On Wed, May 18, 2005 at 12:05:53PM +0800, alex wrote:

I test the sample in the section "Nesting STL algorithm invocations": --------------------------------------

int a[100][200]; int sum = 0;

std::for_each(a, a + 100, bind(ll::for_each(), _1, _1 + 200, protect(sum += _1))); ------------------------------------------------If I remove the protect function, and I will get a complie error.Thx!

It prevents the inner expression from being evaluated too early. There is a _little_ more information here: http://boost.org/libs/bind/bind.html#nested_binds (*) The _1 in "sum += _1" is not the same _1, so you protect it so it isn't evaluated until processing the inner expression. Without the protect it tries to add each array in the range [a,a+100) to sum, but adding int[] to int is an error. jon (*) Bind maintainers, is the first "not" in this text an mistake? Sometimes it is necessary not to evaluate the first argument, but *not* to evaluate some of the other arguments, even when they are nested *bind* subexpressions.