同时使用多个仿函式（第二部分）

一元判定是判定某个值是否满足某个条件的函数，它接收一个数值参数，如果该值满足判定条件则返回“真”，否则返回“否”。

当你需要对一个范围（range）进行筛选时，一元判定就派上用场了。例如，下面的代码将打印出给定范围内的偶数：

不过在实际应用中，你往往需要组合多个一元判定。例如，你也许希望打印正值的偶数，或者负值偶数。

多个一元判定的组合可以通过使用multiple_or/multiple_and函数实现。每个函数最多可接收六个参数；每个参合素都必须是可修改的一元判定。使一元判定获得可修改性的最简单的方法就是继承std::unary_function< type, bool>，“type”值得是你希望测试的数值的类型。

注意t multiple_or/multiple_and函数可以组合使用。例如，multiple_or( a, b, multiple_and( c, d, e), f)等于a || b || (c && d && e) || f。下面是multiple_or/multiple_and的代码和一个例子：

#include <functional> ///////////////////////////////////////////////////// // multiple_or template< class FuncFirst, class FuncSecond> struct multiple_or_func_t : public std::unary_function< FuncFirst::argument_type, bool> { typedef multiple_or_func_t< FuncFirst, FuncSecond> this_class; //注意：从std::unary_function< FuncFirst::argument_type...>导出 // 并且用到了typedef,我们需要确保 // FuncFirst和FuncSecond都是可修改的仿函数！ typedef FuncSecond::argument_type argument_type; multiple_or_func_t( FuncFirst funcFirst, FuncSecond funcSecond) : m_first( funcFirst), m_second( funcSecond) {} bool operator()( const argument_type & value) { if ( m_first( value)) return true; return m_second( value) != false; } template< class FuncThird> multiple_or_func_t< this_class, FuncThird> add( FuncThird funcThird) const { return multiple_or_func_t< this_class, FuncThird>( *this, funcThird); } private: FuncFirst m_first; FuncSecond m_second; }; template< class FuncFirst, class FuncSecond> multiple_or_func_t< FuncFirst, FuncSecond> multiple_or( FuncFirst funcFirst, FuncSecond funcSecond) { return multiple_or_func_t< FuncFirst, FuncSecond>( funcFirst, funcSecond); } template< class FuncFirst, class FuncSecond, class FuncThird> multiple_or_func_t< multiple_or_func_t< FuncFirst, FuncSecond>, FuncThird > multiple_or( FuncFirst funcFirst, FuncSecond funcSecond, FuncThird funcThird) { return multiple_or( funcFirst, funcSecond).add( funcThird); } template< class FuncFirst, class FuncSecond, class FuncThird, class FuncFourth> multiple_or_func_t< multiple_or_func_t< multiple_or_func_t< FuncFirst, FuncSecond>, FuncThird>, FuncFourth> multiple_or( FuncFirst funcFirst, FuncSecond funcSecond, FuncThird funcThird, FuncFourth funcFourth) { return multiple_or( funcFirst, funcSecond, funcThird).add( funcFourth); } template< class FuncFirst, class FuncSecond, class FuncThird, class FuncFourth, class FuncFifth> multiple_or_func_t< multiple_or_func_t< multiple_or_func_t< multiple_or_func_t< FuncFirst, FuncSecond>, FuncThird>, FuncFourth>, FuncFifth> multiple_or( FuncFirst funcFirst, FuncSecond funcSecond, FuncThird funcThird, FuncFourth funcFourth, FuncFifth funcFifth) { return multiple_or( funcFirst, funcSecond, funcThird, funcFourth).add( funcFifth); } template< class FuncFirst, class FuncSecond, class FuncThird, class FuncFourth, class FuncFifth, class FuncSixth> multiple_or_func_t< multiple_or_func_t< multiple_or_func_t< multiple_or_func_t< multiple_or_func_t< FuncFirst, FuncSecond>, FuncThird>, FuncFourth>, FuncFifth>, FuncSixth> multiple_or( FuncFirst funcFirst, FuncSecond funcSecond, FuncThird funcThird, FuncFourth funcFourth, FuncFifth funcFifth, FuncSixth funcSixth) { return multiple_or( funcFirst, funcSecond, funcThird, funcFourth, funcFifth).add( funcSixth); } ///////////////////////////////////////////////////// // multiple_and template< class FuncFirst, class FuncSecond> struct multiple_and_func_t : public std::unary_function< FuncFirst::argument_type, bool> { typedef multiple_and_func_t< FuncFirst, FuncSecond> this_class; //注意：从std::unary_function< FuncFirst::argument_type...>导出 //并且用到了这个typedef，我们需要确保 // FuncFirst和FuncSecond都是仿函数！ typedef FuncSecond::argument_type argument_type; multiple_and_func_t( FuncFirst funcFirst, FuncSecond funcSecond) : m_first( funcFirst), m_second( funcSecond) {} bool operator()( const argument_type & value) { if ( !m_first( value)) return false; return m_second( value) != false; } template< class FuncThird> multiple_and_func_t< this_class, FuncThird> add( FuncThird funcThird) const { return multiple_and_func_t< this_class, FuncThird>( *this, funcThird); } private: FuncFirst m_first; FuncSecond m_second; }; template< class FuncFirst, class FuncSecond> multiple_and_func_t< FuncFirst, FuncSecond> multiple_and( FuncFirst funcFirst, FuncSecond funcSecond) { return multiple_and_func_t< FuncFirst, FuncSecond>( funcFirst, funcSecond); } template< class FuncFirst, class FuncSecond, class FuncThird> multiple_and_func_t< multiple_and_func_t< FuncFirst, FuncSecond>, FuncThird > multiple_and( FuncFirst funcFirst, FuncSecond funcSecond, FuncThird funcThird) { return multiple_and( funcFirst, funcSecond).add( funcThird); } template< class FuncFirst, class FuncSecond, class FuncThird, class FuncFourth> multiple_and_func_t< multiple_and_func_t< multiple_and_func_t< FuncFirst, FuncSecond>, FuncThird>, FuncFourth> multiple_and( FuncFirst funcFirst, FuncSecond funcSecond, FuncThird funcThird, FuncFourth funcFourth) { return multiple_and( funcFirst, funcSecond, funcThird).add( funcFourth); } template< class FuncFirst, class FuncSecond, class FuncThird, class FuncFourth, class FuncFifth> multiple_and_func_t< multiple_and_func_t< multiple_and_func_t< multiple_and_func_t< FuncFirst, FuncSecond>, FuncThird>, FuncFourth>, FuncFifth> multiple_and( FuncFirst funcFirst, FuncSecond funcSecond, FuncThird funcThird, FuncFourth funcFourth, FuncFifth funcFifth) { return multiple_and( funcFirst, funcSecond, funcThird, funcFourth).add( funcFifth); } template< class FuncFirst, class FuncSecond, class FuncThird, class FuncFourth, class FuncFifth, class FuncSixth> multiple_and_func_t< multiple_and_func_t< multiple_and_func_t< multiple_and_func_t< multiple_and_func_t< FuncFirst, FuncSecond>, FuncThird>, FuncFourth>, FuncFifth>, FuncSixth> multiple_and( FuncFirst funcFirst, FuncSecond funcSecond, FuncThird funcThird, FuncFourth funcFourth, FuncFifth funcFifth, FuncSixth funcSixth) { return multiple_and( funcFirst, funcSecond, funcThird, funcFourth, funcFifth).add( funcSixth); }

下面是一个例子：

#include <iostream> #include <algorithm> template<class InputIterator, class OutputIterator, Pred> inline OutputIterator copy_if(InputIterator _F, InputIterator _L, OutputIterator _X, Pred p) { for (; _F != _L; ++_X, ++_F) if ( p( *_F)) *_X = *_F; return (_X); } bool is_even( int i) { return ( i % 2) == 0; } bool is_positive( int i) { return i >= 0; } int main(int argc, char* argv[]) { int aNumbers[] = { 22, 5, 28, 1, 0, -1, 3, -3, -2, 4 }; int nCount = sizeof( aNumbers) / sizeof( int); std::cout << "ALL: " << std::endl; std::copy( aNumbers, aNumbers + nCount, std::ostream_iterator< int>( std::cout, " ")); std::cout << " Even OR Positive: " << std::endl; copy_if( aNumbers, aNumbers + nCount, std::ostream_iterator< int>( std::cout, " "), multiple_or( std::ptr_fun( is_even), std::ptr_fun( is_positive)) ); std::cout << " Even AND Positive: " << std::endl; copy_if( aNumbers, aNumbers + nCount, std::ostream_iterator< int>( std::cout, " "), multiple_and( std::ptr_fun( is_even), std::ptr_fun( is_positive)) ); return 0; }