C++ Copy elements from an object vector into a vector with this element

Question

I want to copy the a values from the foos vector into another vector with just the int value. What's the fastest way to do this?

#include <vector>

struct Foo {
   int a;
};

int main() {
   std::vector<Foo> foos;
   for(int i = 0; i < 100; i++)
      foos[i].a = i;

   std::vector<int> somenumbers;
   //Fastest way to copy the ints from the struct into the somenumbers vector?  

   return 0;
}

use [std::transform](http://en.cppreference.com/w/cpp/algorithm/transform), and then profile to make sure it is actually a bottleneck before trying to optimize it. — Borgleader, Feb 22 '17 at 13:58
Whatever you do, call `reserve` on `somenumbers` before you add anything to it. — NathanOliver, Feb 22 '17 at 13:59

score 3 · Accepted Answer · edited Feb 22 '17 at 14:10

3

What do you mean fastest? Just loop, or algorithm, or something else. For example:

std::transform
(
    foos.begin(), foos.end(), std::back_inserter(somenumbers),
    [](const Foo& v) { return v.a; }
);

Also, since you know size of vector - you should callreserve.

edited Feb 22 '17 at 14:10

NathanOliver

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answered Feb 22 '17 at 13:58

ForEveR

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score 3 · Answer 2 · edited May 23 '17 at 12:16

somenumbers.reserve(foos.size());
for(const auto& f : foos) somenumbers.emplace_back(f.a);

If you're doing this operation multiple times in your codebase, introduce an abstraction:

template <typename TRange, typename TF>
auto map_vector(TRange&& range, TF&& f)
{
    using type = std::decay_t<decltype(f(*std::begin(range)))>;

    std::vector<type> result;
    result.reserve(range.size());

    for(auto&& x : range) result.emplace_back(forward_like<TRange>(x));
    return result;
}

(forward_like can be found here.)

Usage:

auto somenumbers = map_vector(foos, [](const auto& x){ return x.a; });

score 0 · Answer 3 · edited May 28 '21 at 10:11

There is also dense/one-liner solution when you comit to using Boost:

#include <boost/iterator/transform_iterator.hpp>

const std::vector<int> extraction{
    boost::make_transform_iterator(instances.cbegin(), std::mem_fn(&Foo::a)),
    boost::make_transform_iterator(instances.cend(), std::mem_fn(&Foo::a))
};

The function object that is passed to make_transform_iterator can be a lambda, too.

C++ Copy elements from an object vector into a vector with this element

3 Answers3

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