How to use c++ unique_ptr with the map container?

Question

Is it possible to use the unique_ptr with the map container? For example,

#include<iostream>
#include<string>
#include<map>
#include<memory>
using namespace std;

int main(){
    
    unique_ptr<map<string, int>> person;
    person = make_unique<map<string, int>>(make_pair("Tom", 34));

    cout << (*person)["Tom"] << endl;  
}

person = make_unique<map<string, int>>(map<string, int>{ { "Tom", 34 } }); would work, and will use move semantics to make it relatively efficient. — Eljay
– Eljay, Commented Jan 22, 2021 at 16:36
Unrelated to your question. But why do you want to use unique_ptr<map<string, int>> instead of map<string, int>? Moving a std::map won't be more expensive than moving a unique_ptr. There are only rare cases where unique_ptr<map<string, int>> could be usefull. — t.niese
– t.niese, Commented Jan 22, 2021 at 16:53
Yes, it is possible. What problems did you have? Does this code work? If not, how not? — underscore_d
– underscore_d, Commented Jan 22, 2021 at 17:19

Giogre · Accepted Answer · 2021-01-22 17:15:27Z

2

How about defining:

#include<iostream>
#include<string>
#include<map>
#include<memory>
using namespace std;

int main(){
    unique_ptr<map<string, int>> person(new map<string, int>); 
    (*person)["Tom"] = 34;

    cout << (*person)["Tom"] << endl;  
}

Output is:

See for yourself: https://wandbox.org/permlink/CNaWMLYJTxQSfKwt .

edited Jan 22, 2021 at 17:15

answered Jan 22, 2021 at 16:34

Giogre

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9 Comments

t.niese Over a year ago

And what is the benefit over writing unique_ptr<map<string, int>> person = make_unique<map<string, int>>(); (*person)["Tom"] = 34;? Why do you suggest an additional map in your solution?

John Cvelth Over a year ago

Wait, doesn't it just create an additional COPY of the map for no reason?

Giogre Over a year ago

@t.niese I am quite tired today, so much so that in writing my answer I have glossed over these very fine points you make. I have modified my very flawed answer accordingly.

John Cvelth Over a year ago

@Lingo yep, good job. But, if I were nitpicking, I'd still prefer std::make_unique, as it looks better than a naked new, as well as using it to set the initial value.

John Cvelth Over a year ago

@Lingo Can't say I disagree :)

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