Negate boost is_directory in std algorithm

Question

boost::filesystem::recursive_directory_iterator end, begin(directory);

auto num_of_files=std::count_if(begin, end, 
        std::not1(boost::filesystem::is_directory)));

I am trying to negate the function is_directory on the above directory iterator but am hitting a brick wall. I have tried specifying the template for not1 as bool(*)(const boost::filesystem::path&) and tried static casting the function both without success.

I know I can resort to a lamdba but this is cleaner if it works.

Thanks

score 6 · Accepted Answer · answered Sep 19 '11 at 13:06

6

std::not1 needs a function object as its argument. This function object can be obtained with std::ptr_fun, so this should work:

auto num_of_files=std::count_if(begin, end, 
        std::not1(std::ptr_fun((bool(*)(const boost::filesystem::path&))boost::filesystem::is_directory)));

(the number of parentheses is probably incorrect). BTW, you need the cast because is_directory is an overloaded function.

However, since you tag you question c++11, you could use lambdas:

auto num_of_files=std::count_if(begin, end, [](const boost::filesystem::path& p) { return !boost::filesystem::is_directory(p); });

answered Sep 19 '11 at 13:06

jpalecek

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Ah, I am some 30 seconds late :) – Anton Daneyko Sep 19 '11 at 13:08
Worked a charm I prefer this syntax it reads better to me: ...std::ptr_fun(boost::filesystem::is_directory)... – 111111 Sep 19 '11 at 14:09
@111111, you are insane, or possibly a robot. I mean to say that as kindly as possible. – deft_code Sep 20 '11 at 14:44

score 0 · Answer 2 · answered Sep 19 '11 at 13:07

not1 accepts an instance of functor class, which should be an Adaptable Predicate (i.e. with typedefs for return value etc.), while you are trying to feed it with a function pointer. So you need to wrap it in a functor and ptr_fun might help. Perhaps this would work (assume using namespace std; using namespace boost;):

auto num_of_files=count_if(begin, end, not1(ptr_fun(filesystem::is_directory)));

score 0 · Answer 3 · answered Sep 19 '11 at 13:07

You need ptr_fun,

this rather elaborate illustration should print 1 three times: (see also http://ideone.com/C5HTR)

#include <functional>
#include <string>
#include <algorithm>
#include <iostream>

bool pred(const std::string& s)
{
    return s.size() % 2;
}

int main()
{
    std::string data[] = { "hello", "world!" };

    std::cout << std::count_if(data, data+2, 
            pred) << std::endl;

    std::cout << std::count_if(data, data+2, 
            std::ptr_fun(pred) ) << std::endl;

    std::cout << std::count_if(data, data+2, 
            std::not1(std::ptr_fun(pred))) << std::endl;

    return 0;
}

Negate boost is_directory in std algorithm

3 Answers3