Compiler problem or feature?

[NMTop40]

Ok, what I have tried to do here is probably one of those "reasonable-looking but faulty" uses of STL.

Here is my "sample" code which illustrates the problem;


template < typename T >
class H
{
private:
map< string, T > m_map;
public:
template< typename Predicate >
RemoveCopyIf( H< T >& rhs, Predicate pr )
{
m_map.clear();
remove_copy_if( rhs.m_map.begin(), rhs.m_map.end(),
inserter( m_map, m_map.begin() ), pr );
}
void Insert( const string& s, const T& t )
{
m_map[s] = t;
}
};

class J
{
private:
int m_x;
public:
J( int x=0 ) : m_x( x )
{
}
int x() const { return m_x; }

// keep default copy constructor and operator=
template < typename K >
static bool IsOdd( const std:air< K, J > & kjPair )
{
return ( (kjPair.second.m_x & 1) != 0);
}
};

template < typename K >
bool IsOdd( const std:air< K, J > & kjPair )
{
return ( ( kjPair.second.x() & 1 ) != 0 );
}


int main()
{
H< J > h1;
H< J > h2;

h1.Insert( "one", J(1) );
h1.Insert( "two", J(2) );
h1.Insert( "three", J(3) );
h1.Insert( "four", J(4) );

h2.RemoveCopyIf( h1, J::IsOdd< string > ); // internal compiler error
h2.RemoveCopyIf( h1, IsOdd< string > ); // link error if above line commented out
return 0;
}



attempting to compile with VC6

My guess is that the predicate must be a proper function pointer for the predicate and the compiler can't find one. (Is that the cause?)

btw. in my real code I used the static member function with the key filled in (non-template) after the code above failed with the internal compiler error, and I also used Nicolai M. Josuttis's asso_inserter() implementation.

I just posted this here because I'm curious (and it would have been nice to do what I was trying above)

Add the below, for example, and the above works (whether as static in the class or in global space)


bool IsOdd( const std:air< string, J > & kjPair )
{
return ( ( kjPair.second.x() & 1 ) != 0 );
}





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[Paul McKenzie]

Here is the error when compiled with the on-line compiler at http://www.comeaucomputing.com


Thank you for testing your code with Comeau C/C++!
Your Comeau C/C++ test results are as follows:

Comeau C/C++ 4.3 BETA#2 (Feb 17 2002 22:07:43)
Copyright 1988-2002 Comeau Computing. All rights reserved.

"12412.c", line 62: error: no instance of function template
"H<T>::RemoveCopyIf [with T=J]" matches the argument list
The argument types that you used are: (H<J>, <unknown-type&gt
object type is: H<J>
h2.RemoveCopyIf( h1, J::IsOdd< string > );
^

1 error detected in the compilation of "12412.c".

Tell others about http://www.comeaucomputing.com/tryitout !
In strict mode, with -tused, Compile failed
Hit the Back Button to review your code and compile options.
Go Buy Comeau C/C++!! Check out the beta for new C compiler backends

The error points to the same location as VC++. Try this on another compiler and see if it works.

Regards,

Paul McKenzie

[NMTop40]

thanks for the info about the compiler but I still don't get it.

How is template function J::IsOdd<string> any different from the standalone IsOdd with string hard-coded in?

Same line as VC++ but that gave me an "internal compiler error" which means nothing.


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[Tim Tsui]

You should do like this

typedef bool (*PF)( const pair< string , J >& );
h2.RemoveCopyIf<PF>( h1, IsOdd<string> );
h2.RemoveCopyIf<PF>( h1, J::IsOdd<string> );

[Tim Tsui]

The previous post cannot be compiled,
I think it has to be the following.

typedef bool (*PF)( const pair< string , J >& );
PF p = IsOdd<string>;
PF p1 = J::IsOdd;
h2.RemoveCopyIf( h1, p );
h2.RemoveCopyIf( h1, p1 );

[NMTop40]

I got my code to compile by using string (hard coded into J::IsOdd - well the equivalent anyway)

How do you make it work with a template function where string is a variable type?

typedefs can make the code look neater but they don't add extra functionality.


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[Tim Tsui]

type PF has declared the key type of pair is string, so that the compiler can instantiate the template.

typedef can tell the compiler the assured type .

[Paul McKenzie]

The code now also compiles with the Comeau on-line compiler with the changes made. So there must be something in typedef-ing non-trivial types if you intend to make them template arguments. Two compilers (VC++ and Comeau) coming up with the same errors, and then get corrected with the same change -- doesn't seem to be a coincidence.

BTW, the Comeau compiler adheres to the current standard much better than VC++, so there is no chance of some common compiler code being shared.

Regards,

Paul McKenzie

[NMTop40]

which surprises me. I know sometimes typedef'ing fixes problems on VC++ but didn't expect it to be part of a "standard".

I assume Tim Tsui meant that the typedefs should be made in the local function (main in the example), so that J can still have the static template member function with a general key type.


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[NMTop40]

(it still failed with internal compiler error in VC++ though )




The best things come to those who rate

[Paul McKenzie]

Hmm...I have Borland C++ Builder 5.0 at home. Maybe I'll try it there to see what it says. So far, only Comeau handles the typedef solution.

Regards,

Paul McKenzie

[Tim Tsui]

h2.RemoveCopyIf( h1, &IsOdd<string> );
h2.RemoveCopyIf( h1, &J::IsOdd<string> );
The above can be compiled under gcc.
While under SGI CC , these must be
typedef bool (*PF)( const pair< string , J >& );
h2.RemoveCopyIf<PF>( h1, IsOdd<string> );
h2.RemoveCopyIf<PF>( h1, J::IsOdd<string> );

In VC6 , they have to be
typedef bool (*PF)( const pair< string , J >& );
PF p = IsOdd<string>;
PF p1 = J::IsOdd;
h2.RemoveCopyIf( h1, p );
h2.RemoveCopyIf( h1, p1 );


It seems a templat argument of a function must be deduced from a call in VC , while it can be given in SGI CC and gcc.

[cpitis]

The internal compiler error occures very often, when there is some syntax error in template class definitions. Unfortunately, I cannot say what's wrong in this code without trying it.

It is like the static method declaration is not what VC++ compiler likes. If you modify not to be template, it works:


static bool IsOdd( const std:air< string, J > & kjPair )
{
return ( (kjPair.second.m_x & 1) != 0);
}




Your static method implementation may be considered as partial template specialization, which, unfortunately, is not supported by VC.

[NMTop40]

by using a functor class instead of a static template member function. I did, however, have a problem trying to make the nested template class a friend (so it calls a public method)


template < typename T >
class H
{
private:
map< string, T > m_map;
public:
template< typename Predicate >
void RemoveCopyIf( H< T >& rhs, Predicate pr )
{
m_map.clear();
remove_copy_if( rhs.m_map.begin(), rhs.m_map.end(),
inserter( m_map, m_map.begin() ), pr );
}
void Insert( const string& s, const T& t )
{
m_map[s] = t;
}
};

class J
{
private:
int m_x;
public:
J( int x=0 ) : m_x( x )
{
}
int x() const { return m_x; }

// keep default copy constructor and operator=

template < typename K >
struct IsOdd
{
bool operator() ( const std:air< K, J > & kjPair )
{
return ( (kjPair.second.x() & 1) != 0);
}
};
};

int main()
{
H< J > h1;
H< J > h2;

h1.Insert( "one", J(1) );
h1.Insert( "two", J(2) );
h1.Insert( "three", J(3) );
h1.Insert( "four", J(4) );

h2.RemoveCopyIf( h1, J::IsOdd< string >() );
return 0;
}



This compiles fine on Visual C++, althugh I haven't tried running it outputting
the results (of h2)

(Anyone resolve how I could make the inner template class a friend of J ? )


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[NMTop40]

I got it to work in the end by using a nested template functor class instead of a function. I haven't yet worked out how to make this class a friend. I tried

template < typename K > friend J::IsOdd< K >;



but got a compiler error.


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