#include <iostream>
#include <vector>
#include <algorithm>
#include <utility>
 
struct MyIndexGenerator
{
    int value;
    int index;
 
    MyIndexGenerator(int _value, int _first = 0) : value(_value), index(_first) {}
 
    std::pair<int, int> operator()()
    { 
        return std::make_pair(value, index++);
    }
};
 
class Foo
{
public:
    int x;
    int Index;
 
    Foo(const std::pair<int, int> &_x) : x(_x.first), Index(_x.second) {} 
};
 
int main()
{
    int a = 5;
    std::vector<Foo> Instances;
    Instances.reserve(30);
    std::generate_n(std::back_inserter(Instances), 30, MyIndexGenerator(a));
 
    for(auto &f : Instances)
        std::cout << f.Index << ' ';
 
    return 0;
}

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