定义自定义迭代器，将调用函数的元素传递给传递的元素

Question

I am often writing code like the following (here int and double are just example, types are usually pointers)

std::vector<int> va {1 ,2, 3};
std::vector<double> vb;

bool valid(int i) { return i % 2 == 0;}

for(auto a : va)
   if (valid(a))
      vb.push_back(static_cast<double>(a));

Since we don't have transform_if I would like to know if there is a way to do this by definining some special iterator:

template<typename T>
struct inserterStaticCast
{
    // not sure what to put here...
}

Then I could write code like

std::vector<double> vc(a.size());
std::copy_if(a.begin(), a.end(), inserterStaticCast<double>(vc), &valid);

I would also be interested in the backInserterStaticCast version.

Is this possible in C++11?

Thank you

A few clarifications...

I cannot use boost. And to again in this case int and double are just for illustration, in the general case I do need a cast, usually something like static_cast<derived *>(base_ptr) .

Answer 1

I think you want boost's function_output_iterator . In your example it would be

auto cast_back_iterator = make_function_output_iterator([&b](int i){ b.push_back(static_cast<double>(i)); });
std::copy_if(a.begin(), a.end(), cast_back_iterator, valid);

Which makes more sense when there isn't an implicit conversion between the two types, such as with Base * -> Derived * conversion

Answer 2

You can do filtering with std::copy_if and std::back_inserter :

std::vector<double> vc;
std::copy_if(va.begin(), va.end(), std::back_inserter(vc), valid);

Or use boost::range::adaptors::filtered (same example problem as yours):

#include <boost/range/adaptor/filtered.hpp>
#include <boost/range/algorithm/copy.hpp>

#include <vector>

bool valid(int i) { return i % 2 == 0;}

int main() {
    std::vector<int> va {1 ,2, 3};

    using namespace boost::adaptors;
    auto vb = boost::copy_range<std::vector<double>>(va | filtered(valid));
}

Note that conversion of int to double is implicit and doesn't require a cast.

If you do need a conversion, just throw in transformed :

auto vb = boost::copy_range<std::vector<double>>(va
    | filtered(valid)
    | transformed([](int a) { return static_cast<double>(a); })
    );