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nick · Guest

Hi All!

I have code:
#include <iostream>
#include <map>
#include <set>

using namespace std;

template < template
C<T> make_cnt(const T& param) {
C<T> r; r.insert(param); return r;
}
template < template
C<K,V> make_cnt(const pair<K,V>& param) {
C<K,V> r; r.insert(param); return r;
}

int main() {
cout << make_cnt cout << make_cnt }

Under gcc 3.3.2 compiles and works fine.
Under free MS VC++ toolkit 2003 I get such error:

make_cnt.cpp(17) : error C3208: 'make_cnt' : template parameter list
for class template 'std::set' does not match template parameter list
for template template parameter 'C'
make_cnt.cpp(12) : see declaration of 'make_cnt'
make_cnt.cpp(17) : error C3208: 'make_cnt' : template parameter list
for class template 'std::set' does not match template parameter list
for template template parameter 'C'
make_cnt.cpp( Cool

: see declaration of 'make_cnt'

Where is problem?

Thanks.
Dmitriy.

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Victor Bazarov · Guest

nick wrote:

nick · Guest

Oops, I found typo:

werasmus · Guest

Hi Dmitriy,

Gcc 3.3.2 probably uses some form of relaxed matching of template
template parameters (basically boils down to ignoring the defaults).
The definition of set (from SGI STL documentation, as well as the MS
VC++ .net 2003 sourcecode) is something like:

set<Key, Compare, Alloc> or...
template<
class _Kty, //Key
class _Pr = less<_Kty>, //Compare, default being less
class _Alloc = allocator<_Kty> > //Allocator
class set...

Clearly, from this it can be seen that set is a class template
requiring 3 template parameters (I think this is std).

Your template function make_cnt...

/////
template < template
C<T> make_cnt(const T& param)
{
C<T> r; r.insert(param); return r;
}
/////

....requires a class template having only one template parameter.

According to "C++ Templates, The Complete Guide" by Josuttis and
Vandevoorde, p111.

"A template template argument must be a class template with parameters
that exactly match the parameters of the template template parameter it
substitutes."

std::set has 3 parameters, make_cnt only specifies one, therefore no
match. Because 2 of std::set params have default values, Gnu seems to
ignore them, whilst the other compiler does not (which is more iaw.
what the std specifies). The fix is to add 2 parameters with defaults
to the template template param, making their defaults the same as that
of set's defaults (this is defined in the std as being std::less and
std::allocator).

Therefore:

template <
template <
class key,
class compare = std::less class alloc = std::allocator<key> > class contT,
class T >
contT<T> make_cnt(const T& param)
{
contT<T> r; //Uses defaults of contT
r.insert(param);
return r;
}

As they state in "C++ templates", this is not satisfactory, but it
handles the std container types...

Cheers,

Werner

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Ismail Pazarbasi · Guest

As far as I can see, yes, there are problems. What about C, K and V?
You have not specified them. There is no specialization either. Also,
set is a template, too. But you have not specified that. Finally, how
do you construct "pair" like that? Something similar to your code could
be:

template<typename T, typename U>
class Dummy
{
};

template<typename T, template class V>
C<K, V> make_cnt(const pair<K,V>& param)
{
C<K, V> c;
return c;
}

make_cnt<set(pair<int, int>(1, 2));

Ismail

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Carl Barron · Guest

In article <1118740519.603029.225240 (AT) f14g2000cwb (DOT) googlegroups.com>,
nick <nick (AT) bel (DOT) ru> wrote:

nick · Guest

But what about this:
class C {};
template<class A, class B = C> class TAB {};
template<template class BOO {};
template<template class OK {};

BOO<TAB> booboo; // no error in g++, but in VC++ error
OK<TAB> okiedokie;

Who is right?

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Victor Bazarov · Guest

nick wrote: