C++Talk.NET

G. Wang · Guest

Hi,

I tried but failed to find a solution to a real problem that I
experienced. I am hoping that your gurus out there can help me out or
point me to the right direction.

I striped down the real code to the barebone to demonstrate the
problem. I know the posting is still long, so I appreciate your time
and efforts. You can find the code at the end of the post.

Here is the situation. I have a meta class A that has a member
variable whose type is an instance of a meta class B determined by a
meta function ( get_B in the code ). Any instance of meta class B
logically owns a value whose type is an instance of meta class A. If B
holds the value by pointer as demonstrated in Line B, everything works
fine, except that I pay the performance penalty of dynamic memory
allocation.

I don't want to pay the performance penalty, so I want to hold the
value inside B whenever possible, as demonstrated by line A. If I do
so, I will have compile time error. The instantiation of A<0> requires
the compiler to instantiate A<1>, which requires the instantiation of
B_Hold_A_ByValue<1>, which requires the full definition of A<1>, which
we donot have because we are currently instantiate A<1>!.

Here is my question. How can I apply SFINAE principle to determine
case-by-case on when B should hold the value by pointer, when directly
by value? If SFINAE won't help me here, what technique would you
recommend for the situation?

I tried various things on VC 7.1. All failed. I donot want to claim
that the compiler does not support SFINAE properly without hearing
from your guys first.

Thanks a lot and have a good day. Hope to hear from you soon.

===========================================================================

template<int N>
struct determine_N
{
enum { value = 1 };
};

template<int N>
struct get_B;

template< int N >
struct A
{
enum { value = determine_N<N>::value };

typename get_B<value>::type content;
};

template<int N>
struct B_Hold_A_ByValue
{
struct self {};

A<N> m_content;

A<N>& Content()
{
return m_content;
}
};

template<int N>
struct B_Hold_A_ByPointer
{
struct self {};

std::auto_ptr< A m_pContent;

A<N>& Content()
{
if( m_pContent.get() == NULL )
{
m_pContent = std::auto_ptr< A( new A<N> );
}

return *m_pContent;
}
};

template<int N>
struct get_B
{
// how to use SFINAE to determine if "type" should be
// B_Hold_A_ByValue<N> or
// B_Hold_A_ByPointer<N>?

// typedef B_Hold_A_ByValue<N> type; // <----- line A
typedef B_Hold_A_ByPointer };

void main()
{
A<0> a;
}

============================================================================

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Michiel Salters · Guest

[email]visionsoft (AT) hotmail (DOT) com[/email] (G. Wang) wrote in message news:<59199ce9.0404201501.1566e36c (AT) posting (DOT) google.com>...