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Template resolve

Started by surabaya Oct 6, 2002 at 9:43 AM 6 replies 1.1k views

Original Post

Oct 6, 2002 at 9:43 AM

Hi I tried:

    
...
Functor func; // a functor object

typedef std::vector<long> Array;
Array arr;
...
std::sort<Array::iterator, Functor&>( arr.begin(), arr.end(), func );

Didn't compile on VS.NET -> "too many template arguments." But why? std::sort does have a template version that takes an iterator and a predicate type. Could somebody help? I am trying to avoid passing the predicate by value for efficiency consideration. edit: the template parameters inside the less than greater than brackets were not shown. added the source tag section [edited by - surabaya on October 6, 2002 11:00:31 AM] [edited by - surabaya on October 6, 2002 11:01:54 AM] [edited by - surabaya on October 6, 2002 11:10:54 AM]

surabaya

Author

surabaya

Oct 6, 2002 at 10:19 AM

Please help.

Fruny

203

Fruny

Oct 6, 2002 at 3:23 PM

quote:

I am trying to avoid passing the predicate by value for efficiency consideration.

DirkGerrits

Oct 6, 2002 at 6:01 PM

STL is built on the fact that most function pointers are small and always takes them by value. If you don''t like that, do this:

std::sort( arr.begin(), arr.end(), boost::cref(func) );

See also: http://www.boost.org/libs/bind/ref.html

Regards,
Dirk Gerrits

surabaya

Author

surabaya

Oct 6, 2002 at 8:38 PM

quote:

Premature optimisation. What is the size of your functor ?

Agreed. But still I don''t find the reason for the program not to compile.

By the way I still don''t know the functor''s size since I needed to know whether pass-by-ref is possible or not before designing it.

I''ll try the boost solution.

Thanks,

Xai

344

Xai

Oct 6, 2002 at 8:50 PM

pass by ref is NOT possible for functors in the standard librsry, hence the need for boost:cref template ...

the standard supports ref and value versions of functions where absolutely necessary to allow implementation, but not in many cases where it would only be an efficiency issue, because there would then need to be twice as many versions in the standard, each version by ref and by value ...

surabaya

Author

surabaya

Oct 8, 2002 at 11:39 AM

quote:

pass by ref is NOT possible for functors in the standard librsry, hence the need for boost:cref template ...

  
template<typename T>
void Func( T arg ) { ...... }

class SomeClass { ........ };

//.....

SomeClass obj;

Func( obj ); // compiles

Func<SomeClass&>( obj ); // compiles and pass by ref as expected


// below doesn''t compile (too many template args")

typedef std::vector<int> Arr;
std::sort<Arr::iterator, SomeClass&>( arr.begin(), arr.end(), obj );

Without bothering to discuss STL, std::sort is basically just another function template; why can''t it take my template arguments just like normal functions?

Fruny

203

Fruny

Oct 8, 2002 at 11:54 AM

That being said, have you tried directly passing a reference instead of forcing the template type, and seen what error you got ?


Functor f;
Functor &rf = f;
std::sort( foo, bar, rf );

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