Using back_inserter to create vector from single member of struct

Question

I'd like to know if it is possible to use std::back_inserter to create a vector of only a single element from a more complex struct. For example, in the following code:

struct Person {
  std::string first_name;
  std::string last_name;
  int id;
};
std::vector<Person> people;
// Populate 'people'...

// Extract last names into single vector
std::vector<std::string> last_names;
std::copy(begin(people), end(people), std::back_inserter(last_names)); // <... ?

The only solution I've found so far is to create a casting operator Person -> std::string :

struct Person {
  // ...
  operator std::string() const { return last_name; }
};

But is not a good solution if I want to extract first_name and last_name into two vectors, not to mention how obscure becomes such implicit conversion.

Is there any way to indicate std::back_inserter how to construct the element to insert? Any other way to create such vector? Obviously, I'm not referring to the raw way:

std::vector<std::string> last_names;
last_names.reserve(people.size());
for (const auto& person : people) {
  last_names.push_back(person.last_name);
}

but to some <algorithm> -like one ;)

I'd prefer a C++-only answer, but I'm open to Boost solutions if needed.

Answer 1

You should use std::transform instead, with an unary operation function object to perform the transformation on every element. eg

std::transform(std::begin(people), 
               std::end(people), 
               std::back_inserter(last_names), 
               [](const Person& person) { return person.last_name; }
              );

Answer 2

Just to add alternatives you could also use std/boost::bind

  // Extract first names into single vector
  std::vector<std::string> last_names;

  last_names.reserve(people.size());
  std::transform(people.begin(), people.end(), std::back_inserter(last_names), 
                 std::bind(&Person::last_name, std::placeholders::_1));

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