senf::mpl::rv< n > Struct Template Reference
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Return-value type used to implement overload selection. More...

#include <senf/Utils/mpl.hh>

List of all members.

Detailed Description

template<unsigned n>
struct senf::mpl::rv< n >

Return-value type used to implement overload selection.

The senf::mpl::rv type is used together with SENF_MPL_RV() to select template specializations based on a set of overloads:

template <unsigned _> struct select {};

// Case 0
template <>
struct select<0> {
    static bool const has_int_value = true;
    void frobble();
};
template <class T>
senf::mpl::rv<0> select_(int, senf::mpl::take_int<T::value> * = 0);

// Case 1
template <>
struct select<1> {
    static bool const has_int_value = false;
    void dazzle();
};
template <class T>
senf::mpl::rv<1> select_(...);

template <class T>
struct choice : public select<SENF_MPL_RV( select_<T>(0) )> {};

struct A { static const int value = 0; };
struct B {};

choice<A> a; a.frobble();
choice<B> b; b.dazzle();

The selection is always based on two components: A selector class specialized for each of the possible choices and an overloaded function (only signatures, no implementation needed) to provide the conditions.

When instantiatinv choice<T>, we forward T to the select_ set of overloads. Because of SFINAE, the overload set will only contain those instantiations, for which template expansion does not fail.

So, if T has an integer value member, both select_ overloads are ok and the call select_<T>(0) will choose the first (case 0) variant, since the argument 0 is better matched by int than by ....

However, if T does not have an integer value member, expansion for the first overload fails and the overload set only contains the second case.

SENF_MPL_RV() internally uses sizeof to find out, which overload was selected and returns the senf::mpl::rv-argument of that overloads return type. For this to work, the select_ functions need not be implemented since no code is generated and select_ is never called.

This number is than forwarded as template argument to select which is specialized for each case. Therefore, choice<A> has a frobble() member whereas choice<B> has a dazzle() member.

See also:
SENF_MPL_RV

Definition at line 116 of file mpl.hh.


Public Attributes
char  _ [SENF_MPL_RV_ALIGNMENT][n+1]

Member Data Documentation

template<unsigned n>
char senf::mpl::rv< n >::_
_[SENF_MPL_RV_ALIGNMENT][n+1]

Definition at line 117 of file mpl.hh.

The documentation for this struct was generated from the following file: