vector::push_back vs vector::emplace_back

stdout

Constructor 0x7fffad4b75d0:A0,0
--1--
Copy constructor 0x55b854a6ee70:A0,0
16
--2--
Constructor 0x7fffad4b7600:push,10
Copy constructor 0x55b854a6ee98:push,10
Destructor 0x7fffad4b7600:push,10
16
--3--
Constructor 0x55b854a6eec0:emplace_0,20
16
--4--
Constructor 0x7fffad4b7600:emplace_1,30
Copy constructor 0x55b854a6eee8:emplace_1,30
Destructor 0x7fffad4b7600:emplace_1,30
16
--5--
Copy constructor 0x55b854a6ef10:A0,0
16
--end--
5
Destructor 0x7fffad4b75d0:A0,0
Destructor 0x55b854a6ee70:A0,0
Destructor 0x55b854a6ee98:push,10
Destructor 0x55b854a6eec0:emplace_0,20
Destructor 0x55b854a6eee8:emplace_1,30
Destructor 0x55b854a6ef10:A0,0

t0.cpp

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

using namespace std;


class A {
    public:
    A(string a, int b):a_(a), b_(b) {
        cout << "Constructor " <<(void*)this <<":" << a_ << "," << b_ << endl;
    }
    A(const A& a):a_(a.a_), b_(a.b_) {
        cout << "Copy constructor " << (void*)this << ":"  << a_ << "," << b_ << endl;
    }
    ~A() {
        cout << "Destructor " << (void*)this << ":" << a_ << "," << b_ << endl;
    }
    private:
    string a_;
    int b_;
};
int main()
{

    vector<A> as;
    as.reserve(16);
    // as.reserve(16);
    
    A a0("A0", 0);
    cout << "--1--" << endl;
    as.push_back(a0);
    cout << as.capacity() << endl;
    
    cout << "--2--" << endl;
    as.push_back(A("push", 10));
    cout << as.capacity() << endl;
    
    cout << "--3--" << endl;
    as.emplace_back("emplace_0", 20);
    cout << as.capacity() << endl;
    
    cout << "--4--" << endl;
    as.emplace_back(A("emplace_1", 30));
    cout << as.capacity() << endl;
    
    cout << "--5--" << endl;
    as.emplace_back(a0);
    cout << as.capacity() << endl;
    
    cout << "--end--" << endl;
    cout << as.size() << endl;

    return 0;
}