...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
template< typename Sequence , typename State , typename BinaryOp , typename Pred > struct copy_if { typedef unspecified type; };
Returns the result of the successive application of BinaryOp
to the result of the previous BinaryOp
invocation (State
if it's the first call) and every element in the range [begin<Sequence>::type,end<Sequence>::type)
that satisfies the predicate Pred
, in the linear order. A typical application for copy_if
is to conditionally copy the content of one sequence into another - see the example below.
#include "boost/mpl/copy_if.hpp"
Parameter | Requirement | Description |
---|---|---|
Sequence | A model of Sequence | A sequence to iterate. |
State | A type | The initial state for the first BinaryOp application. |
BinaryOp | A model of [Lambda Function] | The operation to be executed on forward traversal. |
Pred | An unary Predicate [Lambda Expression] | The copying condition. |
Expression | Expression type | Precondition | Semantics | Postcondition |
---|---|---|---|---|
typedef copy_if<Sequence,T,Op,Pred>::type s; | A type | Equivalent to typedef lambda<Op>::type op; typedef lambda<Pred>::type pred; typedef fold< Sequence,T,if_< apply<pred,_2>, apply<op,_1,_2>, _1 > >::type s; . |
Linear. Exactly size<Sequence>::type::value
applications of Pred
, and at most size<Sequence>::type::value
applications of BinaryOp
.
typedef list_c<int,0,1,2,3,4,5,6,7,8,9>::type numbers; typedef list_c<int,0,1,2,3,4>::type answer; typedef copy_if< numbers , vector_c<int> , push_back<_1,_2> , less<_1,int_<5> > >::type result;BOOST_STATIC_ASSERT(size<result>::value == 5); BOOST_STATIC_ASSERT((equal<result,answer>::type::value));
Algorithms, copy
, copy_backward_if
, copy_backward
, fold
, iter_fold