Cuatom class | unordered map | map

Question

Hi

Please refer code below :

1. Why temporary object on map causes

Constructor
Copy contructor two times ?

One constructor and one copy contructor (to copy from main to map data) is ok but why third ?

Refer objects m1 and um1 in example code.

2. What's the need of reserve on unordered_map ? Isnt hash alone is enough ? What role reserve plays ? Reserve on vector is ok but why for unordered_map ?

3. Why object m2 and um2 calls different number of special functions (unlike m1 and m2 calls same nos.)?

https://code.sololearn.com/c3Ac0scIcKmn/?ref=app

Answer 1

1.try building the map element "in place" with emplace(), like this, to avoid creating first a pair :

map<int,test> m1;
m1.emplace(1,test())

to answer your second question i will refer you to a well explained post in stack overflow: https://stackoverflow.com/questions/42373232/why-does-stdunordered-map-have-a-reserve-method

I don't understand your 3rd question, i'm sorry (i am kinda in a rush today in real life 😅, sorry but maybe someone can explain better )

Answer 2

Thanks