从 std::vector 创建/填充 std::set 的 STL 方式

Question

I want to create and fill a set from the contents of a member variable of each entry of a vector . This is what I am doing:

struct S { int i; };

int main()
{
    std::vector<S*> structPtrs;

    // code to fill the above vector

    // create set from the above vector
    std::set<int> setInts;
    for (auto it = structPtrs.begin(); it != structPtrs.end(); ++it)
    {
        setInts.insert((*it)->i);
    }
}

Is there an STL way to do it? Or via any available method(s) in <algorithm> ?

Answer 1

You can always apply std::transform from the range defined by the vector onto the "range" defined by an std::inserter :

transform(begin(structPtrs), end(structPtrs),
          inserter(setInts, end(setInts)), [] (S* s) {
  return s->i;
});

That should be more than enough use of the standard library.

---

If you are willing to look beyond the standard library, there is also the option of using something like boost::transform_iterator , which will allow you to move the range transformation into the set's initialization:

auto transfomer = [](S* s) { return s->i; };
std::set<int> setInts(
  boost::make_transform_iterator(begin(structPtrs), transfomer),
  boost::make_transform_iterator(end(structPtrs), transfomer)
);

Answer 2

You could use std::transform with an appropriate lambda and an insert iterator:

std::transform(structPtrs.begin(), structPtrs.end(), std::inserter(setInts, setInts.end()),
    [](S* sp) { return sp->i; });

But personally, I find a simple range for loop to be much easier to follow:

for (S* sp : structPtrs)
    setInts.insert(sp->i);

Answer 3

There is one other way you can do this. If you add conversion operator to int to your struct you can just use range constructor directly

#include <iostream>
#include <set>
#include <vector>
using namespace std;

struct test {int i; operator int() {return i;}};

int main() {
    vector<test> v;
        v.push_back(test{433});
        v.push_back(test{533});
        set<int> s(v.begin(), v.end());
        cout << *(++s.begin());
    return 0;
}

https://www.ideone.com/qJwtwc