How to get array of data members from array of objects in c++?

Question

Imagine that I have an array of "person" objects:

#include <iostream>
class person{
public:
   int age;
   std::string name;
};

Is it possible to get an array of the same size with the age (or the name) of each person? I know how to do it with a loop but I hoped there was some kind of "map" that could get me the same in one line.

Answer 1

Yes, you could, using transform both for legacy arrays and for standard containers such as std::vector .

Using legacy arrays

person persons[2] = { { 20, "name1" }, { 21, "name2" } };
for (auto & person : persons)
    cout << person.age << " " << person.name << endl;

decltype(person::age) ages[sizeof(persons)/sizeof(persons[0])];
std::transform(std::begin(persons), std::end(persons), std::begin(ages), [](person & p) -> decltype(person::age) { return p.age; });
for (auto & age : ages)
    cout << age << endl;

Using std::vector

std::vector<person> persons = { { 20, "name1" }, { 21, "name2" } };
for (auto & person : persons)
    cout << person.age << " " << person.name << endl;

std::vector<decltype(person::age)> ages(persons.size());
std::transform(persons.begin(), persons.end(), ages.begin(), [](person & p) -> decltype(person::age) { return p.age; });
for (auto & age : ages)
    cout << age << endl;

This dynamically extracts the ages of all the persons in the original container.

Answer 2

For the sake of generality I assume with array you mean a dynamically sized array, then the full example would be:

#include <string>
struct person {
   int age;
   std::string name;
};

std::vector<int> getAge(std::vector<person> p) { 
   std::vector<int> result;
   result.reserve(p.size());
   for (auto& e : p) result.push_back(e.age);
   return result;
}

or you using std::transform that would be

#include <algorithm>
int  main() {
    std::vector<person> foo(10);
    std::vector<int>    ages(10);
    std::transform(foo.begin(),foo.end(),ages.begin(),[](person& p){return p.age;});
}

In any case, there is no magic way to get an array of members from an array of instances. There must be a loop somewhere (transform just does a very good job at hiding that loop from you).

Answer 3

Here's how it can be done with std::transform , std::back_inserter , and a lambda:

#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>

struct person{
   int age;
   std::string name;
};

int main()
{
    std::vector<person> pp = {{16, "Bob"},{32, "Alice"}};
    std::vector<int> ages;
    std::transform(
        pp.begin(), 
        pp.end(), 
        std::back_inserter(ages),
        [](const person& p) -> int { return p.age; });
    for(auto v: ages)
        std::cout << v << " ";
}

Output:

16 32 

Live demo

UPDATE: to avoid unnecessary re-allocations you can call ages.reserve(pp.size()) before a call of std::transform