变量超出范围后如何保留矢量值？

Question

Assigning a local variable vector to a map<int , std::vector<int>> m1 in foo(), hoping that the value of s1 cannot be accessible once it goes out of scope. But that is not the case. Looks like the elements in vector are stored in heap memory and local variable s1 is stored in stack. when s1 was stored in map, it looks like it allocated a new heap memory and copied the values to it. Is my understanding right? I am printing the address of each vector element in foo and also the address of each vector element in map.

#include <iostream>
#include <map>
#include <set>
#include<vector>
using namespace std;


std::map<int , std::vector<int>> m1;

void foo(){
    vector<int> s1 = { 10, 20, 30, 40 };
    cout << "local var address: " << &s1 << "\n";
    cout << "Element address " << &s1[0] << "  " << &s1[1] << " "
         << &s1[3] << "  " << &s1[4] << "\n";
    m1[1] = s1;
}


int main() {
    foo();

    cout << "\nElement value and address in map:\n";
    for (auto it = m1[1].begin(); it != m1[1].end();it++) {
        cout << *it << " " << &m1[1][*it] << "\n";  
    }

    return 0;
}

output:

local var address: 0x7fff41714400
Element address 0xc07c20  0xc07c24 0xc07c2c  0xc07c30

Element value and address in map:
10 0xc08cc8
20 0xc08cf0
30 0xc08d18
40 0xc08d40

Answer 1

When you do m1[1] = s1; , you're calling m1[1] 's assignment operator . If you follow that link, you're calling the first instance, which cppreference describes as:

1) Copy assignment operator. Replaces the contents with a copy of the contents of other.

^{(emphasis mine)}

So you're looking at the addresses of two completely different vectors and two completely different sets of items. It makes no sense to compare them.

Answer 2

std::map<int , std::vector<int>>

has a value_type of std::vector<int> . That means that each item stored in the map contains such a vector object.

It doesn't contain a reference to a vector, or a pointer to a vector, it's actually an object.

That means that if you create a vector outside the map, and assign it to a map element, it must be moved or copied.