struct Flags { int active : 1; };

int main() {
  Flags f;
  f.active = 1;

  if (f.active == 1) 
    std::cout << "ok\n";   // не печатает!
}

#include <iostream>
#include <vector>

int main() {
    std::vector<bool> flags{false};
    bool snapshot = flags[0];
    auto proxy = flags[0];

    flags[0] = true;

    std::cout << snapshot << ' '
              << static_cast<bool>(proxy);
}

#include <iostream>

struct Base {
    virtual void show(int n = 1) {
        std::cout << "Base " << n;
    }
};

struct Derived : Base {
    void show(int n = 2) override {
        std::cout << "Derived " << n;
    }
};

int main() {
    Derived object;
    Base& ref = object;
    ref.show();
}

#include <iostream>

int main() {
    auto next = [n = 0]() mutable {
        return ++n;
    };
    auto copy = next;

    std::cout << next() << ' ';
    std::cout << next() << ' ';
    std::cout << copy();
}

#include <iostream>
#include <map>
#include <string>

int main() {
    std::map<std::string, int> counts;

    int value = counts["go"];

    std::cout << value << ' '
              << counts.size();
}