Comparator template argument in recursive templates?

  • Thread starter Steven T. Hatton
  • Start date
S

Steven T. Hatton

While thunbing through _C++ Templates, The Complete Guide_ (reckon I aught
to read it?) I came across a discussion of using templates to "unroll"
loops. I thought that looked like a good idea, so I decided to try it on
some code I'm writing. The idea is to use template objects to perform what
happens in traditional nested for() loops.

for(int row = 0; row < num_rows; row++)
{
for(int col = 0; col < num_cols; col++)
{
operation(a1[row][col], a2[row][col]);
}
}

This code (I believe) accomplishes my objective for '==':
/* gnuBoilerplate.hh */

#ifndef _GNU_BOILERPLATE_HH_
#define _GNU_BOILERPLATE_HH_
/***************************************************************************
* Copyright (C) 2004 by Steven T. Hatton *
* (e-mail address removed) *
* *
* This program is free software; you can redistribute it and/or modify *
* it under the terms of the GNU General Public License as published by *
* the Free Software Foundation; either version 2 of the License, or *
* (at your option) any later version. *
* *
* This program is distributed in the hope that it will be useful, *
* but WITHOUT ANY WARRANTY; without even the implied warranty of *
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the *
* GNU General Public License for more details. *
* *
* You should have received a copy of the GNU General Public License *
* along with this program; if not, write to the *
* Free Software Foundation, Inc., *
* 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. *
***************************************************************************/
#endif
/* Array2Equal.hh */
#ifndef STH_VMATH_ARRAY2EQUAL_HH_
#define STH_VMATH_ARRAY2EQUAL_HH_
#include "gnuBoilerplate.hh"
#include "ArrayEqual.hh"

namespace sth{
namespace vmath{

template <unsigned NUMBER_OF_ROWS,unsigned NUMBER_OF_COLUMNS, typename
T >
class Array2Equal{
public:
static bool equal(const T a1[][NUMBER_OF_COLUMNS], const T a2[
[NUMBER_OF_COLUMNS])
{
return ArrayEqual<NUMBER_OF_COLUMNS, T>::equal(a1[NUMBER_OF_ROWS-1], a
[NUMBER_OF_ROWS-1])
&& Array2Equal<NUMBER_OF_ROWS - 1, NUMBER_OF_COLUMNS, T>::equal(a1, a2);
}
};

template <unsigned NUMBER_OF_COLUMNS, typename T >
class Array2Equal<1, NUMBER_OF_COLUMNS, T>{
public:
static bool equal(const T a1[][NUMBER_OF_COLUMNS], const T a2[
[NUMBER_OF_COLUMNS])
{
return ArrayEqual<NUMBER_OF_COLUMNS, T>::equal(a1[0], a2[0]);
}
};

template<unsigned NUMBER_OF_ROWS, unsigned NUMBER_OF_COLUMNS, typename
T >
inline bool equal2Array(const T a1[][NUMBER_OF_COLUMNS], const T a2[
[NUMBER_OF_COLUMNS])
{
return Array2Equal<NUMBER_OF_ROWS, NUMBER_OF_COLUMNS, T >::equal(a1,
a2);
}

}
}

#endif

/* ArrayEqual.hh */
#ifndef ARRAYEQUAL_HH
#define ARRAYEQUAL_HH
#include "gnuBoilerplate.hh"

namespace sth{
namespace vmath{

template <unsigned NUMBER_OF_ELEMENTS, typename T>
class ArrayEqual{
public:
static bool equal(const T* a1, const T* a2)
{
return (*a2 == *a1)
&& ArrayEqual<NUMBER_OF_ELEMENTS - 1, T>::equal(a1 + 1, a2 + 1);
}
};

template <typename T >
class ArrayEqual<1,T>{
public:
static bool equal(const T* a1, const T* a2)
{
return *a2 == *a1;
}
};

template <unsigned NUMBER_OF_ELEMENTS, typename T >
inline bool equalArray(const T* a1, const T* a2)
{
return ArrayEqual<NUMBER_OF_ELEMENTS, T>::equal(a1, a2);
}

}
}

#endif

I tried very hard to pass a comparator function object as an additional
argument to the templates. For example I created

template <typename T>
struct Equal{
bool operator()(T a, T b) {return a==b;}
};

But no matter how I tried to use it, I kept getting errors saying there was
no funciton Equal(T a, T b);

This is an example:

#ifndef ARRAYCOMPARE_HH
#define ARRAYCOMPARE_HH
#include "gnuBoilerplate.hh"

#include <functional>

namespace sth{
namespace vmath{

template <typename T>
struct Equal: public std::binary_function<bool,T,T>
{
bool operator()( T a, T b) const {return a == b;}
};

template <unsigned NUMBER_OF_ELEMENTS, typename T, typename COMPARATOR>
class ArrayCompare{
public:
static bool compare(const T* a1, const T* a2)
{
return COMPARATOR(*a2, *a1)
&& ArrayCompare<NUMBER_OF_ELEMENTS - 1, T, COMPARATOR>::compare(a1 + 1,
a2 + 1);
}
};

template <typename T, typename COMPARATOR >
class ArrayCompare<1,T, COMPARATOR>{
public:
static bool compare(const T* a1, const T* a2)
{
return COMPARATOR(*a2, *a1);
}
};

template <unsigned NUMBER_OF_ELEMENTS, typename T, typename COMPARATOR >
inline bool compareArray(const T* a1, const T* a2)
{
return ArrayCompare<NUMBER_OF_ELEMENTS, T, COMPARATOR>::compare(a1,
a2);
}

}
}

#endif

/* Array2Compare.hh */
#ifndef STH_VMATH_ARRAY2COMPARE_HH_
#define STH_VMATH_ARRAY2COMPARE_HH_
#include "gnuBoilerplate.hh"

#include "ArrayCompare.hh"

#include <functional>
namespace sth{
namespace vmath{

template <unsigned NUMBER_OF_ROWS,unsigned NUMBER_OF_COLUMNS, typename
T, typename COMPARATOR >
class Array2Compare{
public:
static bool compare(const T a1[][NUMBER_OF_COLUMNS], const T a2[
[NUMBER_OF_COLUMNS])
{
return ArrayCompare<NUMBER_OF_COLUMNS, T, COMPARATOR>::compare(a
[NUMBER_OF_ROWS-1], a2[NUMBER_OF_ROWS-1])
&& Array2Compare<NUMBER_OF_ROWS - 1, NUMBER_OF_COLUMNS, T,
COMPARATOR>::compare(a1, a2);
}
};

template <unsigned NUMBER_OF_COLUMNS, typename T, typename COMPARATOR >
class Array2Compare<1, NUMBER_OF_COLUMNS, T, COMPARATOR>{
public:
static bool compare(const T a1[][NUMBER_OF_COLUMNS], const T a2[
[NUMBER_OF_COLUMNS])
{
return ArrayCompare<NUMBER_OF_COLUMNS, T, COMPARATOR>::compare(a1[0], a
[0]);
}
};

template<unsigned NUMBER_OF_ROWS, unsigned NUMBER_OF_COLUMNS, typename
T, typename COMPARATOR >
inline bool compare2Array(const T a1[][NUMBER_OF_COLUMNS], const T a2[
[NUMBER_OF_COLUMNS])
{
return Array2Compare<NUMBER_OF_ROWS, NUMBER_OF_COLUMNS, T, COMPARATOR
::compare(a1, a2);
Click to expand...
}
}
}

#endif

/* main.cc */
#include "gnuBoilerplate.hh"
#include <sth/vmath/Array2Compare.hh>
#include <iostream>

using sth::vmath::compare2Array;
using sth::vmath::Equal;

using std::cout;
using std::boolalpha;
using std::noboolalpha;
main()
{
float a1[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
float b1[3][3] = {{10,20,30},{40,50,60},{70,80,90}};
cout
<< boolalpha
(a1,b1) << "\n"
Click to expand...
<< noboolalpha;
return 0;
}
//EOF

Wed Oct 06 15:05:53:> g++ -o compare main.cc -I/home/hattons/code/c++
/home/hattons/code/c++/sth/vmath/ArrayCompare.hh:20: error: no matching
function for call to `sth::vmath::Equal<float>::Equal(const float&, const
float&)'
/home/hattons/code/c++/sth/vmath/ArrayCompare.hh:11: error: candidates are:
sth::vmath::Equal<float>::Equal()
/home/hattons/code/c++/sth/vmath/ArrayCompare.hh:11: error:
sth::vmath::Equal<float>::Equal(const sth::vmath::Equal<float>&)


Suggestions?
--
"If our hypothesis is about anything and not about some one or more
particular things, then our deductions constitute mathematics. Thus
mathematics may be defined as the subject in which we never know what we
are talking about, nor whether what we are saying is true." - Bertrand
Russell
 
J

John Harrison

Steven T. Hatton said:
While thunbing through _C++ Templates, The Complete Guide_ (reckon I aught
to read it?) I came across a discussion of using templates to "unroll"
loops. I thought that looked like a good idea, so I decided to try it on
some code I'm writing. The idea is to use template objects to perform
what
happens in traditional nested for() loops.

for(int row = 0; row < num_rows; row++)
{
for(int col = 0; col < num_cols; col++)
{
operation(a1[row][col], a2[row][col]);
}
}
[snip]

I tried very hard to pass a comparator function object as an additional
argument to the templates. For example I created

template <typename T>
struct Equal{
bool operator()(T a, T b) {return a==b;}
};

But no matter how I tried to use it, I kept getting errors saying there
was
no funciton Equal(T a, T b);
Click to expand...

I think its a simple error (but haven't tried compiling anything). I think
you are mixing object with type.

This is an example:

#ifndef ARRAYCOMPARE_HH
#define ARRAYCOMPARE_HH
#include "gnuBoilerplate.hh"

#include <functional>

namespace sth{
namespace vmath{

template <typename T>
struct Equal: public std::binary_function<bool,T,T>
{
bool operator()( T a, T b) const {return a == b;}
};

template <unsigned NUMBER_OF_ELEMENTS, typename T, typename
COMPARATOR>
class ArrayCompare{
public:
static bool compare(const T* a1, const T* a2)
{
return COMPARATOR(*a2, *a1)
Click to expand...

COMPARATOR is a type, so you must construct an object and then call your
operator(), i.e.

return COMPARATOR()(*a2, *a1)

john
 
S

Steven T. Hatton

John said:
COMPARATOR is a type, so you must construct an object and then call your
operator(), i.e.

return COMPARATOR()(*a2, *a1)
Click to expand...

Wow! That is SO obvious (in retrospect). I should have caught it. Thanks!

This compiles and runs, producing the expected results. I still need to
think it all through. I believe it may be traversing backwards. That's
fine for symmetrical operations, but if I want a lexicographic comparrison,
it would not perform as expected.

I did catch another stupid mistake while fixing the one you pointed out. I
now only have one call to COMPARATOR cmp; return cmp(*a2, *a1);

/* ArrayCompare.hh */
#ifndef ARRAYCOMPARE_HH
#define ARRAYCOMPARE_HH
#include "gnuBoilerplate.hh"
#include <functional>

namespace sth{
namespace vmath{

template <typename T>
struct Equal: public std::binary_function<bool,T,T>
{
bool operator()( T a, T b) const {return a == b;}
};

template <unsigned NUMBER_OF_ELEMENTS, typename T, typename COMPARATOR>
class ArrayCompare{
public:
static bool compare(const T* a1, const T* a2)
{
return ArrayCompare<1,T,COMPARATOR>::compare(a2, a1)
&& ArrayCompare<NUMBER_OF_ELEMENTS - 1, T, COMPARATOR>::compare(a1 + 1,
a2 + 1);
}
};

template <typename T, typename COMPARATOR >
class ArrayCompare<1,T, COMPARATOR>{
public:
static bool compare(const T* a1, const T* a2)
{
COMPARATOR cmp;
return cmp(*a2, *a1);
}
};

template <unsigned NUMBER_OF_ELEMENTS, typename T, typename COMPARATOR >
inline bool compareArray(const T* a1, const T* a2)
{
return ArrayCompare<NUMBER_OF_ELEMENTS, T, COMPARATOR>::compare(a1,
a2);
}

}
}

#endif


/* Array2Compare.hh */
#ifndef STH_VMATH_ARRAY2COMPARE_HH_
#define STH_VMATH_ARRAY2COMPARE_HH_
#include "ArrayCompare.hh"
#include "gnuBoilerplate.hh"

#include <functional>
namespace sth{
namespace vmath{

template <unsigned NUMBER_OF_ROWS,unsigned NUMBER_OF_COLUMNS, typename
T, typename COMPARATOR >
class Array2Compare{
public:
static bool compare(const T a1[][NUMBER_OF_COLUMNS], const T a2[
[NUMBER_OF_COLUMNS])
{
return ArrayCompare<NUMBER_OF_COLUMNS, T, COMPARATOR>::compare(a
[NUMBER_OF_ROWS-1], a2[NUMBER_OF_ROWS-1])
&& Array2Compare<NUMBER_OF_ROWS - 1, NUMBER_OF_COLUMNS, T,
COMPARATOR>::compare(a1, a2);
}
};

template <unsigned NUMBER_OF_COLUMNS, typename T, typename COMPARATOR >
class Array2Compare<1, NUMBER_OF_COLUMNS, T, COMPARATOR>{
public:
static bool compare(const T a1[][NUMBER_OF_COLUMNS], const T a2[
[NUMBER_OF_COLUMNS])
{
return ArrayCompare<NUMBER_OF_COLUMNS, T, COMPARATOR>::compare(a1[0], a
[0]);
}
};

template<unsigned NUMBER_OF_ROWS, unsigned NUMBER_OF_COLUMNS, typename
T, typename COMPARATOR >
inline bool compare2Array(const T a1[][NUMBER_OF_COLUMNS], const T a2[
[NUMBER_OF_COLUMNS])
{
return Array2Compare<NUMBER_OF_ROWS, NUMBER_OF_COLUMNS, T, COMPARATOR
::compare(a1, a2);
Click to expand...
}
}
}

#endif


/*main.cc*/
#include "gnuBoilerplate.hh"
#include <sth/vmath/ArrayCompare.hh>
#include <sth/vmath/Array2Compare.hh>
#include <iostream>

using sth::vmath::compareArray;
using sth::vmath::compare2Array;
using sth::vmath::Equal;

using std::cout;
using std::boolalpha;
using std::noboolalpha;
int main()
{

float a[3] = {1,2,3};
float b[3] = {10,20,30};
Equal<float> ef;
cout
<< boolalpha
<< "compareArray(a,b): " << compareArray<3,float, Equal<float> >(a,b) <<
"\n"
<< noboolalpha;


float a1[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
float a2[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
float b1[3][3] = {{10,20,30},{40,50,60},{70,80,90}};

cout
<< boolalpha
(a1,b1) << "\n"
Click to expand...
(a1,a2) << "\n"
Click to expand...
<< noboolalpha;

return 0;
}


/* gnuBoilerplate.hh */

#ifndef _GNU_BOILERPLATE_HH_
#define _GNU_BOILERPLATE_HH_
/***************************************************************************
 *   Copyright (C) 2004 by Steven T. Hatton                                *
 *   (e-mail address removed)                                            *
 *                                                                         *
 *   This program is free software; you can redistribute it and/or modify  *
 *   it under the terms of the GNU General Public License as published by  *
 *   the Free Software Foundation; either version 2 of the License, or     *
 *   (at your option) any later version.                                   *
 *                                                                         *
 *   This program is distributed in the hope that it will be useful,       *
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of        *
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
 *   GNU General Public License for more details.                          *
 *                                                                         *
 *   You should have received a copy of the GNU General Public License     *
 *   along with this program; if not, write to the                         *
 *   Free Software Foundation, Inc.,                                       *
 *   59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.             *
 ***************************************************************************/
#endif

Now I need to generalize the code for n dimensional arrays...
--
"If our hypothesis is about anything and not about some one or more
particular things, then our deductions constitute mathematics. Thus
mathematics may be defined as the subject in which we never know what we
are talking about, nor whether what we are saying is true." - Bertrand
Russell
 

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