Hello,
I created the following composite class using mpl::inherit_linearly:
template <class T>
struct DataField {
T data;
}
typedef mpl::inherit_linearly<
mpl::vector< int, int, int>
mpl::inherit< DataField<_2>, _1 >
Int3Composite;
Int3Composite int3comp;
Since the type of each data field is an int, I cannot use the type of the field to access the
data in the field.
Q: Is there a way to write a template (call it get_field) to access the data using numeric
Hi!
you could use an mpl::pair type to produce a static mapping of int value to type:
mpl::pair, int>
This is a little bit ugly, to write such a big construct, so you can write the following construct
template
struct pair_c : mpl::pair
{};
Actually pair_c could be your DataField struct, so you can add the T instance to it:
template
struct pair_c : mpl::pair
{
T dataField;
};
Now you can define a vector of pair types as follows:
mpl::vector, pair_c<1, int>, pair_c<2, int> > my_vector;
we use in this sample pair_c to make any type unique, dependent on the position in the vector.
The next step would be to define the class with linear inheritance:
template<class Seq>
struct linear_inherited :
boost::mpl::inherit_linearly
<
Seq
, boost::mpl::inherit
>::type
{};
But now you would need the access function. I suggest to integrate this function as a
linear_inherited member:
template<class Seq>
struct linear_inherited :
boost::mpl::inherit_linearly
<
Seq
, boost::mpl::inherit
>::type
{
template<long N>
typename boost::mpl::at_c::type::second& nth_field()
{
typedef typename boost::mpl::at_c::type* ptr_type;
ptr_type ptr = static_cast(this);
return ptr->dataField;
}
};
When this function is a member you save some effor on passing the sequence type to its
specialization, otherwise it will not know which type is meant.
Let me explain this function:
mpl has the at metafunction, which can access the Nth element of a static vector. Here this
function is used to calculate the result_type (in our case it is always int, but we want to be
generic ;), so you can make pair_c<0, double>, pair_c<1, SomeStruct> etc.). The at metafunction
returns the type which was in the vector at this position, in our case this type is pair_c. Now we
need to access the field type T. mpl::pair is defined as
template
struct pair
{
typedef X first;
typedef Y second;
};
so, the type of our datafield in pair_c is of type second. Therefore the function nth_field
returns reference to second.
In the function itself we use the static_cast operator to calculate the offset to our datafield
within the class. And then we return the the field contained there.
Below is the full listing I used to test and compile the scenario:
linear_inh_test.h
---------- START -------------------
#ifndef __OM_LINEAR_INHERITANCE_TEST_H__
#define __OM_LINEAR_INHERITANCE_TEST_H__
#include
#include
#include
#include
#include
#include
#include
#include
template
struct pair_c : boost::mpl::pair
{
T dataField;
};
template<class Seq>
struct linear_inherited :
boost::mpl::inherit_linearly
<
Seq
, boost::mpl::inherit
>::type
{
template<long N>
typename boost::mpl::at_c::type::second& nth_field()
{
typedef typename boost::mpl::at_c::type* ptr_type;
ptr_type ptr = static_cast(this);
return ptr->dataField;
}
};
#endif
__________ END ___________________
main.cpp
---------- START -------------------
#include "linear_inh_test.h"
#include <cassert>
#include
int main()
{
using namespace boost;
typedef mpl::vector, pair_c<1, int>, pair_c<2, int> > my_vector;
linear_inherited test_instance;
int& ref0 = test_instance.nth_field<0>();
ref0 = 0;
int& ref1 = test_instance.nth_field<1>();
ref1 = 1;
int& ref2 = test_instance.nth_field<2>();
ref2 = 2;
assert(
test_instance.nth_field<0>()
!= test_instance.nth_field<1>()
!= test_instance.nth_field<2>()
);
return 0;
}
__________ END ___________________
With Kind Regards,
Ovanes Markarian
On Tue, January 9, 2007 21:51, Khalil Shalish wrote:
position in the type sequence. Something like this:
int field_0 = get_field<0>(in3comp); //This returns the first int data field. int field_1 =
get_field<1>(int3comp);
I am very new to C++ template programming and any help on this will be very much appreciated.
Thank you,
Khalil Shalish
kshalish@austin.rr.com
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With Kind Regards,
Ovanes
