CODEWITHSUNDEEP
c++vector
JamesSin
JamesSin

Reputation: 39

C++, 2D std::vector, do I need to explicitly reserve and push back empty vectors?

When I make a 2D vector. std::vector<std::vector<Pokemon>> pokemons;

Can I run this straight away: Pokemon pikachu = ... pokemons[23].push_back(Pikachu);

Or do I need to: pokemons.reserve(100); for (int i =0;i<100;i++) pokemons.push_back(vector<Pokemon>());

Thank you

Upvotes: 2

Views: 884

Answers (3)

pierre.V
pierre.V

Reputation: 16

Are you sure the vector is the most appropriate container you are looking for? Check How can I efficiently select a Standard Library container in C++11?

If '23' is really important, then you could for example use associative container:

 typedef std::map<int, pokemon> TPokemons;
 TPokemons pokemons;
 pokemons[23] = pikachu;

Maybe the order is not important and an unordered map would be better

Upvotes: 0

Infiltrator
Infiltrator

Reputation: 1681

Can I run this straight away:

Pokemon pikachu = ...
pokemons[23].push_back(Pikachu);

Not straight away, because your pokemons array does not have any elements yet, so pokemons[23] will be an out-of-bounds error.

Otherwise, once your pokemons array is populated, then yes, you can just push_back on one of its elements.

Or do I need to:

pokemons.reserve(100);
for (int i =0;i<100;i++) pokemons.push_back(vector<Pokemon>());

reserve() is only to allocate (reserve) memory for your vector, if you have a good idea of how many elements you will end up having, to avoid multiple memory allocations and potential moves. You do not need to reserve(), but it is a good idea if you will need a decent amount of memory.

Upvotes: 0

Bo Persson
Bo Persson

Reputation: 92231

You can set the initial size of the outer vector during construction

std::vector<std::vector<Pokemon>> pokemons(100);

and it will build 100 inner empty vectors.

Upvotes: 5

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