为什么未调用复制构造函数？

Question

我有以下代码：

#include<iostream>
using namespace std;
class A{
    int size;
    double *arr;

public:
    A(int len):size(len)
    {
        cout<<"ctor called"<<endl;
        arr=new double[size];
    }
    A(const A &rhs)
    {
        cout<<"calling copy ctor"<<endl;
        size=rhs.size;
        arr=new double[size];
    }
    ~A()
    {
        cout<<"calling dtor"<<endl;
        delete[] arr;
        arr=NULL;
        size=0;
    }
};
A createVector()
{
    cout<<"Creating object"<<endl;
    A a(10);
    cout<<"returning after creating object a"<<endl;
    return a;
}
void foo(A v)
{
    cout<<"Class A object rcvd"<<endl;
    //return v;
}
int main()
{
    A a=createVector();
    cout<<"a has been rcvd"<<endl;
    foo(a);
    return 0;
}

我有以下输出：

Creating object
ctor called
returning after creating object a
a has been rcvd
calling copy ctor
Class A object rcvd
calling dtor
calling dtor

我在使用createVector（）函数时遇到麻烦。当我按值返回对象时，应调用其复制构造函数，但在行与行之间看不到任何输出

returning after creating object a
a has been rcvd

为什么没有调用复制构造函数？
（当我将rcvd对象作为值传递给foo（）时，将调用Copy构造函数）