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Lars Jordan
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 Posted: Thu Feb 16, 2006 3:06 am Post subject: how to avoid to explicitly specify template arguments? |
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Hello,
I am thinking about the following issue. When writing a function template which
takes a parameter I want pass this parameter as efficiently as possible. (e.g.
using some traits template to figure out whether its a primitive type or not).
Something like this comes to my mind:
template<typename T>
void func( Loki::TypeTraits<T>::ParameterType t )
{ /* do something with t */ }
This is written fast, but unfortunately it can't be used so conveniently,
because the dependent type is a non-deduced context. And thus I have to specify
the type as a template argument. I don't want that. OK, the next idea: I could
try to miinimize copying by using a wrapper around the parameter type:
// please: this could also be a wrapper provided by boost or so
template<typename T>
struct MySmallSizedWrapper
{
MySmallSizedWrapper( T const& t ) : pT_( &t ) {}
operator T const& () const { return *pT_; }
private:
T const* pT_;
};
template<typename T>
void func( MySmallSizedWrapper<T> t )
{ /* do something with t */ }
My hope was, that I am now able to pass a T which is then converted to
MySmallSizedWrapper<T> (because I haven't made the constructor explicit), but
this doesn't work because conversions are not applied when matching the function
template.
What I don't want to do is neither to specify the template parameter explicitly
(as before) nor do I want to mention MySmallSizedWrapper when using the function
template and despite all this I wan't to pass the parameter in the most
efficient way. My question at this point is: Is there a way to do this?
thx in advance
Lars
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Maxim Yegorushkin
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| Posted: Sat Feb 18, 2006 1:06 am Post subject: Re: how to avoid to explicitly specify template arguments? |
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Lars Jordan wrote:
| Quote: | Hello,
I am thinking about the following issue. When writing a function template which
takes a parameter I want pass this parameter as efficiently as possible. (e.g.
using some traits template to figure out whether its a primitive type or not).
Something like this comes to my mind:
template<typename T
void func( Loki::TypeTraits<T>::ParameterType t )
{ /* do something with t */ }
|
You may create two function templates. One for built-in types, the
other for UDT. Something similar to this:
template<class T>
typename boost::enable_if<boost::is_fundamental<T>, void>::type
foo(T t);
template<class T>
typename boost::disable_if<boost::is_fundamental<T>, void>::type
foo(T const& t);
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werasm
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| Posted: Sat Feb 18, 2006 1:06 am Post subject: Re: how to avoid to explicitly specify template arguments? |
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| Quote: | What I don't want to do is neither to specify the template parameter explicitly
(as before) nor do I want to mention MySmallSizedWrapper when using the function
template and despite all this I wan't to pass the parameter in the most
efficient way. My question at this point is: Is there a way to do this?
|
You could make use of the fact that template parameters can me deduced
from function template arguments. Therefore:
#include <iostream>
template<typename T>
struct MySmallSizedWrapper
{
MySmallSizedWrapper( T const& t ) : pT_( &t ) {}
operator T const& () const { return *pT_; }
private:
T const* pT_;
};
template <class T>
MySmallSizedWrapper<T> mySmallSizedWrapper( T const& t )
{
return MySmallSizedWrapper<T>( t );
}
template<typename T>
void func( MySmallSizedWrapper<T> t )
{ /* do something with t */ }
int main()
{
func( mySmallSizedWrapper( 5 ) );
std::cin.get();
return 0;
}
I compiled something similar under VC++ 7.1.3 and it seemed fine.
Usually only one copy of MySmallSizedWrapper would be made (the
temporary required for the argument of func) due to optimizations. The
solution therefore should be efficient.
Regards,
Werner
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Lars Jordan
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| Posted: Sun Feb 19, 2006 12:06 pm Post subject: Re: how to avoid to explicitly specify template arguments? |
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werasm wrote:
| Quote: | You could make use of the fact that template parameters can me deduced
from function template arguments. Therefore:
|
I know about template argument deduction. Unfortunately it doesn't free me of
the "syntactic noise" when using the 'doSomething()' function. I'd still have to
write down something including 'MySmallSizedWrapper' (which doesn't really
contribute to the problem solution.
On the other hand, the Maxim Yegorushkin's solution _does_ remove this noise
since I don't have to specify anything but the parameters that 'doSomething()'
has to deal with. Nevertheless, thank you very much for your reply.
best regards
Lars
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