Full disclosure, this may be a hammer and nail situation trying to use STL algorithms when none are needed. I have seen a reappearing pattern in some C++14 code I am working with. We have a container that we iterate through, and if the current element matches some condition, then we copy one of the elements fields to another container.
The pattern is something like:
for (auto it = std::begin(foo); it!=std::end(foo); ++it){
auto x = it->Some_member;
// Note, the check usually uses the field would add to the new container.
if(f(x) && g(x)){
bar.emplace_back(x);
}
}
The idea is almost an accumulate where the function being applied does not always return a value. I can only think of a solutions that either
Is this even a good idea?
A quite general solution to your issue would be the following (working example):
#include <iostream>
#include <vector>
using namespace std;
template<typename It, typename MemberType, typename Cond, typename Do>
void process_filtered(It begin, It end, MemberType iterator_traits<It>::value_type::*ptr, Cond condition, Do process)
{
for(It it = begin; it != end; ++it)
{
if(condition((*it).*ptr))
{
process((*it).*ptr);
}
}
}
struct Data
{
int x;
int y;
};
int main()
{
// thanks to iterator_traits, vector could also be an array;
// kudos to @Yakk-AdamNevraumont
vector<Data> lines{{1,2},{4,3},{5,6}};
// filter even numbers from Data::x and output them
process_filtered(std::begin(lines), std::end(lines), &Data::x, [](int n){return n % 2 == 0;}, [](int n){cout << n;});
// output is 4, the only x value that is even
return 0;
}
It does not use STL, that is right, but you merely pass an iterator pair, the member to lookup and two lambdas/functions to it that will first filter and second use the filtered output, respectively.
I like your general solutions but here you do not need to have a lambda that extracts the corresponding attribute.
Clearly, the code can be refined to work with const_iterator
but for a general idea, I think, it should be helpful. You could also extend it to have a member function that returns a member attribute instead of a direct member attribute pointer, if you'd like to use this method for encapsulated classes.
Sure. There are a bunch of approaches.
Find a library with transform_if
, like boost
.
Find a library with transform_range
, which takes a transformation and range or container and returns a range with the value transformed. Compose this with copy_if
.
Find a library with filter_range
like the above. Now, use std::transform
with your filtered range.
Find one with both, and compose filtering and transforming in the appropriate order. Now your problem is just copying (std::copy
or whatever).
Write your own back-inserter wrapper that transforms while inserting. Use that with std::copy_if
.
Write your own range adapters, like 2 3 and/or 4.
Write transform_if
.
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