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Front Page / Metafunctions / Composition and Argument Binding / protect



      typename F
struct protect
    // unspecified
    // ...


protect is an identity wrapper for a Metafunction Class that prevents its argument from being recognized as a bind expression.


#include <boost/mpl/protect.hpp>


Parameter Requirement Description
F Metafunction Class A metafunction class to wrap.

Expression semantics

For any Metafunction Class f:

typedef protect<f> g;
Return type:

Metafunction Class.


If f is a bind expression, equivalent to

struct g
          typename U1 = unspecified,... typename Un = unspecified
    struct apply
        : apply_wrapn<f,U1,...Un>

otherwise equivalent to typedef f g;.


struct f
    template< typename T1, typename T2 > struct apply
        typedef T2 type;

typedef bind< quote3<if_>,_1,_2,bind<f,_1,_2> > b1;
typedef bind< quote3<if_>,_1,_2,protect< bind<f,_1,_2> > > b2;

typedef apply_wrap2< b1,false_,char >::type r1;
typedef apply_wrap2< b2,false_,char >::type r2;

BOOST_MPL_ASSERT(( is_same<r1,char> ));
BOOST_MPL_ASSERT(( is_same<r2,protect< bind<f,_1,_2> > > ));

See also

Composition and Argument Binding, invocation, bind, quote, apply_wrap