lambda parameter with optional return value

Question

I'm trying to write a function like std::for_each, that in addition to the normal usage, can also take a std::function<bool (param)>. A false return value means that I want to break out of the loop. The code below is what I've gotten so far.

The second call to a.visit([&](int) -> void) does not compile when evaluating !visitor(i). Is it possible to make this work or am I barking up the wrong tree?

I'm using MSVC 2010 but want the code to be generally C++11 compatible.

#include <list>
#include <string>
#include <iostream>

struct A 
{
    std::list<int> _lst;

    template<typename _F>
    void visit(_F visitor) 
    {
        for(std::list<int>::const_iterator it = _lst.begin(), end = _lst.end() ; it != end ; it++) {
            int i = *it;
            if (std::is_void<decltype(visitor(i))>::value) {
                visitor(i);
            } else {
               if (!visitor(i)) { // <----- error C2171: '!' : illegal on operands of type 'void'
                   break;
               }
            }
        }
    }

};

int main(int argc, char* argv[])
{
    A a;
    // populate a
    for (int i = 0 ; i < 10 ; i++) { 
        a._lst.push_back(i); 
    }

    a.visit([](int i) -> bool {
        std::cout << i << std::endl;
        return i < 5;
    });

    a.visit([](int i) {
        std::cout << i << std::endl;
    });
}

Answer 1

Here's how I would implement for_almost_each; I'm using namespace std plus type aliases for readability purposes.

#include <algorithm>
#include <iterator>
#include <functional>

using namespace std;

template<class Iter, class Func>
Iter
for_almost_each_impl(Iter begin, Iter end, Func func, std::true_type)
{
    for (auto i = begin; i!=end; ++i)
        if (!func(*i))
            return i;
    return end;
}

template<class Iter, class Func>
Iter
for_almost_each_impl(Iter begin, Iter end, Func func, std::false_type)
{
    for_each(begin, end, func);
    return end;
}


template<class Iter, class Func>
Iter for_almost_each(Iter begin, Iter end, Func func)
{
    using Val = typename iterator_traits<Iter>::value_type;
    using Res = typename result_of<Func(Val)>::type;
    return for_almost_each_impl(begin, end,
                                func,
                                is_convertible<Res, bool>{} );
}

I used is_convertible, as it seems to make more sense than is_same.

Answer 2

Your std::is_void needs to be done at compile time and can't be done inside the function body. This use of function overloading will work:

#include <list>
#include <string>
#include <iostream>
#include <type_traits> // missing header

struct A 
{
    std::list<int> _lst;

    // wrapper for bool returning visitor
    template<typename _F, typename Iter>
    bool do_visit(_F visitor, Iter it, std::true_type)
    {
      return visitor(*it);
    }

    // wrapper for non-bool returning visitor
    template<typename _F, typename Iter>
    bool do_visit(_F visitor, Iter it, std::false_type)
    {
      visitor(*it);
      return true;
    }

    template<typename _F>
    void visit(_F visitor) 
    {
        for (auto it = _lst.begin(), end = _lst.end() ; it != end ; it++) {
            // select correct visitor wrapper function using overloading
            if (!do_visit(visitor, it, std::is_same<bool, decltype(visitor(0))>())) {
              break;
            }
        }
    }
};

int main(int argc, char* argv[])
{
    A a;
    // populate a
    for (int i = 0 ; i < 10 ; i++) { 
        a._lst.push_back(i); 
    }

    a.visit([](int i) -> bool {
        std::cout << i << std::endl;
        return i < 5;
    });

    a.visit([](int i) {
        std::cout << i << std::endl;
    });
}

Answer 3

This lambda doesn't return a value, which is why you're getting an error that "visitor" is returning void:

a.visit([](int i) {
    std::cout << i << std::endl;
});

You could make this work by rewriting as:

a.visit([](int i) -> bool {
    std::cout << i << std::endl;
    return true;
});