How to use unordered_map of unordered_maps in c++?

Question

I have below data structures problem ? Could you please help me out. So I my requirement is to initialise this data structures to default values when I add the new data items into this map.

How can I do this efficiently ?

I need a1, a2, a3 should be set to zero for each entry I am going to add.

struct a { int a1; int a2; int a3; }; struct A { struct a A1; struct a A2; }; unordered_map<int, unordered_map<int, struct A>> big_map;

Tried running below code.

 unordered_map<int, struct A> inner_map; big_map[0] = inner_map; struct A m; big_map[0][0] = m; cout << "Values: " << big_map[0][0].A1.a1 << ", " << big_map[0][0].A1.a2 << ", " << big_map[0][0].A1.a3 << endl;

Output:

g++ -std=c++11 -o exe b.cc ./exe Values: 0, 0, 1518395376 ./exe Values: 0, 0, -210403408 ./exe Values: 0, 0, -1537331360 ./exe Values: 0, 0, -915603664

So default initialisation is not being done for a3 ?

@SombreroChicken No need for that, after c++11 inserting to a map leads to a default initialisation, even if the type does not have a constructor e.g. ints, floats etc. — KostasRim
– KostasRim, Commented Jun 1, 2018 at 16:02
@KostasRim That was the generalized answer, not including C++11. But then again unordered_map is C++11 only so you're right. — Hatted Rooster
– Hatted Rooster, Commented Jun 1, 2018 at 16:06

Slava · Accepted Answer · 2018-06-01 16:06:36Z

6

Since C++11 you can do this:

struct a { int a1 = 0; int a2 = 0; int a3 = 0; };

answered Jun 1, 2018 at 16:06

Slava

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scohe001 · Accepted Answer · 2018-06-01 16:02:55Z

2

You can add a default constructor to your structs:

struct a { a() : a1(0), a2(0), a3(0) { } ... }; struct A { A() : A1(), A2() { } ... };

And then when you add a new struct you can do:

big_map[5][7] = A();

answered Jun 1, 2018 at 16:02

scohe001

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6 Comments

jpb123 Over a year ago

I did not understand why A1() ?, A1 is variable

scohe001 Over a year ago

@jpb123 A1 is a struct object. A1() in the initialization list is explicitly calling its default constructor to set a1, a2, a3 to 0.

Sean Burton Over a year ago

@jpb123 Do you mean in the initialiser-list of A's constructor? It default initialises the A1 member variable.

jpb123 Over a year ago

@scohe001, Thank you so much. I was not knowing this. It works.

jpb123 Over a year ago

@SeanBurton, A1(some_another_struct_a_variable) initializes A1 with some_another_struct_a_variable and A1() initializes A1 using default constructor. . Am I right ?

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physicist · Accepted Answer · 2018-06-01 16:04:11Z

You can give constructor to struct just like class, and init it to 0

struct a { int a1; int a2; int a3; a() : a1(0), a2(0), a3(0) { } }

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