Boris wrote:
Hi, I have two predicates f1 and f2 and I want to construct the predicate f4 = f1 && f2 using boost::bind. Unfortunately I observe a strange behaviour of boost::bind I can not explain.
struct Test { int a; int b; };
Test t = { 1, 2 };
typedef boost::function< bool (Test const&) > Fn;
Fn f1 = bind( &Test::a, _1 ) == 1; assert (f1( t )); // Ok
Fn f2 = bind( &Test::b, _1 ) == 3; assert (!f2( t )); // ok
Fn f3 = bind( std::logical_and< bool >(), bind( &Test::a, _1 ) == 1, bind( &Test::b, _1 ) == 3 ); assert (!f3( t )); // Ok
Fn f4 = bind( std::logical_and< bool >(), f1, f2 ); assert (!f4( t )); // assertion failed
Why does the predicate f4 behave different from f3?
f4( t ) returns f1 && f2, which is true because a function<> in a boolean context evaluates to true when it's nonempty. The outer bind cannot see inside f1 and f2 to recognize them as subexpressions since you've wrapped them in a function<>, so it treats them as arguments for logical_and. Try Fn f4 = bind( std::logical_and< bool >(), bind( f1, _1 ), bind( f2, _1 ) );