Emplace_back calls constructor move and destructor

Question

I am trying to replace two instances of the cBar class with a vector with the emplace_back function.

According to the reference, calling emplace_back only reserves a place in the vector, and then creates a new instance in place.

Now I tried to experiment with it:

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


class cBar
{
public:

    cBar(const int index);
    cBar(cBar&& other); //needed for emplace_back?
    ~cBar();

private:
    cBar(const cBar& other) = delete;
    cBar& operator=(const cBar& other) = delete;

public:
    int                         mIndex;

};




cBar::cBar(const int index) : mIndex(index)
{
    std::cout << "cBar being created ..." << std::endl;
}
cBar::cBar(cBar&& other) : mIndex(other.mIndex)
{
    std::cout << "cBar being moved ..." << std::endl;
}
cBar::~cBar()
{
    std::cout << "cBar being destroyed ..." << std::endl;
}

int main()
{
    std::vector<cBar> mBars;

    std::cout << "Begin to create 2 cBar instance, I am expecting 2 \"cBar being created ...\" messages here" << std::endl;
    mBars.emplace_back(0);//this supposed to call only cBar(const int index) constructor, and nothing more
    mBars.emplace_back(1);//this works as expected, only one constructor call

    //destroy all
    std::cout << "Destroy the 2 isntances (this works, I see the two expected \"cBar being destroyed ...\" messages here" << std::endl;
    mBars.clear();

    std::cin.get();
    return 0;
}

Conclusion:

Begin to create 2 cBar instance, I am expecting 2 "cBar being created ..." messages here
cBar being created ...
cBar being moved ...
cBar being destroyed ...
cBar being created ...
Destroy the 2 isntances (this works, I see the two expected "cBar being destroyed ..." messages here
cBar being destroyed ...
cBar being destroyed ...

If you run the one above, you will see that the first emplace_back creates an instance "in place", but then immediately calls the move constructor, and then the destructor is called.

What is more strange is that in the case of the second emplace, I see the expected behavior: only one constructor call.

I have two questions:

• , emplace_back push_back.

• , , ? , , , .

Visual Studio 2015.

:

Begin to create 2 cBar instance, I am expecting 2 "cBar being created ..." messages here
Vector size:0
cBar being created ...
Vector size:1
cBar being moved ...
cBar being destroyed ...
cBar being created ...
Vector size:2
Destroy the 2 isntances (this works, I see the two expected "cBar being destroyed ..." messages here
cBar being destroyed ...
cBar being destroyed ...

Answer 1

2. , , ?

emplace_back ; vector , / , .

1. , emplace_back push_back.

, vector /. . emplace_back, push_back, vector .

Answer 2

- , , . , , , .

. , , .