C ++删除地图中的指针

Question

Deleting heap items with a pointer in a map

I have this map:

map<string, Plaats*> plaatsen;

In a function I am adding places to this map like this:

Plaats * fromPlace = new Plaats(from);
Plaats * toPlace = new Plaats(to);
auto insertedFrom = plaatsen.insert(pair<string,Plaats*>(from,fromPlace));
auto insertedTo = plaatsen.insert(pair<string,Plaats*>(to,toPlace));
//delete from or to if they are not inserted
if(!insertedFrom.second){
    delete fromPlace;
}
if(!insertedTo.second){
    delete toPlace;
}

If the element is added to my map, I need to delete it in my destructor.

KortstePad::~KortstePad(){
    //delete every item in plaatsen
    for(pair<string,Plaats*> place : plaatsen){
        //Plaats *p = place.second;
        delete place.second;
        place.second = nullptr;
    }

    for(pair<string,Plaats*> place : plaatsen){
        Plaats *p = place.second;
        cout << (p == nullptr) << endl;
    }
}

It appears my code is not deleting my places, because this is the output of the program:

0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0

Why is this code not setting my pointers to nullptr? Am I setting a local variable to nullptr?

Answer 1

You should use a reference to an element of the value type in the loop instead of using a copy. For example

for ( pair<const string,Plaats*> &place : plaatsen){
    //Plaats *p = place.second;
    delete place.second;
    place.second = nullptr;
}

Also take into account that the key shall have qualifier const pair<const string,Plaats*> .

Answer 2

You need to use a reference to avoid copying.

for(pair<string,Plaats*>& place : plaatsen){

Edit: I missed a const in the above answer. So Vlad's answer is better. Robinson's comment on my answer makes an even better answer.