template instantiation question
Consider
Code:
# include <iostream>
template < typename value_type >
struct your_policy {
static
value_type write_to_register ( value_type value ) {
return ( 1 );
}
static
value_type read_from_register ( value_type value ) {
return ( 2 );
}
}; // your_policy
template < typename value_type >
struct our_policy {
static
value_type write_to_register ( value_type value ) {
return ( 3 );
}
static
value_type read_from_register ( value_type value ) {
return ( 4 );
}
}; // our_policy
template < typename unsigned_type, typename policy >
struct lets_see {
public: // only for testing
lets_see ()
{}
static
unsigned_type & dummy ( void ) {
static unsigned_type x;
return ( x );
}
public:
lets_see & operator= ( unsigned_type const val ) {
dummy() = policy< unsigned_type >::write_to_register( val );
return ( *this );
}
operator unsigned_type ( void ) const {
unsigned_type result =
policy< unsigned_type >::read_from_register( dummy() ) ;
return ( result) ;
}
};
I'd like for the user to be able to specify the desired policy during template instantiation. For instance:
Code:
int main ( void ) {
lets_see < unsigned int, your_policy < unsigned int > > a;
lets_see < unsigned int, our_policy < unsigned int > > b;
a.dummy() = 5;
std::cout << a << '\n';
std::cin.get();
}
I'm getting errors though that I'm unsure how to resolve. How would I achieve this?
Re: template instantiation question
Quote:
Originally Posted by
mop65715
Code:
lets_see & operator= ( unsigned_type const val ) {
dummy() = policy< unsigned_type >::write_to_register( val );
return ( *this );
}
operator unsigned_type ( void ) const {
unsigned_type result =
policy< unsigned_type >::read_from_register( dummy() ) ;
return ( result) ;
}
The policy in this context doesn't need a template argument. That is the error that Comeau C++ reports.
The lets_see struct shouldn't be templatizing the policy as your code is doing. The lets_see struct has no idea what the policy is, but you're giving it template arguments within the lets_see struct, as if lets_see knows more about the policy argument than it should.
Those arguments were already taken care of in main() when you instantiated the template. This code compiles correctly using Comeau:
Code:
# include <iostream>
template < typename value_type >
struct your_policy {
static
value_type write_to_register ( value_type value ) {
return ( 1 );
}
static
value_type read_from_register ( value_type value ) {
return ( 2 );
}
}; // your_policy
template < typename value_type >
struct our_policy {
static
value_type write_to_register ( value_type value ) {
return ( 3 );
}
static
value_type read_from_register ( value_type value ) {
return ( 4 );
}
}; // our_policy
template < typename unsigned_type, typename policy >
struct lets_see
{
public: // only for testing
lets_see ()
{}
static
unsigned_type & dummy ( void )
{
static unsigned_type x;
return ( x );
}
public:
lets_see & operator=( unsigned_type const val )
{
dummy()=policy::write_to_register( val );
return ( *this );
}
operator unsigned_type ( void ) const {
unsigned_type result =
policy::read_from_register( dummy() ) ;
return ( result) ;
}
};
int main ()
{
lets_see < unsigned int, your_policy < unsigned int > > a;
lets_see < unsigned int, our_policy < unsigned int > > b;
a.dummy() = 5;
std::cout << a << '\n';
std::cin.get();
}
Regards,
Paul McKenzie
Re: template instantiation question
Quote:
Originally Posted by
Paul McKenzie
The policy in this context doesn't need a template argument. That is the error that Comeau C++ reports.
For starters, thanks Paul. Two questions:
a) Can you show me a context where it does matter?
b) On an slightly unrelated note. Consider the case where the layout defined in a requirements document is as follows:
[ 0..3 ] (Hd 1 = 0x0)
[ 4..7 ] (Hd 2 = 0x0)
[8..11] (Hd 3 =0x0)
[12..15 ] (Hd 4 =0x2).
Where the HD representation is the field names and MSB is bit 0. When I read this data from _raw_ memory I obtain 0x0002. Trouble I'm on a little endian machine so as part of my policy class the raw value gets converted to 0x0200. In doing so HD 2 is now a 2 which does not align with the requirement. That tells me the requirements document claim of MSB being bit 0 is FALSE. To me if the implementation that's transmitting the data is sending the data with MSB being bit 0 (big endian) and I'm on a little endian machine. That translation will result in HD 4 on an LE not being set to a 2 but to a zero and HD2 will equate to a 2. Am I off on this?
