
by: Marek Vondrak | last post by:
Hello. I have written the following program and am curious why it prints "1" "2". What are the exact effects of explicitly providing function template parameters at the call? Is the second...
2
by: Barry | last post by:
The following code compiles with VC8 but fails to compiles with Comeau online, I locate the standard here: An explicit specialization of any of the following:
4
by: David Sanders | last post by:
Hi, I have a class with an integer template parameter, taking values 1, 2 or 3, and a function 'calc' in that class which performs calculations. Some calculations need only be performed if the...
5
by: chgans | last post by:
Hi all, I'm having difficulties with some template static member, especially when this member is a template instance, for example: ---- template<typename T> class BaseT { public: static...
0
isladogs
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 3 Apr 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome former...
0
by: ryjfgjl | last post by:
In our work, we often need to import Excel data into databases (such as MySQL, SQL Server, Oracle) for data analysis and processing. Usually, we use database tools like Navicat or the Excel import...
0
by: Charles Arthur | last post by:
How do i turn on java script on a villaon, callus and itel keypad mobile phone
0
by: aa123db | last post by:
Variable and constants Use var or let for variables and const fror constants. Var foo ='bar'; Let foo ='bar';const baz ='bar'; Functions function $name$ ($parameters$) { } ...
0
by: ryjfgjl | last post by:
If we have dozens or hundreds of excel to import into the database, if we use the excel import function provided by database editors such as navicat, it will be extremely tedious and time-consuming...
0
by: ryjfgjl | last post by:
In our work, we often receive Excel tables with data in the same format. If we want to analyze these data, it can be difficult to analyze them because the data is spread across multiple Excel files...
0
by: emmanuelkatto | last post by:
Hi All, I am Emmanuel katto from Uganda. I want to ask what challenges you've faced while migrating a website to cloud. Please let me know. Thanks! Emmanuel
1
by: nemocccc | last post by:
hello, everyone, I want to develop a software for my android phone for daily needs, any suggestions?
0
by: Hystou | last post by:
There are some requirements for setting up RAID: 1. The motherboard and BIOS support RAID configuration. 2. The motherboard has 2 or more available SATA protocol SSD/HDD slots (including MSATA, M.2...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.