From: "Duane Murphy"

Bind has been working very well for some of the things I have been wanting to do. I have run into a rather puzzling problem this time and was wondering what was happening:

Here is the example:

void f( int ) { }

struct array_obj { int operator[]( int ){ return NULL; } };

int main() { array_obj a; int array[1]; int value = a[0]; boost::bind( f, _1 )( 3 ); boost::bind( f, _1 )( value ); boost::bind( f, _1 )( array[0] ); boost::bind( f, _1 )( a[0] ); return 0; }

The first three bind statements compile just fine. The third one issues an error:

function call 'operator()(int)' does not match.

Along with all 20 variations of operator() generated by bind.

Obviously there is something amiss with going through the return value of operator[](). I also get the same problem using an iterator which would be going through operator*().

The problem here is that a[0] is an rvalue of type int. This is a known limitation of bind(); it needs to pass the arguments unmodified to f (since f may have a reference argument) and uses signatures of the form template<class T> result_type operator()(T & t) const; Unfortunately this doesn't work for rvalues (temporaries.) I don't know of a good solution. Making operator[] return int & (or int const & in the const case) would solve this specific problem, if that's an option. -- Peter Dimov Multi Media Ltd.