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Conditional templates

Started by Kest Aug 13, 2006 at 7:03 AM 6 replies 850+ views

Original Post

147

Aug 13, 2006 at 7:03 AM

Very quick simple question. Is it possible to 'ask' the compiler if a template type has a specific function, and generate different code if it does? Example:

template <typename type>
void SaveObject(const type &obj,fileptr *file)
{
   #iftemplatehasfunction< type, void Save(fileptr*) const >
      obj.Save( file );
   #else
      file->write( &obj, sizeof( obj ) );
   #endif
}

If not, is there another way around this type of situation? I use a templated function for resource management. But recently I realized that I need to allow some resource types to 'compare' themselves to other resources of the same type to see if the resources match completely regarding specifics for that type (IE, a skinmesh might need to compare morph targets). The only alternative would be to go back and force every object that can be loaded as a resource to implement a 'return true' version of this function. Thanks for any information.

sturmritter

Aug 13, 2006 at 7:05 AM

you could use #ifndef and #define compiler directives to solve this problem I believe.

- Sturm

ToohrVyk

238

ToohrVyk

Aug 13, 2006 at 7:07 AM

The closest I can get to this is to specialize the template for a class that has said Save method:

template <typename type> void SaveObject(const type &obj,fileptr *file){      file->write( &obj, sizeof( obj ) );}class Saveable { public: virtual void Save(fileptr* file) = 0; };template <> void SaveObject<Saveable>(const Saveable & obj,fileptr * file) {      obj.Save( file );}

This way, all classes inherited from Saveable will use the second one, while all other types will use the first one.

ToohrVyk

238

ToohrVyk

Aug 13, 2006 at 7:08 AM

Quote:
Original post by sturmritter
you could use #ifndef and #define compiler directives to solve this problem I believe.

There are no #ifndef and #define compiler directives, sorry.

sturmritter

Aug 13, 2006 at 7:21 AM

Meant Pre-processor directive. Sorry for confusion. I bag all directives into one lump.

Anyways, you could put your templates into a header file, and use the Pre-processor directives.

For example:

template
void SaveObject(const type &obj,fileptr *file)
{
#ifndef _Template_Specific_Function_
#define _Template_Specific_Function_
< type, void Save(fileptr*) const >
obj.Save( file );
#else
file->write( &obj, sizeof( obj ) );
#endif
}

Anyways, just a thought, I haven't tested it out.

- Sturm

ToohrVyk

238

ToohrVyk

Aug 13, 2006 at 7:24 AM

Quote:
Original post by sturmritter
Meant Pre-processor directive. Sorry for confusion. I bag all directives into one lump.

Anyways, you could put your templates into a header file, and use the Pre-processor directives.

No problem. Say, how does your code work?

sturmritter

Aug 13, 2006 at 7:44 AM

Well, I haven't actually run into Kest's problem myself, so it's more of a logic exercise for me.

the method above would seem rather cumbersome as you would have to change the #define line to fit whatever function you were looking for at compile time.

I'm not sure how to do it dynamically during run-time.

Perhaps using equal_to template? it's c-standard library template

[Source]Standard C++ Library Reference   equal_toSee Also<functional> MembersA binary predicate that tests whether a value of a specified type is equal to another value of that type.template<class Type>   struct equal_to : public binary_function<Type, Type, bool>    {      bool operator()(         const Type& _Left,          const Type& _Right      ) const;   };Parameters_Left The left operand of type Type in the equality to be tested. _Right The right operand of type Type in the equality to be tested. Return Valuetrue if _Left equals _Right; false if _Left does not equal _Right.RemarksThe objects of type Type must be equality comparable. This requires that the operator== defined on the set of objects satisfies the mathematical properties of an equivalence relation All the built-in numeric and pointer types satisfy this requirement.Example// functional_equal_to.cpp// compile with: /EHsc#include <vector>#include <functional>#include <algorithm>#include <iostream>using namespace std;int main( ){   vector <double> v1, v2, v3 ( 6 );   vector <double>::iterator Iter1, Iter2, Iter3;      int i;   for ( i = 0 ; i <= 5 ; i+=2 )   {      v1.push_back( 2.0 *i );      v1.push_back( 2.0 * i + 1.0 );   }   int j;   for ( j = 0 ; j <= 5 ; j+=2 )   {      v2.push_back( - 2.0 * j );      v2.push_back( 2.0 * j + 1.0 );   }   cout << "The vector v1 = ( " ;   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )      cout << *Iter1 << " ";   cout << ")" << endl;   cout << "The vector v2 = ( " ;   for ( Iter2 = v2.begin( ) ; Iter2 != v2.end( ) ; Iter2++ )      cout << *Iter2 << " ";   cout << ")" << endl;   // Testing for the element-wise equality between v1 & v2   transform ( v1.begin( ),  v1.end( ), v2.begin( ), v3.begin ( ),       equal_to<double>( ) );   cout << "The result of the element-wise equal_to comparison\n"      << " between v1 & v2 is: ( " ;   for ( Iter3 = v3.begin( ) ; Iter3 != v3.end( ) ; Iter3++ )      cout << *Iter3 << " ";   cout << ")" << endl;}OutputThe vector v1 = ( 0 1 4 5 8 9 )The vector v2 = ( 0 1 -4 5 -8 9 )The result of the element-wise equal_to comparison between v1 & v2 is: ( 1 1 0 1 0 1 )[/SOURCE]

Perhaps the operands of Type Type could be member functions rather than values?

Enigma

188

Enigma

Aug 13, 2006 at 7:48 AM

You might be able to use SFINAE, depending on your compiler, i.e.:

#include <iostream>template < typename Type, void (Type:: *)() >struct HasMember{};template < typename Type >void DoSaveObject(Type const & object, HasMember< Type, &Type::Save > const *){	std::cout << "call save member function\n";}template < typename Type >void DoSaveObject(Type const & object, ...){	std::cout << "call alternative save function\n";}template < typename Type >void SaveObject(Type const & object){	DoSaveObject< Type >(object, 0);}struct HasSave{	void Save(){}};struct DoesntHaveSave{};int main(){	SaveObject(HasSave());	SaveObject(DoesntHaveSave());	SaveObject(int());}

Appears to work on VC++ 8.0. Fails on Borland 5.6.4 and GCC 3.3.1, but from what I've read it should work on GCC 3.4.x and presumably GCC 4.x too.

Σnigma

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