c++ copy elision

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#include <iostream>
#include <string>
#include <vector>

class BigObject {
public:
    BigObject() : person_names_(std::vector<std::string>(1000000)) {
        std::cout << "constructor. " << person_names_.size() << std::endl;
    }
    ~BigObject() {
        std::cout << "destructor. " << std::endl;
    }
    BigObject(const BigObject& other) {
        person_names_ = other.person_names_;
        std::cout << "copy constructor. " << person_names_.size() << std::endl;
    }

private:
    std::vector<std::string> person_names_;
};

BigObject Foo() {
    BigObject local_obj;
    return local_obj;
    // return std::move(local_obj);
}

int n {0};

struct C
{
    explicit C(int) {}
    C(const C&) { ++n; }
    int x {1};
};

int main() {
    {
        BigObject obj = Foo();
    }
    //-----Foo()
    //       : BigObject local_obj  -> ctor + copy ctor
    //       : return local_obj     -> dtor
    //BigObject obj
    //       :                      -> copy ctor
    //                              -> dtor
    //       :                      -> dtor

    C c1(42);
    // copy ctor is equivalent to direct-ctor
    C c2 = C(42);
    std::cout << "n:" << n << std::endl;
    return 0;
}

/*
g++ -g -fno-elide-constructors -Wall -std=c++11 main.cpp
main.cpp: In function ‘int main()’:
main.cpp:49:7: warning: variable ‘c2’ set but not used [-Wunused-but-set-variable]
   49 |     C c2 = C(42);
      |       ^~

./a.out
constructor. 1000000
copy constructor. 1000000
destructor.
copy constructor. 1000000
destructor.
destructor.
n:1
*/

/*
g++ -g main.cpp
./a.out

constructor. 1000000
destructor.
n:0
*/