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      typename F1
    , typename F2
    , typename F3 = false_
    , typename Fn = false_
struct or_
    typedef unspecified type;


Returns the result of short-circuit logical or (||) operation on its arguments.


#include "boost/mpl/or.hpp"


 Parameter  Requirement  Description  
F1, F2, .., FnA model of nullary Metafunction

Expression semantics

 Expression  Expression type  Precondition  Semantics  Postcondition 
or_<f1,f2,..,fn>A model of bool Integral Constanttrue_ if either of f1::type::value, f2::type::value, .., fn::type::value expressions evaluates to true, and false_ otherwise; guarantees left-to-right evaluation; moreover, the operands subsequent to the first fi metafunction that evaluates to true are not evaluated.


// will generate compile-time error if invoked with T == any fundamental type
template< typename T > struct fail
   typedef typename T::nonexistent type;

BOOST_STATIC_ASSERT((or_< false_,true_ >::value == true)); BOOST_STATIC_ASSERT((or_< true_,fail<int> >::value == true)); // OK, fail<int> is never invoked BOOST_STATIC_ASSERT((or_< false_,true_,fail<int> >::value == true)); // OK too

See also

Metafunctions, and_, not_

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Last edited September 6, 2003 3:14 am