C++ Vector 何时push_back深度复制对象？

问：

我创建了一个向量，并使用 push_back 将多个节点对象放入其中。但是，我无法预测它何时会使用移动构造函数或复制构造函数。

push_back使用复制构造函数或移动构造函数时有什么模式吗？C++ 参考资料说它有时会复制，有时会移动，但从未详细说明他们何时做什么。

#include <iostream>
#include <vector>
#include <unordered_set>
#include <set>
#include <unordered_map>
#include <map>
#include <queue>
using namespace std;
struct Node {
    int val;
    
    Node(int val) : val(val) {
        cout<<"created object" << val<<endl;
    }
    
    Node(const Node& m) : val(m.val) {
        cout<<"copy constructor is called on value " << m.val << endl;
    }
    
    ~Node() {
        cout<<"destroyed val" << val<<endl;
    }


    Node(Node&& other) noexcept 
        : val(other.val) {
            cout<<"moved val " << other.val << endl;
    }


};


void f(vector<Node>& a) {
    cout<<"______________________"<<endl;
    Node tmp(12);
    cout<<"12 established"<<endl;
    a.push_back(tmp);
    cout<<"a pushed back 12"<<endl;
    a.push_back(Node(14));
    
    cout<<"a pushed back tmp obj 14"<<endl;
    
    tmp.val+=5;
    
    cout<<"increased tmp.val"<<endl;
    cout<<tmp.val<<endl;
    cout<<a[1].val<<endl;
    cout<<"two prints"<<endl;
    cout<<"_______________"<<endl;
    cout<<"end of f"<<endl;
    // return a;
}

int main() {
    
    vector<Node> a = {Node(125)}; //copied since initialized temp var.
    a.reserve(4000);
    cout<<"start of f"<<endl;
    f(a);

    
    cout<<"program ended"<<endl;


    //noteiced: Copy constructor called upon local variable (12) that the vector knows will not stay with it --- and belongs to local scope.
    //copy constructor not called upon temporary variable that soon belonged to vector (14).
    //same thing with std::queue, std::stack, and many others.
//but it this a pattern or a coincidence?
}

输出：

copy constructor is called on value 125
destroyed val125
moved val 125
destroyed val125
start of f
______________________
created object12
12 established
copy constructor is called on value 12
a pushed back 12
created object14
moved val 14
destroyed val14
a pushed back tmp obj 14
increased tmp.val
17
12
two prints
_______________
end of f
destroyed val17
program ended
destroyed val125
destroyed val12
destroyed val14```