#include <iostream>
    #include <vector>

    struct A {
        std::vector<int> v = { 1, 2, 3 }; // default population
    };

    struct B {
        std::vector<int> v;
        B() : v(4) {}
    };

    int main() {
        A a;
        B b;
        std::cout << a.v.size() << ", " << b.v.size() << "\n";
        std::cout << "\n";
        for (int v : a.v) { std::cout << v << "\n"; }
        std::cout << "\n";
        for (int v : b.v) { std::cout << v << "\n"; }
    }

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