Assigning of unordered_map to pair of objects

Question

I am trying to understand the unordered_map assignment, I get the following error: no matching function for call to std::pair<foo, foo>::pair(), according to the doc for unordered_map operator[]:

If k does not match the key of any element in the container, the function inserts a new element with that key and returns a reference to its mapped value.

So I am trying to assign an object (from make_pair) to this reference, which I am guessing is not allowed. However with the pair<int,int>, it works, then I am wondering if I must declare some other operators for foo to make this work.

#include <bits/stdc++.h>
using namespace std;

struct foo {
  int n;
  foo(int n): n(n) {};
};

int main(){
  unordered_map<int, pair<foo,foo>> m;
  //m[3] = make_pair(foo(1),foo(2));         <--- error here

  unordered_map<int, pair<int,int>> ii;
  ii[3] = make_pair(1,2);
}

-bitmask, yes I have been staring at it for a while, I would post it here (quite long), if someone can help me decipher it, that would be great. — Frankie Y. Liu
– Frankie Y. Liu, Commented Mar 20, 2020 at 2:34
Well, it would be useful to be able to help you decipher it, if you'd post it. — bitmask
– bitmask, Commented Mar 20, 2020 at 14:31
-bitmask, I posted a follow up question, it is not the whole set of errors, let me know if I should add more/less. (stackoverflow.com/questions/60776989/…) — Frankie Y. Liu
– Frankie Y. Liu, Commented Mar 20, 2020 at 15:26
I provided you with a step-by-step guide of how to understand such an error under that question. Hope it helps. — bitmask
– bitmask, Commented Mar 20, 2020 at 17:03

Stephen Newell · Accepted Answer · 2020-03-20 02:06:15Z

2

The problem is that operator [] might have to construct a value object, in your case std::pair<foo, foo>. Since foo doesn't have a default constructor, it can't construct the default std::pair.

You can either provide a default constructor for foo (including adding a default value for n), or you'll have to use another method to insert values into m.

answered Mar 20, 2020 at 2:06

Stephen Newell

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2 Comments

Frankie Y. Liu Over a year ago

Thanks Stephen, if I use m.insert(make_pair(key,make_pair(foo(1),foo(2)))), then the default constructor is not invoked, is that correct? Or are there better ways to do this, thanks!

Stephen Newell Over a year ago

You're correct. You probably want to do some error-checking, but otherwise your suggestion is fine.

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