Re: Using type traits with const

From:

SG <s.gesemann@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 10 May 2012 11:48:26 -0700 (PDT)

Message-ID:

<251ed977-3ef6-4941-a4b9-4ea0fa39fd58@n13g2000vbl.googlegroups.com>

On May 10, 10:17 am, David Barrett-Lennard wrote:

In the following example I want foo() to select the implementation based
on a trait is_blah which is true for struct X. When foo() is passed a
const X*, VS2008 and VS2010 select the implementation assuming is_blah is
false. Is this the expected behaviour?

Yes.

template< typename T >
struct is_blah { static const bool value = false; };

struct X {};
template<> struct is_blah<X> { static const bool value = true; };

Since you did not provide another specialization for const X we have

is_blah<X>::value --> true
is_blah<const X>::value --> false

template< typename T > void foo(T* p);

void test()
{
X* p1 = 0;
foo(p1); // true

Correct. T=X ==> is_blah<T>::value=true

const X* p2 = 0;
foo(p2); // false
}

Correct. T=const X ==> is_blah<T>::value=false

Cheers!
SG

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