97 lines
2.8 KiB
Plaintext
97 lines
2.8 KiB
Plaintext
[/
|
|
(C) Copyright Edward Diener 2012
|
|
Distributed under the Boost Software License, Version 1.0.
|
|
(See accompanying file LICENSE_1_0.txt or copy at
|
|
http://www.boost.org/LICENSE_1_0.txt).
|
|
]
|
|
|
|
[section:tti_detail_has_data Introspecting inner data]
|
|
|
|
The TTI macro [macroref BOOST_TTI_HAS_DATA] introspects the data of a class.
|
|
The data can be member data or static member data.
|
|
|
|
BOOST_TTI_HAS_DATA macro takes a single parameter, which is the name
|
|
of an inner member data or static member data, whose existence
|
|
the programmer wants to check. The macro generates a metafunction
|
|
called "has_data_'name_of_inner_data'".
|
|
|
|
The metafunction can be invoked by passing it the enclosing type
|
|
to introspect and the type of the data.
|
|
|
|
The metafunction returns a single type called 'type', which is a
|
|
boost::mpl::bool_. As a convenience the metafunction
|
|
returns the value of this type directly as a compile time bool constant
|
|
called 'value'. This is true or false depending on whether the inner
|
|
data, of the specified type, exists or not.
|
|
|
|
[heading Generating the metafunction]
|
|
|
|
You generate the metafunction by invoking the macro with the name
|
|
of an inner data member:
|
|
|
|
BOOST_TTI_HAS_DATA(AData)
|
|
|
|
generates a metafunction called 'has_data_AStaticMemberData' in the current scope.
|
|
|
|
[heading Invoking the metafunction]
|
|
|
|
You invoke the metafunction by instantiating the template with an enclosing
|
|
type to introspect and the type of the data. A return value called
|
|
'value' is a compile time bool constant.
|
|
|
|
has_data_AData<Enclosing_Type,Data_Type>::value
|
|
|
|
[heading Examples]
|
|
|
|
First we generate metafunctions for various inner data names:
|
|
|
|
#include <boost/tti/has_data.hpp>
|
|
|
|
BOOST_TTI_HAS_DATA(data1)
|
|
BOOST_TTI_HAS_DATA(data2)
|
|
BOOST_TTI_HAS_DATA(data3)
|
|
|
|
Next let us create some user-defined types we want to introspect.
|
|
|
|
struct AClass
|
|
{
|
|
};
|
|
struct Top
|
|
{
|
|
int data1;
|
|
static AClass * data2;
|
|
};
|
|
struct Top2
|
|
{
|
|
static long data1;
|
|
Top data3;
|
|
};
|
|
|
|
Finally we invoke our metafunction and return our value.
|
|
This all happens at compile time, and can be used by
|
|
programmers doing compile time template metaprogramming.
|
|
|
|
has_data_data1<Top,int>::value; // true
|
|
has_data_data1<Top,long>::value; // false
|
|
has_data_data1<Top2,int>::value; // false
|
|
has_data_data1<Top2,long>::value; // true
|
|
|
|
has_data_data2<Top,AClass *>::value; // true
|
|
has_data_data2<Top,int *>::value; // false
|
|
|
|
has_data_data3<Top2,int>::value; // false
|
|
has_data_data3<Top2,Top>::value; // true;
|
|
|
|
[heading Metafunction re-use]
|
|
|
|
The macro encodes only the name of the data for
|
|
which we are searching and the fact that we are introspecting
|
|
for data within an enclosing type.
|
|
|
|
Because of this, once we create our metafunction for
|
|
introspecting an inner data by name, we can reuse
|
|
the metafunction for introspecting any enclosing type, having
|
|
any inner data type, for that name.
|
|
|
|
[endsect]
|