Question

Consider the following non-working code:

typedef map<int, unsigned> mymap;
mymap m;
for( int i = 1; i < 5; ++i )
    m[i] = i;
// 'remove' all elements from map where .second < 3
remove_if(m.begin(), m.end(), bind2nd(less<int>(), 3));

I’m trying to remove elements from this map where .second < 3. This obviously isn’t written correctly. How do I write this correctly using:

1. Standard STL function objects & techniques using bind + less<> but without having to write a custom functor
2. Boost.Bind
3. C++0x Lambdas

I know I’m not eraseing the elements. Don’t worry about that; I’m just simplifying the problem to solve.

Answer 1

I’m not sure how to do this using just the STL binders but I think your main problem is that what’s being passed into the functor you give to remove isn’t just an int but a pair<int, unsigned>.

Using boost::bind you’d do it like this:

remove_if(m.begin(), m.end(), bind(&std::pair<int, unsigned>::second, _1) < 3);

Using a lambda function it’s something like this:

remove_if(m.begin(), m.end(), [](const std::pair<int, unsigned>& p) { return p.second < 3; } );

I haven’t checked that this compiles, sorry.