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Introduces enable_if_type

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enable_if_type allow to perform SFINAE check on the existence
of a dependent type.

It has been used here and there in various boost library but it's
useful enough to warrant an autonomous existence.
This commit is contained in:
Joel Falcou 2015-07-24 20:13:32 +02:00
parent d9b28783e8
commit ad513c1641
3 changed files with 53 additions and 1 deletions
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20
doc/enable_if.qbk
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@ -304,6 +304,26 @@ depends on the template arguments of the class. Note that
again, the second argument to `enable_if` is not needed; the
default (`void`) is the correct value.
The `enable_if_type` template is usable this scenario but instead of
using a type traits to enable or disable a specialization, it use a
SFINAE context to check for the existence of a dependent type inside
its parameter. For example, the following structure extracts a dependent
`value_type` from T if and only if `T::value_type` exists.
``
template <class T, class Enable = void>
class value_type_from
{
typedef T type;
};
template <class T>
class value_type_from<T, typename enable_if_type<typename T::value_type>::type>
{
typedef typename T::value_type type;
};
``
[endsect]
[section Overlapping enabler conditions]

9
include/boost/core/enable_if.hpp
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@ -23,6 +23,11 @@
namespace boost
{
template<typename T, typename R=void>
struct enable_if_type
{
typedef R type;
};
template <bool B, class T = void>
struct enable_if_c {
@ -80,6 +85,10 @@ namespace boost {
template <typename T>
struct enable_if_does_not_work_on_this_compiler;
template<typename T, typename R=void>
struct enable_if_type : enable_if_does_not_work_on_this_compiler<T>
{ };
template <bool B, class T = detail::enable_if_default_T>
struct enable_if_c : enable_if_does_not_work_on_this_compiler<T>
{ };

25
test/eif_partial_specializations.cpp
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@ -14,6 +14,7 @@
#include <boost/type_traits/is_arithmetic.hpp>
#include <boost/detail/lightweight_test.hpp>
using boost::enable_if_type;
using boost::enable_if_c;
using boost::disable_if_c;
using boost::enable_if;
@ -46,9 +47,28 @@ struct tester2<T, typename disable_if<is_arithmetic<T> >::type> {
BOOST_STATIC_CONSTANT(bool, value = false);
};
template <class T, class Enable = void>
class tester3
{
typedef T type;
BOOST_STATIC_CONSTANT(bool, value = false);
};
template <class T>
class tester3<T, typename enable_if_type<typename T::value_type>::type>
{
typedef typename T::value_type type;
BOOST_STATIC_CONSTANT(bool, value = true);
};
struct sample_value_type
{
typedef float***& value_type;
};
int main()
{
BOOST_TEST(tester<int>::value);
BOOST_TEST(tester<double>::value);
@ -61,6 +81,9 @@ int main()
BOOST_TEST(!tester2<char*>::value);
BOOST_TEST(!tester2<void*>::value);
BOOST_TEST(!tester3<char*>::value);
BOOST_TEST(tester3<sample_value_type>::value);
return boost::report_errors();
}