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A probably simple question about pick semantics [message #34794]

Mon, 12 December 2011 16:58

peek
Messages: 13
Registered: March 2010

Promising Member

I wanted to pass a Vector by reference to a template function, but the Vector is lost Sad

:

.cpp file

Vector<double> v;
v << 1;

Test(v);

.h file

template <class T>
void Test(T v)
{
	// Nothing done
}

After calling to Test(), Vector v contents disappear.

When debugging you can see than before jumping to Test(), the compiler call to Vector pick constructor:

Vector(pick_ Vector& v)          { Pick(v); }

and before leaving Test(), the compiler calls Vector destructor:

~Vector()  { ....

However if Test is called with a pointer to Vector, v contents remains.

Test(&v)

What could we do to call the templated function without loosing Vector contents?

Thank you Smile

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Re: A probably simple question about pick semantics [message #34795 is a reply to message #34794]

Mon, 12 December 2011 17:08

cbpporter
Messages: 1428
Registered: September 2007

Ultimate Contributor

Use a reference parameter: Test(T& v) or Test(const T& v).

Pick semantics kick in on a copy operation. Even without pick semantics using reference would be advised, since otherwise you would be needlessly copying around data when passed to the function.

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Re: A probably simple question about pick semantics [message #34915 is a reply to message #34795]

Fri, 16 December 2011 22:35

peek
Messages: 13
Registered: March 2010

Promising Member

Hello cbpporter

I have tried it doing it by reference but the problem remains Sad

template <class T>
void Test(T &v)
{
	// Nothing done
}

In fact I forgot to put the "&" in the sample code.

Is there a way to avoid this copy in a Vector passed by reference to a templated function?

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Re: A probably simple question about pick semantics [message #34917 is a reply to message #34915]

Fri, 16 December 2011 22:49

mirek
Messages: 14290
Registered: November 2005

Ultimate Member

peek wrote on Fri, 16 December 2011 16:35

Hello cbpporter

I have tried it doing it by reference but the problem remains Sad

template <class T>
void Test(T &v)
{
	// Nothing done
}

In fact I forgot to put the "&" in the sample code.

Is there a way to avoid this copy in a Vector passed by reference to a templated function?

More complex (read "compilable") example would be needed...

In general, this has to work, because such templates are widespread in the U++ code (read "theide would not run" Smile

Mirek

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Re: A probably simple question about pick semantics [message #34918 is a reply to message #34917]

Sat, 17 December 2011 00:26

peek
Messages: 13
Registered: March 2010

Promising Member

Sorry... there is no problem.

Thank you all !!

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